C++函数对象-运算符函数对象 - 比较 - 实现 x == y 的函数对象（std::equal_to）

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：

比较

实现 x == y 的函数对象

std::equal_to

进行比较的函数对象。调用类型 T 上的 operator== ，除非特化。

进行比较的函数对象。调用类型 T 上的 operator== ，除非特化。

特化

成员函数

std::equal_to::operator()

检查 lhs 是否等于 rhs 。

参数

返回值

若 lhs == rhs 则为 true ，否则为 false 。

异常

（无）

可能的实现

constexpr bool 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;
    }

    friend Cell operator /(const Cell &cell1, const Cell &cell2)
    {
        Cell cell = {cell1.x / cell2.x, cell1.y / cell2.y};
        return cell;
    }

    friend Cell operator %(const Cell &cell1, const Cell &cell2)
    {
        Cell cell = {cell1.x % cell2.x, cell1.y % cell2.y};
        return cell;
    }

    friend bool operator ==(const Cell &cell1, const Cell &cell2)
    {
        return cell1.x == cell2.x && cell1.y == cell2.y;
    }
};

std::ostream &operator<<(std::ostream &os, const Cell &cell)
{
    os << "{" << cell.x << "," << cell.y << "}";
    return os;
}

int main()
{
    std::cout << std::boolalpha;

    int *ptr = nullptr;
    std::cout << "std::equal_to<int*>()(1023, 1024):        "
              << std::equal_to<int*>()(ptr, nullptr) << std::endl;

    std::cout << "std::equal_to<char>()(50, 2):             "
              << std::equal_to<char>()(50, 2) << std::endl;
    std::cout << "std::equal_to<char>()('a', 97):           "
              << std::equal_to<char>()('a', 97) << std::endl;
    std::cout << "std::equal_to<int>()(1023, 1024):         "
              << std::equal_to<int>()(1023, 1024) << std::endl;
    std::cout << "std::equal_to<long>()(1023, 1024):        "
              << std::equal_to<long>()(1023, 1024) << std::endl;
    std::cout << "std::equal_to<long long>()(1023, 1024):   "
              << std::equal_to<long long>()(1023, 1024) << std::endl;

    std::cout << "std::equal_to<uint8_t>()(1023, 1024):     "
              << std::equal_to<uint8_t>()(8, 32) << std::endl;
    std::cout << "std::equal_to<uint16_t>()(123, 456):      "
              << std::equal_to<uint16_t>()(123, 456) << std::endl;
    std::cout << "std::equal_to<uint32_t>()(101, 202):      "
              << std::equal_to<uint32_t>()(101, 202) << std::endl;
    std::cout << "std::equal_to<uint64_t>()(10230, 10240):  "
              << std::equal_to<uint64_t>()(10230, 10240) << std::endl;

    std::cout << "std::equal_to<int8_t>()(1023, 1024):      "
              << std::equal_to<int8_t>()(8, 32) << std::endl;
    std::cout << "std::equal_to<int16_t>()(123, 456):       "
              << std::equal_to<int16_t>()(123, 456) << std::endl;
    std::cout << "std::equal_to<int32_t>()(101, 202):       "
              << std::equal_to<int32_t>()(101, 202) << std::endl;
    std::cout << "std::equal_to<int64_t>()(10230, 10240):   "
              << std::equal_to<int64_t>()(10230, 10240) << std::endl;

    std::cout << "std::equal_to<double>()(3.14, 3.14):      "
              << std::equal_to<double>()(3.14, 3.14) << std::endl;
    std::cout << "std::equal_to<float>()(3.14, 3.14):       "
              << std::equal_to<float>()(3.14, 3.14) << std::endl;
    std::cout << "std::equal_to<float>()(3, 3):             "
              << std::equal_to<float>()(3, 3) << std::endl;
    std::cout << "std::equal_to<float>()(3.56, 3.14):       "
              << std::equal_to<float>()(3.56, 3.14) << std::endl;
    std::cout << "std::equal_to<int>()(3.14, 3.14):         "
              << std::equal_to<int>()(3.34, 3.34) << std::endl;

    std::cout << "std::equal_to<Cell>()(Cell{101, 101}, Cell{202, 202}):       "
              << std::equal_to<Cell>()(Cell{101, 101}, Cell{202, 202}) << std::endl;

    std::cout << "std::equal_to<std::string>()(\"I am a \", \"handsome programmer\"):"
              << std::equal_to<std::string>()("I am a ", "handsome programmer") << std::endl;
    return 0;
}

输出

std::equal_to<int*>()(1023, 1024):        true
std::equal_to<char>()(50, 2):             false
std::equal_to<char>()('a', 97):           true
std::equal_to<int>()(1023, 1024):         false
std::equal_to<long>()(1023, 1024):        false
std::equal_to<long long>()(1023, 1024):   false
std::equal_to<uint8_t>()(1023, 1024):     false
std::equal_to<uint16_t>()(123, 456):      false
std::equal_to<uint32_t>()(101, 202):      false
std::equal_to<uint64_t>()(10230, 10240):  false
std::equal_to<int8_t>()(1023, 1024):      false
std::equal_to<int16_t>()(123, 456):       false
std::equal_to<int32_t>()(101, 202):       false
std::equal_to<int64_t>()(10230, 10240):   false
std::equal_to<double>()(3.14, 3.14):      true
std::equal_to<float>()(3.14, 3.14):       true
std::equal_to<float>()(3, 3):             true
std::equal_to<float>()(3.56, 3.14):       false
std::equal_to<int>()(3.14, 3.14):         true
std::equal_to<Cell>()(Cell{101, 101}, Cell{202, 202}):       false
std::equal_to<std::string>()("I am a ", "handsome programmer"):false