Assignment Operators

The assignment operator = stores a value into a variable. C++ also provides **compound assignment operators** that combine an arithmetic or bitwise operation with an assignment in a single, more concise step.

Basic Assignment

CPP

int score = 0; // this = is initialization, not assignment (see note below)
score = 10;    // this = is assignment — score already existed

Compound Assignment Operators

Operator	Equivalent To	Example
+=	a = a + b	total += 10;
-=	a = a - b	total -= 5;
*=	a = a * b	price *= 1.1;
/=	a = a / b	average /= count;
%=	a = a % b	remainder %= 7;
&=	a = a & b	flags &= mask;
\|=	a = a \| b	flags \|= FLAG_VISIBLE;
^=	a = a ^ b	flags ^= FLAG_SELECTED;
<<=	a = a << b	value <<= 2;
=	a = a >> b	value >>= 1;

CPP

#include <iostream>

int main() {
    int total = 100;
    total += 20;   // total = 120
    total -= 5;    // total = 115
    total *= 2;    // total = 230
    total /= 10;   // total = 23
    total %= 5;    // total = 3

    std::cout << total << std::endl;
    return 0;
}

Chained Assignment

Assignment is an expression that evaluates to the assigned value, and it associates right-to-left, so you can chain several assignments in one statement.

CPP

int a, b, c;
a = b = c = 0; // evaluated right-to-left: c = 0, then b = (c's value), then a = (b's value)
// all three variables are now 0

Assignment vs Initialization

These look the same but are different operations

In int x = 5;, the = is initialization — it is giving the variable its very first value as part of its creation. In x = 10; (on a later line, where x already exists), the = is assignment — it replaces an existing value. The distinction matters because some types behave differently for each: a type can define a constructor for initialization and a separate operator= for assignment, and the rules for what is legal (e.g. with const or reference members) differ between the two.

CPP

#include <string>

int main() {
    std::string s1 = "hello"; // initialization — calls a constructor
    std::string s2;           // default-initialized (empty string)
    s2 = "world";              // assignment — calls operator=

    const int x = 5;          // initialization — legal even though x is const
    // x = 10;                // ERROR: assignment to a const variable is illegal
    return 0;
}