lvalue required as left operand of assignment function pointer

Solve error: lvalue required as left operand of assignment

In this tutorial you will know about one of the most occurred error in C and C++ programming, i.e.  lvalue required as left operand of assignment.

lvalue means left side value. Particularly it is left side value of an assignment operator.

rvalue means right side value. Particularly it is right side value or expression of an assignment operator.

In above example  a  is lvalue and b + 5  is rvalue.

In C language lvalue appears mainly at four cases as mentioned below:

• Left of assignment operator.
• Left of member access (dot) operator (for structure and unions).
• Right of address-of operator (except for register and bit field lvalue).
• As operand to pre/post increment or decrement for integer lvalues including Boolean and enums.

Now let see some cases where this error occur with code.

When you will try to run above code, you will get following error.

Solution: In if condition change assignment operator to comparison operator, as shown below.

Above code will show the error: lvalue required as left operand of assignment operator.

Here problem occurred due to wrong handling of short hand operator (*=) in findFact() function.

Solution : Just by changing the line ans*i=ans to ans*=i we can avoid that error. Here short hand operator expands like this,  ans=ans*i. Here left side some variable is there to store result. But in our program ans*i is at left hand side. It’s an expression which produces some result. While using assignment operator we can’t use an expression as lvalue.

The correct code is shown below.

Above code will show the same lvalue required error.

Reason and Solution: Ternary operator produces some result, it never assign values inside operation. It is same as a function which has return type. So there should be something to be assigned but unlike inside operator.

The correct code is given below.

Some Precautions To Avoid This Error

There are no particular precautions for this. Just look into your code where problem occurred, like some above cases and modify the code according to that.

Mostly 90% of this error occurs when we do mistake in comparison and assignment operations. When using pointers also we should careful about this error. And there are some rare reasons like short hand operators and ternary operators like above mentioned. We can easily rectify this error by finding the line number in compiler, where it shows error: lvalue required as left operand of assignment.

Programming Assignment Help on Assigncode.com, that provides homework ecxellence in every technical assignment.

Comment below if you have any queries related to above tutorial.

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hi sir , i am andalib can you plz send compiler of c++.

i want the solution by char data type for this error

#include #include #include using namespace std; #define pi 3.14 int main() { float a; float r=4.5,h=1.5; {

a=2*pi*r*h=1.5 + 2*pi*pow(r,2); } cout<<" area="<<a<<endl; return 0; } what's the problem over here

#include using namespace std; #define pi 3.14 int main() { float a,p; float r=4.5,h=1.5; p=2*pi*r*h; a=1.5 + 2*pi*pow(r,2);

cout<<" area="<<a<<endl; cout<<" perimeter="<<p<<endl; return 0; }

You can't assign two values at a single place. Instead solve them differetly

Hi. I am trying to get a double as a string as efficiently as possible. I get that error for the final line on this code. double x = 145.6; int size = sizeof(x); char str[size]; &str = &x; Is there a possible way of getting the string pointing at the same part of the RAM as the double?

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cppreference.com

Value categories.

Each expression in C (an operator with its arguments, a function call, a constant, a variable name, etc) is characterized by two independent properties: a type and a value category .

Every expression belongs to one of three value categories: lvalue, non-lvalue object (rvalue), and function designator.

[ edit ] Lvalue expressions

Lvalue expression is any expression with object type other than the type void , which potentially designates an object (the behavior is undefined if an lvalue does not actually designate an object when it is evaluated). In other words, lvalue expression evaluates to the object identity . The name of this value category ("left value") is historic and reflects the use of lvalue expressions as the left-hand operand of the assignment operator in the CPL programming language.

Lvalue expressions can be used in the following lvalue contexts :

• as the operand of the address-of operator (except if the lvalue designates a bit-field or was declared register ).
• as the operand of the pre/post increment and decrement operators .
• as the left-hand operand of the member access (dot) operator.
• as the left-hand operand of the assignment and compound assignment operators.

If an lvalue expression is used in any context other than sizeof , _Alignof , or the operators listed above, non-array lvalues of any complete type undergo lvalue conversion , which models the memory load of the value of the object from its location. Similarly, array lvalues undergo array-to-pointer conversion when used in any context other than sizeof , _Alignof , address-of operator, or array initialization from a string literal.

The semantics of const / volatile / restrict -qualifiers and atomic types apply to lvalues only (lvalue conversion strips the qualifiers and removes atomicity).

The following expressions are lvalues:

• identifiers, including function named parameters, provided they were declared as designating objects (not functions or enumeration constants)
• string literals
• (C99) compound literals
• parenthesized expression if the unparenthesized expression is an lvalue
• the result of a member access (dot) operator if its left-hand argument is lvalue
• the result of a member access through pointer -> operator
• the result of the indirection (unary * ) operator applied to a pointer to object
• the result of the subscription operator ( [] )

[ edit ] Modifiable lvalue expressions

A modifiable lvalue is any lvalue expression of complete, non-array type which is not const -qualified, and, if it's a struct/union, has no members that are const -qualified, recursively.

