C++函数对象-运算符函数对象-算术运算-实现 x * y 的函数对象（std::multiplies）_c++ multiplies

任何定义了函数调用操作符的对象都是函数对象。C++ 支持创建、操作新的函数对象，同时也提供了许多内置的函数对象。

运算符函数对象

C++ 针对常用的算术和逻辑运算定义了很多函数对象：

算术运算

实现 x * y 的函数对象

std::multiplies

进行乘法的函数对象。等效地在二个 T 类型实例上调用 operator* 。

特化

成员函数

std::multiplies::operator()

返回 lhs 与 rhs 的积。

参数

返回值

lhs * rhs 的结果。

异常

（无）

可能的实现

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

调用示例

#include <iostream>
#include <functional>
 
struct Cell
{
    int x;
    int y;
 
    Cell() = default;
    Cell(int a, int b): x(a), y(b) {}
    Cell(const Cell &cell)
    {
        x = cell.x;
        y = cell.y;
    }
 
    bool operator <(const Cell &cell) const
    {
        if (x == cell.x)
        {
            return y < cell.y;
        }
        else
        {
            return x < cell.x;
        }
    }
 
    Cell &operator+(const Cell &cell)
    {
        x += cell.x;
        y += cell.y;
        return *this;
    }
 
    Cell &operator+=(const Cell &cell)
    {
        x += cell.x;
        y += cell.y;
        return *this;
    }
 
    Cell &operator*=(int n)
    {
        x *= n;
        y *= n;
        return *this;
    }
 
    Cell &operator++()
    {
        x += 1;
        y += 1;
        return *this;
    }
 
    friend Cell operator +(const Cell &cell1, const Cell &cell2)
    {
        Cell cell = cell1;
        cell += cell2;
        return cell;
    }
 
    friend Cell operator *(const Cell &cell1, const Cell &cell2)
    {
        Cell cell = {cell1.x * cell2.x, cell1.y * cell2.y};
        return cell;
    }
};
 
std::ostream &operator<<(std::ostream &os, const Cell &cell)
{
    os << "{" << cell.x << "," << cell.y << "}";
    return os;
}
 
int main()
{
    std::cout << "std::multiplies<char>()(50, 2):             "
              << std::multiplies<char>()(50, 2) << std::endl;
    std::cout << "std::multiplies<int>()(1023, 1024):         "
              << std::multiplies<int>()(1023, 1024) << std::endl;
    std::cout << "std::multiplies<long>()(1023, 1024):        "
              << std::multiplies<long>()(1023, 1024) << std::endl;
    std::cout << "std::multiplies<long long>()(1023, 1024):   "
              << std::multiplies<long long>()(1023, 1024) << std::endl;
 
    std::cout << "std::multiplies<uint8_t>()(1023, 1024):     "
              << std::multiplies<uint8_t>()(8, 32) << std::endl;
    std::cout << "std::multiplies<uint16_t>()(123, 456):      "
              << std::multiplies<uint16_t>()(123, 456) << std::endl;
    std::cout << "std::multiplies<uint32_t>()(101, 202):      "
              << std::multiplies<uint32_t>()(101, 202) << std::endl;
    std::cout << "std::multiplies<uint64_t>()(10230, 10240):  "
              << std::multiplies<uint64_t>()(10230, 10240) << std::endl;
 
    std::cout << "std::multiplies<int8_t>()(1023, 1024):      "
              << std::multiplies<int8_t>()(8, 32) << std::endl;
    std::cout << "std::multiplies<int16_t>()(123, 456):       "
              << std::multiplies<int16_t>()(123, 456) << std::endl;
    std::cout << "std::multiplies<int32_t>()(101, 202):       "
              << std::multiplies<int32_t>()(101, 202) << std::endl;
    std::cout << "std::multiplies<int64_t>()(10230, 10240):   "
              << std::multiplies<int64_t>()(10230, 10240) << std::endl;
 
    std::cout << "std::multiplies<double>()(3.14, 3.14):      "
              << std::multiplies<double>()(3.14, 3.14) << std::endl;
    std::cout << "std::multiplies<float>()(3.14, 3.14):       "
              << std::multiplies<float>()(3.14, 3.14) << std::endl;
    std::cout << "std::multiplies<float>()(3, 3):             "
              << std::multiplies<float>()(3, 3) << std::endl;
    std::cout << "std::multiplies<int>()(3.14, 3.14):         "
              << std::multiplies<int>()(3.34, 3.34) << std::endl;
 
    std::cout << "std::multiplies<Cell>()(Cell{101, 101}, Cell{202, 202}):       "
              << std::multiplies<Cell>()(Cell{101, 101}, Cell{202, 202}) << std::endl;
 
    //编译失败
//    std::cout << "std::multiplies<std::string>()(\"I am a \", \"handsome programmer\"):"
//              << std::multiplies<std::string>()("I am a ", "handsome programmer") << std::endl;
    return 0;
}

输出

std::multiplies<char>()(50, 2):             d
std::multiplies<int>()(1023, 1024):         1047552
std::multiplies<long>()(1023, 1024):        1047552
std::multiplies<long long>()(1023, 1024):   1047552
std::multiplies<uint8_t>()(1023, 1024):
std::multiplies<uint16_t>()(123, 456):      56088
std::multiplies<uint32_t>()(101, 202):      20402
std::multiplies<uint64_t>()(10230, 10240):  104755200
std::multiplies<int8_t>()(1023, 1024):
std::multiplies<int16_t>()(123, 456):       -9448
std::multiplies<int32_t>()(101, 202):       20402
std::multiplies<int64_t>()(10230, 10240):   104755200
std::multiplies<double>()(3.14, 3.14):      9.8596
std::multiplies<float>()(3.14, 3.14):       9.8596
std::multiplies<float>()(3, 3):             9
std::multiplies<int>()(3.14, 3.14):         9
std::multiplies<Cell>()(Cell{101, 101}, Cell{202, 202}):       {20402,20402}