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std::plus

Defined in header <functional> 
template< class T >
struct plus;
 (until C++14) 
template< class T = void >
struct plus;
 (since C++14)

Function object for performing addition. Effectively calls operator+ on two instances of type T.

Specializations

The standard library provides a specialization of std::plus when T is not specified, which leaves the parameter types and return type to be deduced.

function object implementing x + y deducing argument and return types
(class template specialization)
(since C++14)

Member types

Type Definition
result_type(deprecated in C++17) T
first_argument_type(deprecated in C++17) T
second_argument_type(deprecated in C++17) T
(until C++20)

Member functions

operator()
 returns the sum of two arguments
(public member function)

std::plus::operator() 

T operator()( const T& lhs, const T& rhs ) const;
 (until C++14) 
constexpr T operator()( const T& lhs, const T& rhs ) const;
 (since C++14)

Returns the sum of lhs and rhs.

Parameters

lhs, rhs - values to sum

Return value

The result of lhs + rhs.

Exceptions

(none).

Possible implementation

constexpr T operator()(const T &lhs, const T &rhs) const 
{
    return lhs + rhs;
}

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http://en.cppreference.com/w/cpp/utility/functional/plus