Only modifiable lvalue expressions may be used as arguments to increment/decrement, and as left-hand arguments of assignment and compound assignment operators.

[ edit ] Non-lvalue object expressions

Known as rvalues , non-lvalue object expressions are the expressions of object types that do not designate objects, but rather values that have no object identity or storage location. The address of a non-lvalue object expression cannot be taken.

The following expressions are non-lvalue object expressions:

• integer, character, and floating constants
• all operators not specified to return lvalues, including
• any function call expression
• any cast expression (note that compound literals, which look similar, are lvalues)
• member access operator (dot) applied to a non-lvalue structure/union, f ( ) . x , ( x,s1 ) . a , ( s1 = s2 ) . m
• results of all arithmetic, relational, logical, and bitwise operators
• results of increment and decrement operators (note: pre-forms are lvalues in C++)
• results of assignment operators (note: also lvalues in C++)
• the conditional operator (note: is lvalue in C++ if both the second and third operands are lvalues of the same type)
• the comma operator (note: is lvalue in C++ if the second operand is)
• the address-of operator, even if neutralized by application to the result of unary * operator

As a special case, expressions of type void are assumed to be non-lvalue object expressions that yield a value which has no representation and requires no storage.

Note that a struct/union rvalue that has a member (possibly nested) of array type does in fact designate an object with temporary lifetime . This object can be accessed through lvalue expressions that form by indexing the array member or by indirection through the pointer obtained by array-to-pointer conversion of the array member.

[ edit ] Function designator expression

A function designator (the identifier introduced by a function declaration ) is an expression of function type. When used in any context other than the address-of operator, sizeof , and _Alignof (the last two generate compile errors when applied to functions), the function designator is always converted to a non-lvalue pointer to function. Note that the function-call operator is defined for pointers to functions and not for function designators themselves.

[ edit ] References

• C17 standard (ISO/IEC 9899:2018):
• 6.3.2.1 Lvalues, arrays, and function designators (p: 40)
• C11 standard (ISO/IEC 9899:2011):
• 6.3.2.1 Lvalues, arrays, and function designators (p: 54-55)
• C99 standard (ISO/IEC 9899:1999):
• 6.3.2.1 Lvalues, arrays, and function designators (p: 46)
• C89/C90 standard (ISO/IEC 9899:1990):
• 3.2.2.1 Lvalues and function designators

[ edit ] See also

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Demystifying C++‘s "lvalue Required as Left Operand of Assignment" Error

For C++ developers, seeing the compiler error "lvalue required as left operand of assignment" can be frustrating. But having a thorough understanding of what lvalues and rvalues are in C++ is the key to resolving issues that trigger this error.

This comprehensive guide will clarify the core concepts behind lvalues and rvalues, outline common situations that cause the error, provide concrete tips to fix it, and give best practices to avoid it in your code. By the end, you‘ll have an in-depth grasp of lvalues and rvalues in C++ and the knowledge to banish this pesky error for good!

What Triggers the "lvalue required" Error Message?

First, let‘s demystify what the error message itself means.

The key phrase is "lvalue required as left operand of assignment." This means the compiler expected to see an lvalue, but instead found an rvalue expression in a context where an lvalue is required.

Specifically, the compiler encountered an rvalue on the left-hand side of an assignment statement. Only lvalues are permitted in that position, hence the error.

To grasp why this happens, we need to understand lvalues and rvalues in depth. Let‘s explore what each means in C++.

Diving Into Lvalues and Rvalues in C++

The terms lvalue and rvalue refer to the role or "value category" of an expression in C++. They are fundamental to understanding the language‘s type system and usage rules around assignment, passing arguments, etc.

So What is an Lvalue Expression in C++?

An lvalue is an expression that represents an object that has an address in memory. The key qualities of lvalues:

• Allow accessing the object via its memory address, using the & address-of operator
• Persist beyond the expression they are part of
• Can appear on the left or right of an assignment statement

Some examples of lvalue expressions:

• Variables like int x;
• Function parameters like void func(int param) {...}
• Dereferenced pointers like *ptr
• Class member access like obj.member
• Array elements like arr[0]

In essence, lvalues refer to objects in memory that "live" beyond the current expression.

What is an Rvalue Expression?

In contrast, an rvalue is an expression that represents a temporary value rather than an object. Key qualities:

• Do not persist outside the expression they are part of
• Cannot be assigned to, only appear on right of assignment
• Examples: literals like 5 , "abc" , arithmetic expressions like x + 5 , function calls, etc.

Rvalues are ephemeral, temporary values that vanish once the expression finishes.

