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

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::less_equal

template< class T > struct less_equal;		(C++14 前)
template< class T = void > struct less_equal;		(C++14 起)

进行比较的函数对象。调用类型 T 上的 operator<= ，除非特化。

特化

std::less_equal 的特化为任何指针类型产生严格全序，即使内建的 operator<= 不如此。严格全序在 std::less 、 std::greater 、 std::less_equal 和 std::greater_equal 对该指针类型的特化间一致，且亦与对应的内建运算符（ < 、 > 、 <= 及 >= ）所强加的部分顺序一致。

若特化 std::less_equal 的函数调用运算符调用内建运算符比较指针，则它产生严格全序，即使内建的 operator<= 不如此。此严格全序在特化 std::less 、 std::greater 、 std::less_equal 和 std::greater_equal 间一致，亦与对应的内建运算符所强加的部分顺序一致。

(C++14 起)

标准库提供 std::less_equal 在不指定 T 时的特化，这使得参数类型和返回类型留待推导。 less_equal 实现 x <= y 并推导参数和返回类型的函数对象 (类模板特化)

(C++14 起

 成员类型

类型 定义 result_type(C++17 中弃用) bool first_argument_type(C++17 中弃用) T second_argument_type(C++17 中弃用) T

(C++20 前

成员函数

operator()

检查第一参数是否小于或等于第二个 (公开成员函数)

 

std::less_equal::operator()

bool operator()( const T& lhs, const T& rhs ) const;		(C++14 前)
constexpr bool operator()( const T& lhs, const T& rhs ) const;		(C++14 起)

检查 lhs 是否小于或等于 rhs 。

参数

lhs, rhs

-

要比较的值

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 
#include 

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;
    }

    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;
    }

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

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

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

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

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

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::less_equal()(1023, 1024):        "
              << std::less_equal()(ptr, nullptr) << std::endl;

    std::cout << "std::less_equal()(50, 2):             "
              << std::less_equal()(50, 2) << std::endl;
    std::cout << "std::less_equal()('a', 97):           "
              << std::less_equal()('a', 97) << std::endl;
    std::cout << "std::less_equal()(1023, 1024):         "
              << std::less_equal()(1023, 1024) << std::endl;
    std::cout << "std::less_equal()(1023, 1024):        "
              << std::less_equal()(1023, 1024) << std::endl;
    std::cout << "std::less_equal()(1023, 1024):   "
              << std::less_equal()(1023, 1024) << std::endl;

    std::cout << "std::less_equal()(1023, 1024):     "
              << std::less_equal()(8, 32) << std::endl;
    std::cout << "std::less_equal()(123, 456):      "
              << std::less_equal()(123, 456) << std::endl;
    std::cout << "std::less_equal()(101, 202):      "
              << std::less_equal()(101, 202) << std::endl;
    std::cout << "std::less_equal()(10230, 10240):  "
              << std::less_equal()(10230, 10240) << std::endl;

    std::cout << "std::less_equal()(1023, 1024):      "
              << std::less_equal()(8, 32) << std::endl;
    std::cout << "std::less_equal()(123, 456):       "
              << std::less_equal()(123, 456) << std::endl;
    std::cout << "std::less_equal()(101, 202):       "
              << std::less_equal()(101, 202) << std::endl;
    std::cout << "std::less_equal()(10230, 10240):   "
              << std::less_equal()(10230, 10240) << std::endl;

    std::cout << "std::less_equal()(3.14, 3.14):      "
              << std::less_equal()(3.14, 3.14) << std::endl;
    std::cout << "std::less_equal()(3.14, 3.14):       "
              << std::less_equal()(3.14, 3.14) << std::endl;
    std::cout << "std::less_equal()(3, 3):             "
              << std::less_equal()(3, 3) << std::endl;
    std::cout << "std::less_equal()(3.56, 3.14):       "
              << std::less_equal()(3.56, 3.14) << std::endl;
    std::cout << "std::less_equal()(3.14, 3.14):         "
              << std::less_equal()(3.34, 3.34) << std::endl;

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

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

输出

std::less_equal()(1023, 1024):        true
std::less_equal()(50, 2):             false
std::less_equal()('a', 97):           true
std::less_equal()(1023, 1024):         true
std::less_equal()(1023, 1024):        true
std::less_equal()(1023, 1024):   true
std::less_equal()(1023, 1024):     true
std::less_equal()(123, 456):      true
std::less_equal()(101, 202):      true
std::less_equal()(10230, 10240):  true
std::less_equal()(1023, 1024):      true
std::less_equal()(123, 456):       true
std::less_equal()(101, 202):       true
std::less_equal()(10230, 10240):   true
std::less_equal()(3.14, 3.14):      true
std::less_equal()(3.14, 3.14):       true
std::less_equal()(3, 3):             true
std::less_equal()(3.56, 3.14):       false
std::less_equal()(3.14, 3.14):         true
std::less_equal()(Cell{101, 101}, Cell{202, 202}):       true
std::less_equal()("I am a ", "handsome programmer"):true