std::multiplies

std::multiplies

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Prerequisites

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1. What is std::multiplies?

std::multiplies is a function object (functor) defined in <functional> that performs multiplication.

It is part of the standard functional utilities used with STL algorithms.

#include <functional>

std::multiplies<int> mul;

int result = mul(3, 4);  // 12

Key Idea

std::multiplies<T>()(a, b) == a * b

behaves like a function:

int x = std::multiplies<int>()(3, 4);  // 12

2. Function Signature

template<class T>
struct multiplies
{
    constexpr T operator()(const T& lhs, const T& rhs) const;
};

Example with std::accumulate

#include <numeric>
#include <functional>
#include <vector>

std::vector<int> v = {1, 2, 3, 4};

int product = std::accumulate(v.begin(), v.end(),
                              1,
                              std::multiplies<>());

24

Example with std::reduce

#include <numeric>
#include <execution>
#include <functional>

int product = std::reduce(std::execution::par,
                          v.begin(), v.end(),
                          1,
                          std::multiplies<>());

3. Why Use std::multiplies

Cleaner Code

std::accumulate(v.begin(), v.end(), 1,
    [](int a, int b)
    {
        return a * b;
    });

vs

std::accumulate(v.begin(), v.end(), 1, std::multiplies<>());

shorter and clearer

Generic Programming

Works well in templates:

template<typename T>
T product(const std::vector<T>& v)
{
    return std::accumulate(v.begin(), v.end(), T{1}, std::multiplies<>());
}

Works with Parallel Algorithms

cpp id="ck8u0p" std::reduce(std::execution::par, v.begin(), v.end(), 1, std::multiplies<>());

associative operation → safe for parallel

Related Function Objects

Functor	Operation
std::plus	a + b
std::minus	a - b
std::multiplies	a * b
std::divides	a / b
std::modulus	a % b