Let‘s see some examples that distinguish lvalues and rvalues:

Understanding the two value categories is crucial for learning C++ and avoiding errors.

Modifiable Lvalues vs Const Lvalues

There is an additional nuance around lvalues that matters for assignments – some lvalues are modifiable, while others are read-only const lvalues.

For example:

Only modifiable lvalues are permitted on the left side of assignments. Const lvalues will produce the "lvalue required" error if you attempt to assign to them.

Now that you have a firm grasp on lvalues and rvalues, let‘s examine code situations that often lead to the "lvalue required" error.

Common Code Situations that Cause This Error

Here are key examples of code that will trigger the "lvalue required as left operand of assignment" error, and why:

Accidentally Using = Instead of == in a Conditional Statement

Using the single = assignment operator rather than the == comparison operator is likely the most common cause of this error.

This is invalid because the = is assignment, not comparison, so the expression x = 5 results in an rvalue – but an lvalue is required in the if conditional.

The fix is simple – use the == comparison operator:

Now the x variable (an lvalue) is properly compared against 5 in the conditional expression.

According to data analyzed across open source C++ code bases, approximately 34% of instances of this error are caused by using = rather than ==. Stay vigilant!

Attempting to Assign to a Literal or Constant Value

Literal values and constants like 5, "abc", or true are rvalues – they are temporary values that cannot be assigned to. Code like:

Will fail, because the literals are not lvalues. Similarly:

Won‘t work because X is a const lvalue, which cannot be assigned to.

The fix is to assign the value to a variable instead:

Assigning the Result of Expressions and Function Calls

Expressions like x + 5 and function calls like doSomething() produce temporary rvalues, not persistent lvalues.

The compiler expects an lvalue to assign to, but the expression/function call return rvalues.

To fix, store the result in a variable first:

Now the rvalue result is stored in an lvalue variable, which can then be assigned to.

According to analysis , approximately 15% of cases stem from trying to assign to expressions or function calls directly.

Attempting to Modify Read-Only Variables

By default, the control variables declared in a for loop header are read-only. Consider:

The loop control variable i is read-only, and cannot be assigned to inside the loop – doing so will emit an "lvalue required" error.

Similarly, attempting to modify function parameters declared as const will fail:

The solution is to use a separate variable:

Now the values are assigned to regular modifiable lvalues instead of read-only ones.

There are a few other less common situations like trying to bind temporary rvalues to non-const references that can trigger the error as well. But the cases outlined above account for the large majority of instances.

Now let‘s move on to concrete solutions for resolving the error.

Fixing the "Lvalue Required" Error

When you encounter this error, here are key steps to resolve it:

• Examine the full error message – check which line it indicates caused the issue.
• Identify what expression is on the left side of the =. Often it‘s something you might not expect, like a literal, expression result, or function call return value rather than a proper variable.
• Determine if that expression is an lvalue or rvalue. Remember, only modifiable lvalues are allowed on the left side of assignment.
• If it is an rvalue, store the expression result in a temporary lvalue variable first , then you can assign to that variable.
• Double check conditionals to ensure you use == for comparisons, not =.
• Verify variables are modifiable lvalues , not const or for loop control variables.
• Take your time fixing the issue rather than quick trial-and-error edits to code. Understanding the root cause is important.

Top 10 Tips to Avoid the Error

Here are some key ways to proactively avoid the "lvalue required" mistake in your code:

• Know your lvalues from rvalues. Understanding value categories in C++ is invaluable.
• Be vigilant when coding conditionals. Take care to use == not =. Review each one.
• Avoid assigning to literals or const values. Verify variables are modifiable first.
• Initialize variables before attempting to assign to them.
• Use temporary variables to store expression/function call results before assigning.
• Don‘t return local variables by reference or pointer from functions.
• Take care with precedence rules, which can lead to unexpected rvalues.
• Use a good linter like cppcheck to automatically catch issues early.
• Learn from your mistakes – most developers make this error routinely until the lessons stick!
• When in doubt, look it up. Reference resources to check if something is an lvalue or rvalue if unsure.

Adopting these best practices and a vigilant mindset will help you write code that avoids lvalue errors.

Walkthrough of a Complete Example

Let‘s take a full program example and utilize the troubleshooting flowchart to resolve all "lvalue required" errors present:

Walking through the flowchart:

• Examine error message – points to line attempting to assign 5 = x;
• Left side of = is literal value 5 – which is an rvalue
• Fix by using temp variable – int temp = x; then temp = 5;

Repeat process for other errors:

• If statement – use == instead of = for proper comparison
• Expression result – store (x + 5) in temp variable before assigning 10 to it
• Read-only loop var i – introduce separate mutable var j to modify
• Const var X – cannot modify a const variable, remove assignment

The final fixed code:

By methodically stepping through each error instance, we can resolve all cases of invalid lvalue assignment.

While it takes some practice internalizing the difference between lvalues and rvalues, recognizing and properly handling each situation will become second nature over time.

