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

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::greater

实现比较的函数对象。调用类型 T 上的 operator> ，除非特化。

特化

std::greater 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。

 成员类型

成员函数

std::greater::operator()

检查 lhs 是否大于 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;}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;}}
};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::greater<int*>()(1023, 1024):        "<< std::greater<int*>()(ptr, nullptr) << std::endl;std::cout << "std::greater<char>()(50, 2):             "<< std::greater<char>()(50, 2) << std::endl;std::cout << "std::greater<char>()('a', 97):           "<< std::greater<char>()('a', 97) << std::endl;std::cout << "std::greater<int>()(1023, 1024):         "<< std::greater<int>()(1023, 1024) << std::endl;std::cout << "std::greater<long>()(1023, 1024):        "<< std::greater<long>()(1023, 1024) << std::endl;std::cout << "std::greater<long long>()(1023, 1024):   "<< std::greater<long long>()(1023, 1024) << std::endl;std::cout << "std::greater<uint8_t>()(1023, 1024):     "<< std::greater<uint8_t>()(8, 32) << std::endl;std::cout << "std::greater<uint16_t>()(123, 456):      "<< std::greater<uint16_t>()(123, 456) << std::endl;std::cout << "std::greater<uint32_t>()(101, 202):      "<< std::greater<uint32_t>()(101, 202) << std::endl;std::cout << "std::greater<uint64_t>()(10230, 10240):  "<< std::greater<uint64_t>()(10230, 10240) << std::endl;std::cout << "std::greater<int8_t>()(1023, 1024):      "<< std::greater<int8_t>()(8, 32) << std::endl;std::cout << "std::greater<int16_t>()(123, 456):       "<< std::greater<int16_t>()(123, 456) << std::endl;std::cout << "std::greater<int32_t>()(101, 202):       "<< std::greater<int32_t>()(101, 202) << std::endl;std::cout << "std::greater<int64_t>()(10230, 10240):   "<< std::greater<int64_t>()(10230, 10240) << std::endl;std::cout << "std::greater<double>()(3.14, 3.14):      "<< std::greater<double>()(3.14, 3.14) << std::endl;std::cout << "std::greater<float>()(3.14, 3.14):       "<< std::greater<float>()(3.14, 3.14) << std::endl;std::cout << "std::greater<float>()(3, 3):             "<< std::greater<float>()(3, 3) << std::endl;std::cout << "std::greater<float>()(3.56, 3.14):       "<< std::greater<float>()(3.56, 3.14) << std::endl;std::cout << "std::greater<int>()(3.14, 3.14):         "<< std::greater<int>()(3.34, 3.34) << std::endl;std::cout << "std::greater<Cell>()(Cell{101, 101}, Cell{202, 202}):       "<< std::greater<Cell>()(Cell{101, 101}, Cell{202, 202}) << std::endl;std::cout << "std::greater<std::string>()(\"I am a \", \"handsome programmer\"):"<< std::greater<std::string>()("I am a ", "handsome programmer") << std::endl;return 0;
}

输出

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