# Increment/decrement operators

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Increment/decrement operators increment or decrement 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 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

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&) |  |

The operand 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.

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

#### 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)(until C++17)
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 (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.

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

#### 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 &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)

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