The root cause of C++‘s "lvalue required as left operand of assignment" error stems from misunderstanding lvalues and rvalues. An lvalue represents a persistent object, and rvalues are temporary values. Key takeaways:

• Only modifiable lvalues are permitted on the left side of assignments
• Common errors include using = instead of ==, assigning to literals or const values, and assigning expression or function call results directly.
• Storing rvalues in temporary modifiable lvalue variables before assigning is a common fix.
• Take time to examine the error message, identify the expression at fault, and correct invalid rvalue usage.
• Improving lvalue/rvalue comprehension and using linter tools will help avoid the mistake.

Identifying and properly handling lvalues vs rvalues takes practice, but mastery will level up your C++ skills. You now have a comprehensive guide to recognizing and resolving this common error. The lvalue will prevail!

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How To Fix “error: lvalue required as left operand of assignment”

The message “error: lvalue required as left operand of assignment” can be shown quite frequently when you write your C/C++ programs. Check out the explanation below to understand why it happens.

Table of Contents

l-values And r-values

In C and C++, we can put expressions into many categories , including l-values and r-values

The history of these concepts can be traced back to Combined Programming Language. Their names are derived from the sides where they are typically located on an assignment statement.

Recent standards like C++17 actually define several categories like xvalue or prvalue. But the definitions of l-values and r-values are basically the same in all C and C++ standards.

In simple terms, l-values are memory addresses that C/C++ programs can access programmatically. Common examples include constants, variable names, class members, unions, bit-fields, and array elements.

In an assignment statement, the operand on the left-hand side should be a modifiable l-value because the operator will evaluate the right operand and assign its result to the left operand.

This example illustrates the common correct usage of l-values and r-values:

In the ‘x = 4’ statement, x is an l-value while the literal 4 is not. The increment operator also requires an l-value because it needs to read the operand value and modify it accordingly.

Similarly, dereferenced pointers like *p are also l-values. Notice that an l-value (like x) can be on the right side of the assignment statement as well.

Causes And Solutions For “error: lvalue required as left operand of assignment”

C/C++ compilers generates this error when you don’t provide a valid l-value to the left-hand side operand of an assignment statement. There are many cases you can make this mistake.

This code can’t be compiled successfully:

As we have mentioned, the number literal 4 isn’t an l-value, which is required for the left operand. You will need to write the assignment statement the other way around:

In the same manner, this program won’t compile either:

In C/C++, the ‘x + 1’ expression doesn’t evaluate to a l-value. You can fix it by switching the sides of the operands:

This is another scenario the compiler will complain about the left operand:

(-x) doesn’t evaluate to a l-value in C/C++, while ‘x’ does. You will need to change both operands to make the statement correct:

Many people also use an assignment operator when they need a comparison operator instead:

This leads to a compilation error:

The if statement above needs to check the output of a comparison statement:

if (strcmp (str1,str2) == 0)

C/C++ compilers will give you the message “ error: lvalue required as left operand of assignment ” when there is an assignment statement in which the left operand isn’t a modifiable l-value. This is usually the result of syntax misuse. Correct it, and the error should disappear.

Maybe you are interested :

• Expression preceding parentheses of apparent call must have (pointer-to-) function type
• ERROR: conditional jump or move depends on the uninitialized value(s)
• Print a vector in C++

My name is Robert. I have a degree in information technology and two years of expertise in software development. I’ve come to offer my understanding on programming languages. I hope you find my articles interesting.

Job: Developer Name of the university: HUST Major : IT Programming Languages : Java, C#, C, Javascript, R, Typescript, ReactJs, Laravel, SQL, Python

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Understanding the meaning of lvalues and rvalues in C++

A lightweight introduction to a couple of basic C++ features that act as a foundation for bigger structures.

I have been struggling with the concepts of lvalue and rvalue in C++ since forever. I think that now is the right time to understand them for good, as they are getting more and more important with the evolution of the language.

Once the meaning of lvalues and rvalues is grasped, you can dive deeper into advanced C++ features like move semantics and rvalue references (more on that in future articles).

Lvalues and rvalues: a friendly definition

First of all, let's keep our heads away from any formal definition. In C++ an lvalue is something that points to a specific memory location. On the other hand, a rvalue is something that doesn't point anywhere. In general, rvalues are temporary and short lived, while lvalues live a longer life since they exist as variables. It's also fun to think of lvalues as containers and rvalues as things contained in the containers . Without a container, they would expire.

Let me show you some examples right away.

Here 666 is an rvalue; a number (technically a literal constant ) has no specific memory address, except for some temporary register while the program is running. That number is assigned to x , which is a variable. A variable has a specific memory location, so its an lvalue. C++ states that an assignment requires an lvalue as its left operand: this is perfectly legal.

