Assignment operators

 

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Assignment operators modify the value of the object.

Operator name	Syntax	Over​load​able	Prototype examples (for class T)
			Inside class definition	Outside class definition
basic assignment	a = b	Yes	T& T::operator =(const T2& b);	N/A
move assignment (C++11)	a = rvalue	Yes	T& T::operator =(T2&& b);	N/A
addition assignment	a += b	Yes	T& T::operator +=(const T2& b);	T& operator +=(T& a, const T2& b);
subtraction assignment	a -= b	Yes	T& T::operator -=(const T2& b);	T& operator -=(T& a, const T2& b);
multiplication assignment	a *= b	Yes	T& T::operator *=(const T2& b);	T& operator *=(T& a, const T2& b);
division assignment	a /= b	Yes	T& T::operator /=(const T2& b);	T& operator /=(T& a, const T2& b);
modulo assignment	a %= b	Yes	T& T::operator %=(const T2& b);	T& operator %=(T& a, const T2& b);
bitwise AND assignment	a &= b	Yes	T& T::operator &=(const T2& b);	T& operator &=(T& a, const T2& b);
bitwise OR assignment	a |= b	Yes	T& T::operator |=(const T2& b);	T& operator |=(T& a, const T2& b);
bitwise XOR assignment	a ^= b	Yes	T& T::operator ^=(const T2& b);	T& operator ^=(T& a, const T2& b);
bitwise left shift assignment	a <<= b	Yes	T& T::operator <<=(const T2& b);	T& operator <<=(T& a, const T2& b);
bitwise right shift assignment	a >>= b	Yes	T& T::operator >>=(const T2& b);	T& operator >>=(T& a, const T2& b);
Notes All built-in assignment operators return *this, and most user-defined overloads also return *this so that the user-defined operators can be used in the same manner as the built-ins. However, in a user-defined operator overload, any type can be used as return type (including void). T2 can be any type including T

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 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 = {}		(since C++11)
E1 = {E2}		(since C++11)

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 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 is a reference type, the assignment operator applies to 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)

If the right operand is a braced-init-list

• the expression E1 = {} is equivalent to E1 = T(), where T is the type of E1.
• the expression E1 = {E2} is equivalent to E1 = T(E2), where T is the type of E1, except that narrowing implicit conversions are prohibited.

For class types, this syntax generates a call to the assignment operator with std::initializer_list as the argument, following the rules of list-initialization

Example

run this code

#include <iostream>
int main()
{
    int n = 0;  // not an assignment
    n = 1;      // direct asignment
    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 << ' ';
    p = &n;      // direct assignment
    p = nullptr; // null-pointer conversion, then assignment 
}

Output:

1 0 97 98 1 2

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 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()).

See also

Operator precedence

Common operators
assignment	increment decrement	arithmetic	logical	comparison	member access	other
a = b a = rvalue a += b a -= b a *= b a /= b a %= b a &= b a |= b a ^= b a <<= b a >>= b	++a --a a++ a--	+a -a a + b a - b a * b a / b a % b ~a a & b a | b a ^ b a << b a >> b	!a a && b a || b	a == b a != b a < b a > b a <= b a >= b	a[b] *a &a a->b a.b a->*b a.*b	a(...) a, b (type) a ? :
Special operators
static_cast converts one type to another compatible type dynamic_cast converts virtual base class to derived class const_cast converts type to compatible type with different cv qualifiers reinterpret_cast converts type to incompatible type new allocates memory delete deallocates memory sizeof queries the size of a type sizeof... queries the size of a parameter pack (since C++11) typeid queries the type information of a type noexcept checks if an expression can throw an exception (since C++11) alignof queries alignment requirements of a type (since C++11)