How to Fix “lvalue required as left operand of assignment” in C++

Short answer: the left side of = has to be something that can store a value, like a variable. Usually you typed = where you meant == in a comparison such as if (n % 2 = 0), or wrote an assignment backwards, as in a + b = total.

What “lvalue” Means

An lvalue is something with its own place in memory that you can store a value into: a variable, an array element, a member of an object. The name comes from being allowed on the left of =.

Everything else is just a value: a number like 10, the result of a + b, or whatever a function returns. You can read those values, but there’s nowhere to store anything in them:

score = 10;    // fine: score is a variable
10 = score;    // error: 10 is just a value

Every cause below puts a plain value on the left of =.

Cause 1: = Instead of == in a Condition

The classic. = assigns and == compares:

#include <iostream>

int main() {
    int n = 7;
    if (n % 2 = 0) {
        std::cout << "even\n";
    }
}

GCC:    error: lvalue required as left operand of assignment
Clang:  error: expression is not assignable
MSVC:   error C2106: '=': left operand must be l-value

n % 2 is a calculated value, so C++ can’t store 0 in it.

Fix: compare with ==:

#include <iostream>

int main() {
    int n = 8;
    if (n % 2 == 0) {
        std::cout << "even\n";
    }
}

even

There’s more on % in the modulo operator, and on conditions in the conditionals tutorial.

Cause 2: An Assignment Written Backwards

In maths, a + b = total and total = a + b say the same thing. In C++, = means “store the right side in the left side”, so the order matters:

#include <iostream>

int main() {
    int a = 2;
    int b = 3;
    int total;
    a + b = total;
    std::cout << total << '\n';
}

GCC:    error: lvalue required as left operand of assignment
Clang:  error: expression is not assignable
MSVC:   error C2106: '=': left operand must be l-value

A number on the left, as in 10 = score;, gives exactly the same three messages.

Fix: put the variable on the left:

#include <iostream>

int main() {
    int a = 2;
    int b = 3;
    int total = a + b;
    std::cout << total << '\n';
}

5

Cause 3: Assigning to a Function’s Return Value

#include <iostream>

class Player {
public:
    int getHealth() const { return health; }
private:
    int health = 100;
};

int main() {
    Player hero;
    hero.getHealth() = 50;
    std::cout << hero.getHealth() << '\n';
}

GCC:    error: lvalue required as left operand of assignment
Clang:  error: expression is not assignable
MSVC:   error C2106: '=': left operand must be l-value

getHealth() hands back a copy of the number, not the health member itself, so there’s nothing to store into.

The same thing happens with the standard library. word.length() = 3; gives the same error, because length() returns a number. To shorten a string, use word.resize(3);.

Fix: add a member function that changes the value, usually called a setter:

#include <iostream>

class Player {
public:
    int getHealth() const { return health; }
    void setHealth(int h) { health = h; }
private:
    int health = 100;
};

int main() {
    Player hero;
    hero.setHealth(50);
    std::cout << hero.getHealth() << '\n';
}

50

See getters and setters for when to use them.

The Trap That Compiles: if (x = 5)

When the left side is a plain variable, = inside a condition is legal. There’s no lvalue error, just a bug:

#include <iostream>

int main() {
    int x = 2;
    if (x = 5) {
        std::cout << "x is 5\n";
    }
}

x is 5

x = 5 stores 5 in x, and the condition then checks that 5. Any number other than zero counts as true, so the program prints “x is 5” even though x started as 2.

Whether you get a warning depends on your compiler and settings:

GCC (with -Wall):     warning: suggest parentheses around assignment used as truth value [-Wparentheses]
Clang:                warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
Visual Studio (/W4):  warning C4706: assignment used as a condition

Without -Wall, GCC says nothing. Visual Studio only warns at warning level 4; at its default level 3 it’s silent. Turning warnings up is the easiest way to catch this mistake.

Similar Messages

Two related errors come from assigning to something that is a variable but isn’t allowed to change:

For the const one, decide whether the value really should change. See const in C++. For arrays, copy them element by element, or use a std::array or std::vector, which you can copy with =. See std::array.

Quick Reference

For other common messages, see C++ error messages explained.

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Related Articles

• C++ Error Messages Explained — the most common errors and what they mean.
• C++ Conditionals Tutorial — if, else and the comparison operators.
• The Modulo Operator in C++ — checking for even and odd numbers.
• Getters and Setters in C++ — changing an object’s data the right way.
• How to Fix ‘expected primary-expression before’ — another error from a small typo.