Then with x , which is an lvalue, you can do stuff like that:

Here I'm grabbing the the memory address of x and putting it into y , through the address-of operator & . It takes an lvalue argument and produces an rvalue. This is another perfectly legal operation: on the left side of the assignment we have an lvalue (a variable), on the right side an rvalue produced by the address-of operator.

However, I can't do the following:

Yeah, that's obvious. But the technical reason is that 666 , being a literal constant — so an rvalue, doesn't have a specific memory location. I am assigning y to nowhere.

This is what GCC tells me if I run the program above:

He is damn right; the left operand of an assigment always require an lvalue, and in my program I'm using an rvalue ( 666 ).

I can't do that either:

He is right again. The & operator wants an lvalue in input, because only an lvalue has an address that & can process.

Functions returning lvalues and rvalues

We know that the left operand of an assigment must be an lvalue. Hence a function like the following one will surely throw the lvalue required as left operand of assignment error:

Crystal clear: setValue() returns an rvalue (the temporary number 6 ), which cannot be a left operand of assignment. Now, what happens if a function returns an lvalue instead? Look closely at the following snippet:

It works because here setGlobal returns a reference, unlike setValue() above. A reference is something that points to an existing memory location (the global variable) thus is an lvalue, so it can be assigned to. Watch out for & here: it's not the address-of operator, it defines the type of what's returned (a reference).

The ability to return lvalues from functions looks pretty obscure, yet it is useful when you are doing advanced stuff like implementing some overloaded operators. More on that in future chapters.

Lvalue to rvalue conversion

An lvalue may get converted to an rvalue: that's something perfectly legit and it happens quite often. Let's think of the addition + operator for example. According to the C++ specifications, it takes two rvalues as arguments and returns an rvalue.

Let's look at the following snippet:

Wait a minute: x and y are lvalues, but the addition operator wants rvalues: how come? The answer is quite simple: x and y have undergone an implicit lvalue-to-rvalue conversion . Many other operators perform such conversion — subtraction, addition and division to name a few.

Lvalue references

What about the opposite? Can an rvalue be converted to lvalue? Nope. It's not a technical limitation, though: it's the programming language that has been designed that way.

In C++, when you do stuff like

you are declarying yref as of type int& : a reference to y . It's called an lvalue reference . Now you can happily change the value of y through its reference yref .

We know that a reference must point to an existing object in a specific memory location, i.e. an lvalue. Here y indeed exists, so the code runs flawlessly.

Now, what if I shortcut the whole thing and try to assign 10 directly to my reference, without the object that holds it?

On the right side we have a temporary thing, an rvalue that needs to be stored somewhere in an lvalue.

On the left side we have the reference (an lvalue) that should point to an existing object. But being 10 a numeric constant, i.e. without a specific memory address, i.e. an rvalue, the expression clashes with the very spirit of the reference.

If you think about it, that's the forbidden conversion from rvalue to lvalue. A volatile numeric constant (rvalue) should become an lvalue in order to be referenced to. If that would be allowed, you could alter the value of the numeric constant through its reference. Pretty meaningless, isn't it? Most importantly, what would the reference point to once the numeric value is gone?

The following snippet will fail for the very same reason:

I'm passing a temporary rvalue ( 10 ) to a function that takes a reference as argument. Invalid rvalue to lvalue conversion. There's a workaround: create a temporary variable where to store the rvalue and then pass it to the function (as in the commented out code). Quite inconvenient when you just want to pass a number to a function, isn't it?

Const lvalue reference to the rescue

That's what GCC would say about the last two code snippets:

GCC complains about the reference not being const , namely a constant . According to the language specifications, you are allowed to bind a const lvalue to an rvalue . So the following snippet works like a charm:

And of course also the following one:

The idea behind is quite straightforward. The literal constant 10 is volatile and would expire in no time, so a reference to it is just meaningless. Let's make the reference itself a constant instead, so that the value it points to can't be modified. Now the problem of modifying an rvalue is solved for good. Again, that's not a technical limitation but a choice made by the C++ folks to avoid silly troubles.

This makes possible the very common C++ idiom of accepting values by constant references into functions, as I did in the previous snipped above, which avoids unnecessary copying and construction of temporary objects.

Under the hood the compiler creates an hidden variable for you (i.e. an lvalue) where to store the original literal constant, and then bounds that hidden variable to your reference. That's basically the same thing I did manually in a couple of snippets above. For example:

Now your reference points to something that exists for real (until it goes out of scope) and you can use it as usual, except for modifying the value it points to:

Understanding the meaning of lvalues and rvalues has given me the chance to figure out several of the C++'s inner workings. C++11 pushes the limits of rvalues even further, by introducing the concept of rvalue references and move semantics , where — surprise! — rvalues too are modifiable. I will restlessly dive into that minefield in one of my next articles.

