Increment/decrement operators

 

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Increment/decrement operators increments or decrements the value of the object.

Operator name	Syntax	Over​load​able	Prototype examples (for class T)
			Inside class definition	Outside class definition
pre-increment	++a	Yes	T& T::operator++();	T& operator++(T& a);
pre-decrement	--a	Yes	T& T::operator--();	T& operator--(T& a);
post-increment	a++	Yes	T T::operator++(int);	T operator++(T& a, int);
post-decrement	a--	Yes	T T::operator--(int);	T operator--(T& a, int);
Notes Prefix forms of the built-in operators return references and postfix forms return values, and typical user-defined overloads follow the pattern 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). The int parameter is a dummy parameter used to differentiate between pre- and post- in versions of the operators. When the user-defined postfix operator is called, the value passed in that parameter is always zero, although it may be changed by calling the operator using function call notation, e.g. a.operator++(2).

Explanation

pre-increment and pre-decrement operators increments or decrements the value of the object and returns a reference to the result.

post-increment and post-decrement creates a copy of the object, increments or decrements the value of the object and returns the copy from before the increment or decrement.

Built-in prefix operators

For every optionally volatile-qualified arithmetic type A other than bool, and for every optionally volatile-qualified pointer P to optionally cv-qualified object type, the following function signatures participate in overload resolution:

A& operator++(A&)
bool& operator++(bool&)		(deprecated)
P& operator++(P&)
A& operator--(A&)
P& operator--(P&)

The operand of a built-in prefix increment or decrement operator must be a modifiable lvalue (non-const reference) of non-boolean arithmetic type or pointer to complete object type. For these operands, the expression ++x is exactly equivalent to x+=1, and the expression --x is exactly equivalent to x-=1, that is, the result is the updated operand, returned as lvalue, and all arithmetic conversion rules and pointer arithmetic rules defined for arithmetic operators apply.

If the operand of the preincrement operator is of type bool, it is set to true (deprecated).

Built-in postfix operators

For every optionally volatile-qualified arithmetic type A other than bool, and for every optionally volatile-qualified pointer P to optionally cv-qualified object type, the following function signatures participate in overload resolution:

A operator++(A&, int)
bool operator++(bool&, int)		(deprecated)
P operator++(P&, int)
A operator--(A&, int)
P operator--(P&, int)

The operand of a built-in postfix increment or decrement operator must be a modifiable lvalue (non-const reference) of non-boolean arithmetic type or pointer to complete object type. The result is a prvalue, which is a copy the original value of the operand. As a side-effect, this operator modifies the value of its argument arg as if by evaluating arg += 1 or arg -= 1, for increment and decrement respectively. All arithmetic conversion rules and pointer arithmetic rules defined for arithmetic operators apply.

If the operand of the postincrement operator is of type bool, it is set to true (deprecated).

Example

#include <iostream>
int main()
{
    int n = 1;
    int n2 = ++n;
    int n3 = ++ ++n;
    int n4 = n++;
//    int n5 = n++ ++; // compile error
//    int n5 = n + ++n; // undefined behavior
    std::cout << "n = " << n << '\n'
              << "n2 = " << n2 << '\n'
              << "n3 = " << n3 << '\n'
              << "n4 = " << n4 << '\n';
}

Output:

n = 5
n2 = 2
n3 = 4
n4 = 4

Notes

Because of the side-effects involved, built-in increment and decrement operators must be used with care to avoid undefined behavior due to violations of sequencing rules.

Because a temporary copy of the object is constructed during the operation, pre-increment or pre-decrement operators are usually more efficient in contexts where the returned value is not used.

Standard library

Increment and decrement operators are overloaded for many standard library types. In particular, every Iterator overloads operator++ and every BidirectionalIterator overloads operator--, even if those operators are no-ops for the particular iterator.

overloads for arithmetic types
operator++ operator++(int) operator-- operator--(int)	increments or decrements the atomic value by one (public member function of std::atomic)
operator++ operator++(int) operator-- operator--(int)	increments or decrements the tick count (public member function of std::chrono::duration)
overloads for iterator types
operator++ operator++(int)	advances the iterator (public member function of std::raw_storage_iterator)
operator++ operator++(int)	advances the iterator (public member function of std::reverse_iterator)
operator-- operator++(int)	decrements the iterator (public member function of std::reverse_iterator)
operator++ operator++(int)	no-op (public member function of std::back_insert_iterator)
operator++ operator++(int)	no-op (public member function of std::front_insert_iterator)
operator++ operator++(int)	no-op (public member function of std::insert_iterator)
operator++ operator++(int)	advances the iterator (public member function of std::move_iterator)
operator-- operator--(int)	decrements the iterator (public member function of std::move_iterator)
operator++ operator++(int)	advances the istream_iterator (public member function of std::istream_iterator)
operator++ operator++(int)	no-op (public member function of std::ostream_iterator)
operator++ operator++(int)	advances the istreambuf_iterator (public member function of std::istreambuf_iterator)
operator++ operator++(int)	no-op (public member function of std::ostreambuf_iterator)
operator++ operator++(int)	advances the regex_iterator (public member function of std::regex_iterator)
operator++ operator++(int)	advances the regex_token_iterator (public member function of std::regex_token_iterator)

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)