Increment/decrement operators

C++

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Technical Specifications

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Expressions

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integer literals

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Operators

Assignment operators: a=b, a+=b, a-=b, a*=b, a/=b, a%=b, a&=b, a|=b, a^=b, a<<=b, a>>=b

Increment and decrement: ++a, --a, a++, a--

Arithmetic operators: +a, -a, a+b, a-b, a*b, a/b, a%b, ~a, a&b, a|b, a^b, a<<b, a>>b

Logical operators: a||b, a&&b, !a

Comparison operators: a==b, a!=b, a<b, a>b, a<=b, a>=b, a<=>b(C++20)

Member access operators: a[b], *a, &a, a->b, a.b, a->*b, a.*b

Other operators: a(...), a,b, a?b:c

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Explicit conversions (T)a, T(a)

User-defined conversion

Increment/decrement operators increment or decrement the value of the object.

Operator name	Syntax	Overloadable	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 versions of the built-in operators return references and postfix versions 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 prefix and postfix 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) or operator++(a, 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

The prefix increment and decrement expressions have the form

++ expr
-- expr

1) prefix increment (pre-increment)

2) prefix decrement (pre-decrement)

The operand expr of a built-in prefix increment or decrement operator must be a modifiable (non-const) lvalue of non-boolean arithmetic type or pointer to complete object type. For non-boolean operands, the expression ++x is exactly equivalent to x += 1, and the expression --x is exactly equivalent to x -= 1, that is, the prefix increment or decrement is an lvalue expression that identifies the modified operand. All arithmetic conversion rules and pointer arithmetic rules defined for arithmetic operators apply and determine the implicit conversion (if any) applied to the operand as well as the return type of the expression.

If the operand of the pre-increment operator is of type bool, it is set to true (deprecated). (until C++17)

In overload resolution against user-defined 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)(until C++17)
P& operator++(P&)
A& operator--(A&)
P& operator--(P&)

Built-in postfix operators

The postfix increment and decrement expressions have the form

expr ++
expr --

1) postfix increment (post-increment)

2) postfix decrement (post-decrement)

The operand expr of a built-in postfix increment or decrement operator must be a modifiable (non-const) lvalue of non-boolean arithmetic type or pointer to complete object type. The result is prvalue copy of the original value of the operand. As a side-effect, for non-boolean operands, the expression x++ modifies the value of its operand as if by evaluating x += 1, and the expression x-- modifies the value of its operand as if by evaluating x -= 1. All arithmetic conversion rules and pointer arithmetic rules defined for arithmetic operators apply and determine the implicit conversion (if any) applied to the operand as well as the return type of the expression.

If the operand of the post-increment operator is of type bool, it is set to true (deprecated). (until C++17)

In overload resolution against user-defined 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)(until C++17)
P operator++(P&, int)
A operator--(A&, int)
P operator--(P&, int)

Example

Run this code

#include <iostream>

int main()
{
    int n1 = 1;
    int n2 = ++n1;
    int n3 = ++ ++n1;
    int n4 = n1++;
//  int n5 = n1++ ++;   // error
//  int n6 = n1 + ++n1; // undefined behavior
    std::cout << "n1 = " << n1 << '\n'
              << "n2 = " << n2 << '\n'
              << "n3 = " << n3 << '\n'
              << "n4 = " << n4 << '\n';
}

Output:

n1 = 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 post-increment and post-decrement, 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)operator+=operator+operator--operator--(int)operator-=operator-	advances or decrements the iterator (public member function of std::reverse_iterator)
operator++operator++(int)operator+=operator+operator--operator--(int)operator-=operator-	advances or decrements the iterator (public member function of std::move_iterator)
operator++operator++(int)	no-op (public member function of std::front_insert_iterator)
operator++operator++(int)	no-op (public member function of std::back_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::istream_iterator)
operator++operator++(int)	no-op (public member function of std::ostream_iterator)
operator++operator++(int)	advances the 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 iterator to the next match (public member function of std::regex_iterator)
operator++operator++(int)	advances the iterator to the next submatch (public member function of std::regex_token_iterator)

See also

Operator precedence

Operator overloading

Common operators
assignment	increment decrement	arithmetic	logical	comparison	member access	other
a = b 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[b] *a &a a->b a.b a->*b a.*b	a(...) a, b ? :
Special operators
static_cast converts one type to another related type dynamic_cast converts within inheritance hierarchies const_cast adds or removes cv qualifiers reinterpret_cast converts type to unrelated type C-style cast converts one type to another by a mix of static_cast, const_cast, and reinterpret_cast new creates objects with dynamic storage duration delete destructs objects previously created by the new expression and releases obtained memory area 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)