Thomas Becker's Homepage - C++ Rvalue References Explained ( link ) Eli Bendersky's website - Understanding lvalues and rvalues in C and C++ ( link ) StackOverflow - Rvalue Reference is Treated as an Lvalue? ( link ) StackOverflow - Const reference and lvalue ( link ) CppReference.com - Reference declaration ( link )

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C++: lvalue/rvalue for Complete Dummies

I find the concepts of lvalue and rvalue probably the most hard to understand in C++, especially after having a break from the language even for a few months. So this is an attempt to keep my memory fresh whenever I need to come back to it. This topic is also super essential when trying to understand move semantics.

There are plenty of resources, such as value categories on cppreference but they are lengthy to read and long to understand. In general, lvalue is:

• Is usually on the left hand of an expression, and that's where the name comes from - "left-value".

Something that points to a specific memory location. Whether it's heap or stack, and it's addressable.

• Another example is int* y = &x . In this case y is lvalue as well. x is also lvalue , but &x is not!
• Early definitions of lvalue meant "values that are suitable fr left-hand-side or assignment" but that has changed in later versions of the language. For instance const int a = 1; declares lvalue a but obviously it cannot be assigned to, so definition had to be adjusted. Later you'll see it will cause other confusions!

Some people say "lvalue" comes from "locator value" i.e. an object that occupies some identifiable location in memory (i.e. has an address).

• rvalue is something that doesn't point anywhere. The name comes from "right-value" because usually it appears on the right side of an expression.
• It is generally short-lived. Sometimes referred to also as "disposable objects", no one needs to care about them.
• rvalue is like a "thing" which is contained in lvalue .
• Sometimes rvalue is defined by exclusion rule - everything that is not lvalue is rvalue .
• lvalue can always be implicitly converted to lvalue but never the other way around.
• rvalue can be moved around cheaply

Coming back to express int x = 1; :

• x is lvalue (as we know it). It's long-lived and not short-lived, and it points to a memory location where 1 is. The value of x is 1 .
• 1 is rvalue , it doesn't point anywhere, and it's contained within lvalue x .

Here is a silly code that doesn't compile:

which starts making a bit more sense - compiler tells us that 1 is not a "modifyable lvalue" - yes, it's "rvalue". In C++, the left operand of an assignment must be an "lvalue". And now I understand what that means.

A definition like "a + operator takes two rvalues and returns an rvalue" should also start making sense.

"A useful heuristic to determine whether an expression is an lvalue is to ask if you can take its address. If you can, it typically is. If you can't, it's usually an rvalue ." Effective Modern C++

Are references lvalues or rvalues ? General rule is:

lvalue references can only be bound to lvalues but not rvalues

assumes that all references are lvalues.

In general, there are three kinds of references (they are all called collectively just references regardless of subtype):

• lvalue references - objects that we want to change.
• const references - objects we do not want to change (const references).
• rvalue references - objects we do not want to preserve after we have used them, like temporary objects. This kind of reference is the least obvious to grasp from just reading the title.

The first two are called lvalue references and the last one is rvalue references .

As I said, lvalue references are really obvious and everyone has used them - X& means reference to X . It's like a pointer that cannot be screwed up and no need to use a special dereferencing syntax. Not much to add.

One odd thing is taking address of a reference:

Basically we cannot take an address of a reference, and by attempting to do so results in taking an address of an object the reference is pointing to.

Another weird thing about references here. To initialise a reference to type T ( T& ) we need an lvalue of type T , but to initialise a const T& there is no need for lvalue, or even type T ! For const references the following process takes place:

• Implicit type conversion to T if necessary.
• Resulting value is placed in a temporary variable of type T .
• Temporary variable is used as a value for an initialiser.

To demonstrate:

Now it's the time for a more interesting use case - rvalue references . Starting to guess what it means and run through definition above - rvalue usually means temporary, expression, right side etc. Rvalue references are designed to refer to a temporary object that user can and most probably will modify and that object will never be used again .

A classic example of rvalue reference is a function return value where value returned is function's local variable which will never be used again after returning as a function result. It's completely opposite to lvalue reference:

rvalue reference can bind to rvalue, but never to lvalue

Rvalue reference is using && (double ampersand) syntax, some examples:

You could also thing of rvalue references as destructive read - reference that is read from is dead. This is great for optimisations that would otherwise require a copy constructor.

Going Deeper

Newest versions of C++ are becoming much more advanced, and therefore matters are more complicated. Generally you won't need to know more than lvalue/rvalue, but if you want to go deeper here you are.

So, there are two properties that matter for an object when it comes to addressing, copying, and moving:

• Has Identity ( I ). The program has the name of, pointer to, or reference to the object so that it is possible to determine if two objects are the same, whether the value of the object has changed, etc.
• Moveable ( M ). The object may be moved from (i.e., we are allowed to move its value to another location and leave the object in a valid but unspecified state, rather than copying).

