Assignment operators

Assignment operators modify the value of the object.

Explanation

copy assignment operator replaces the contents of the object a with a copy of the contents of b (b is not modified). For class types, this is a special member function, described in copy assignment operator.

move assignment operator replaces the contents of the object a with the contents of b, avoiding copying if possible (b may be modified). For class types, this is a special member function, described in move assignment operator. (since C++11).

For non-class types, copy and move assignment are indistinguishable and are referred to as direct assignment.

compound assignment operators replace the contents of the object a with the result of a binary operation between the previous value of a and the value of b.

Builtin direct assignment

For every type T, the following function signatures participate in overload resolution:

T*& operator=(T*&, T*);

T*volatile & operator=(T*volatile &, T*);

For every enumeration or pointer to member type T, optionally volatile-qualified, the following function signature participates in overload resolution:

T& operator=(T&, T );

For every pair A1 and A2, where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signature participates in overload resolution:

A1& operator=(A1&, A2);

For expressions E1 of any scalar type T, the following additional forms of the builtin assignment expression are allowed:

E1 = {}

E1 = {E2}

Note: the above includes all non-class types except reference types, array types, function types, and the type void, which are not directly assignable.

The direct assignment operator expects a modifiable lvalue as its left operand and an rvalue expression or a braced-init-list (since C++11) as its right operand, and returns an lvalue identifying the left operand after modification.

For non-class types, the right operand is first implicitly converted to the cv-unqualified type of the left operand, and then its value is copied into the object identified by left operand.

When the left operand has reference type, the assignment operator modifies the referred-to object.

If the left and the right operands identify overlapping objects, the behavior is undefined (unless the overlap is exact and the type is the same).

Example

#include <iostream>
int main()
{
    int n = 0;  // not an assignment
    n = 1;      // direct assignment
    std::cout << n << ' ';
    n = {};     // zero-initialization, then assignment
    std::cout << n << ' ';
    n = 'a';    // integral promotion, then assignment
    std::cout << n << ' ';
    n = {'b'};   // explicit cast, then assignment
    std::cout << n << ' ';
    n = 1.0;    // floating-point conversion, then assignment
    std::cout << n << ' ';
//    n = {1.0}; // compiler error (narrowing conversion)
 
    int& r = n;  // not an assignment
    int* p;
 
    r = 2;       // assignment through reference
    std::cout << n << '\n';
    p = &n;      // direct assignment
    p = nullptr; // null-pointer conversion, then assignment 
 
    struct {int a; std::string s;} obj;
    obj = {1, "abc"}; // assignment from a braced-init-list
    std::cout << obj.a << ':' << obj.s << '\n';
}

1 0 97 98 1 2
1:abc

Builtin compound assignment

For every pair A1 and A2, where A1 is an arithmetic type (optionally volatile-qualified) and A2 is a promoted arithmetic type, the following function signatures participate in overload resolution:

A1& operator*=(A1&, A2);

A1& operator/=(A1&, A2);

A1& operator+=(A1&, A2);

A1& operator-=(A1&, A2);

For every pair I1 and I2, where I1 is an integral type (optionally volatile-qualified) and I2 is a promoted integral type, the following function signatures participate in overload resolution:

I1& operator%=(I1&, I2);

I1& operator<<=(I1&, I2);

I1& operator>>=(I1&, I2);

I1& operator&=(I1&, I2);

I1& operator^=(I1&, I2);

I1& operator|=(I1&, I2);

For every optionally cv-qualified object type T, the following function signatures participate in overload resolution:

T*& operator+=(T*&, std::ptrdiff_t);

T*& operator-=(T*&, std::ptrdiff_t);

T*volatile & operator+=(T*volatile &, std::ptrdiff_t);

T*volatile & operator-=(T*volatile &, std::ptrdiff_t);

The behavior of every builtin compound-assignment expression E1 op= E2 (where E1 is a modifiable lvalue expression and E2 is an rvalue expression or a braced-init-list (since C++11)) is exactly the same as the behavior of the expression E1 = E1 op E2, except that the expression E1 is evaluated only once and that it behaves as a single operation with respect to indeterminately-sequenced function calls (e.g. in f(a += b, g()), the += is either not started at all or is completed as seen from inside g()).

Example

See also

Operator precedence.

Operator overloading.