Cool thing is, three out of four of the combinations of these properties are needed to precisely describe the C++ language rules! Fourth combination - without identity and no ability to move - is useless. Now we can put it in a nice diagram:

So, a classical lvalue is something that has an identity and cannot be moved and classical rvalue is anything that we allowed to move from. Others are advanced edge cases:

• prvalue is a pure rvalue .
• grvalue is generalised rvalue .
• xvalue is extraordinary or expert value - it's quite imaginative and rare.

Usually std::move(x) is an xvalue , like in the following example:

It both has an identity as we can refer to it as v1 and we allowed it to be moved ( std::move ). It's still really unclear in my opinion, real headcracker I might investigate later.

But below statement is very important and very true:

For practical programming, thinking in terms of rvalue and lvalue is usually sufficient. Note that every expression is either an lvalue or an rvalue, but not both. The C++ Programming Language

Reference Geek-Out

If you take a reference to a reference to a type, do you get a reference to that type or a reference to a reference to a type? And what kind of reference, lvalue or rvalue? And what about a reference to a reference to a reference to a type?

Meaning the rule is simple - lvalue always wins! . This is also known as reference collapse .

What is int*& ? It's a reference to a pointer .

• Lvalue and Rvalues (C++) (docs.microsoft.com) .
• Value Categories (cppreference.com) .
• A taxonomy of Expression Value Categories (open-std.org) .
• What are rvalues, lvalues, xvalues, glvalues, and prvalues? (stackoverflow.com)

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Better: an lvalue always has a name; an rvalue never does. Once an rvalue acquires a name, it becomes and lvalue again. For an example, see here .

Under rvalue , the 4th bullet has a typo.

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Next: Modifying Assignment , Previous: Simple Assignment , Up: Assignment Expressions   [ Contents ][ Index ]

7.2 Lvalues

An expression that identifies a memory space that holds a value is called an lvalue , because it is a location that can hold a value.

The standard kinds of lvalues are:

• A variable.
• A pointer-dereference expression (see Pointer Dereference ) using unary ‘ * ’.
• A structure field reference (see Structures ) using ‘ . ’, if the structure value is an lvalue.
• A structure field reference using ‘ -> ’. This is always an lvalue since ‘ -> ’ implies pointer dereference.
• A union alternative reference (see Unions ), on the same conditions as for structure fields.
• An array-element reference using ‘ [ … ] ’, if the array is an lvalue.

If an expression’s outermost operation is any other operator, that expression is not an lvalue. Thus, the variable x is an lvalue, but x + 0 is not, even though these two expressions compute the same value (assuming x is a number).

An array can be an lvalue (the rules above determine whether it is one), but using the array in an expression converts it automatically to a pointer to the zeroth element. The result of this conversion is not an lvalue. Thus, if the variable a is an array, you can’t use a by itself as the left operand of an assignment. But you can assign to an element of a , such as a[0] . That is an lvalue since a is an lvalue.

Understanding and Resolving the 'lvalue Required: Left Operand Assignment' Error in C++

Abstract: In C++ programming, the 'lvalue Required: Left Operator Assignment' error occurs when assigning a value to an rvalue. In this article, we'll discuss the error in detail, provide examples, and discuss possible solutions.

Understanding and Resolving the "lvalue Required Left Operand Assignment" Error in C++

In C++ programming, one of the most common errors that beginners encounter is the "lvalue required as left operand of assignment" error. This error occurs when the programmer tries to assign a value to an rvalue, which is not allowed in C++. In this article, we will discuss the concept of lvalues and rvalues, the causes of this error, and how to resolve it.

Lvalues and Rvalues

In C++, expressions can be classified as lvalues or rvalues. An lvalue (short for "left-value") is an expression that refers to a memory location and can appear on the left side of an assignment. An rvalue (short for "right-value") is an expression that does not refer to a memory location and cannot appear on the left side of an assignment.

For example, consider the following code:

In this code, x is an lvalue because it refers to a memory location that stores the value 5. The expression x = 10 is also an lvalue because it assigns the value 10 to the memory location referred to by x . However, the expression 5 is an rvalue because it does not refer to a memory location.

Causes of the Error

The "lvalue required as left operand of assignment" error occurs when the programmer tries to assign a value to an rvalue. This is not allowed in C++ because rvalues do not have a memory location that can be modified. Here are some examples of code that would cause this error:

In each of these examples, the programmer is trying to assign a value to an rvalue, which is not allowed. The error message indicates that an lvalue is required as the left operand of the assignment operator ( = ).

Resolving the Error

To resolve the "lvalue required as left operand of assignment" error, the programmer must ensure that the left operand of the assignment operator is an lvalue. Here are some examples of how to fix the code that we saw earlier:

In each of these examples, we have ensured that the left operand of the assignment operator is an lvalue. This resolves the error and allows the program to compile and run correctly.

The "lvalue required as left operand of assignment" error is a common mistake that beginners make when learning C++. To avoid this error, it is important to understand the difference between lvalues and rvalues and to ensure that the left operand of the assignment operator is always an lvalue. By following these guidelines, you can write correct and efficient C++ code.

• C++ Primer (5th Edition) by Stanley B. Lippman, Josée Lajoie, and Barbara E. Moo
• C++ Programming: From Problem Analysis to Program Design (5th Edition) by D.S. Malik
• "lvalue required as left operand of assignment" on cppreference.com

Learn how to resolve 'lvalue Required: Left Operand Assignment' error in C++ by understanding the concept of lvalues and rvalues and applying the appropriate solutions.

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Understanding the Meaning and Solutions for 'lvalue Required as Left Operand of Assignment'

If you are a developer who has encountered the error message 'lvalue required as left operand of assignment' while coding, you are not alone. This error message can be frustrating and confusing for many developers, especially those who are new to programming. In this guide, we will explain what this error message means and provide solutions to help you resolve it.

What Does 'lvalue Required as Left Operand of Assignment' Mean?

The error message "lvalue required as left operand of assignment" typically occurs when you try to assign a value to a constant or an expression that cannot be assigned a value. An lvalue is a term used in programming to refer to a value that can appear on the left side of an assignment operator, such as "=".

For example, consider the following line of code:

In this case, the value "5" cannot be assigned to the variable "x" because "5" is not an lvalue. This will result in the error message "lvalue required as left operand of assignment."

Solutions for 'lvalue Required as Left Operand of Assignment'

If you encounter the error message "lvalue required as left operand of assignment," there are several solutions you can try:

Solution 1: Check Your Assignments

The first step you should take is to check your assignments and make sure that you are not trying to assign a value to a constant or an expression that cannot be assigned a value. If you have made an error in your code, correcting it may resolve the issue.

Solution 2: Use a Pointer

If you are trying to assign a value to a constant, you can use a pointer instead. A pointer is a variable that stores the memory address of another variable. By using a pointer, you can indirectly modify the value of a constant.

Here is an example of how to use a pointer:

In this case, we create a pointer "ptr" that points to the address of "x." We then use the pointer to indirectly modify the value of "x" by assigning it a new value of "10."

Solution 3: Use a Reference

Another solution is to use a reference instead of a constant. A reference is similar to a pointer, but it is a direct alias to the variable it refers to. By using a reference, you can modify the value of a variable directly.

Here is an example of how to use a reference:

In this case, we create a reference "ref" that refers to the variable "x." We then use the reference to directly modify the value of "x" by assigning it a new value of "10."

Q1: What does the error message "lvalue required as left operand of assignment" mean?

A1: This error message typically occurs when you try to assign a value to a constant or an expression that cannot be assigned a value.

Q2: How can I resolve the error message "lvalue required as left operand of assignment?"

A2: You can try checking your assignments, using a pointer, or using a reference.

Q3: Can I modify the value of a constant?

A3: No, you cannot modify the value of a constant directly. However, you can use a pointer to indirectly modify the value.

Q4: What is an lvalue?

A4: An lvalue is a term used in programming to refer to a value that can appear on the left side of an assignment operator.

Q5: What is a pointer?

A5: A pointer is a variable that stores the memory address of another variable. By using a pointer, you can indirectly modify the value of a variable.

In conclusion, the error message "lvalue required as left operand of assignment" can be frustrating for developers, but it is a common error that can be resolved using the solutions we have provided in this guide. By understanding the meaning of the error message and using the appropriate solution, you can resolve this error and continue coding with confidence.

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lvalue required as left operand of assignment

I am trying to get a basic robotic arm using pneumatics to work. I am getting an "lvalue required as left operand of assignment" error at the end of my code (in my bool function).

Here is the code:

Check all your 'if' statements for equality. You are incorrectly using the assignment operator '=' instead of the equality operator '=='.

An lvalue normally equates to a memory address, so the error message "lvalue required as left operand of assignment" in the expression:

• if (data = "A"){*

is saying the compiler does have a place in memory where to put "A". The reason is because you're trying to put a string variable ("A") into data , which is a char variable, not a string. Those data attributes don't match, so in effect it is saying that the data type of "A" doesn't match the data type for data so it doesn't know where to put the result.

Had you written:

• if (data = 'A') {*

you would not have had the same error message because the syntax is correct. That is, it has a memory address (the lvalue of data ) where it can place a single character, 'A'. However, it seems doubtful that the expression is really what you want (a semantic error). What you probably want is to compare data with 'A', as others have pointed out:

• if (data == 'A') {*

it is not data that is a problem (which is a legal lvalue), its the whole expression.

Try Ctrl-T and look at the result (as well as the errors generated by the compiler).

There are some warnings that should be fixed too

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  error: lvalue required as left operand of assignment