可以在 C++17 中模拟 C++20 的 'operator==(const T&) = default' 吗？

Question

在 C++ 20 中，我们可以让编译器像这样自动为我们生成 operator== 的实现（以及所有其他 default comparasions，但我只对 operator== 这里):

#include <compare>
struct Point {
  int x;
  int y;
  bool operator==(const Point&) const = default;
};

有没有办法在 C++17 中实现相同的（自动生成 operator==）？

由于库是一个不错的解决方案，我查看了 boost/operators。下面的和上面的一样吗？

#include <boost/operators.hpp>
struct Point : boost<equality_comparable<Point, Point>> {
  int x;
  int y;
  bool operator==(const Point&) const; // will this get auto-generated too and do the same as above?
};

Answer 1

仅适用于聚合（围绕 POD（琐碎+标准布局）类型的集合）。

使用 PFR 您可以 opt in using a macro:

Boost.PFR adds the following out-of-the-box functionality for aggregate initializable structures:

comparison functions

heterogeneous comparators

hash

IO streaming

access to members by index

member type retrieval

methods for cooperation with std::tuple

methods to visit each field of the structure

示例使用 BOOST_PFR_FUNCTIONS_FOR:

Defines comparison and stream operators for T along with hash_value function.

See Also : 'Three ways of getting operators' for other ways to define operators and more details.

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struct Point { int x, y; };

#include <boost/pfr.hpp>    
BOOST_PFR_FUNCTIONS_FOR(Point)

#include <iostream>
#include <vector>
#include <set>
int main() {
    std::vector pv { Point{1,2}, {2,1}, {-3,4}, {1,4/2}, {-3,-4} };

    for(auto& p : pv) std::cout << p << " ";
    std::cout << "\n";

    std::set ps(begin(pv), end(pv));

    for(auto& p : ps) std::cout << p << " ";
    std::cout << "\n";
}

版画

{1, 2} {2, 1} {-3, 4} {1, 2} {-3, -4} 
{-3, -4} {-3, 4} {1, 2} {2, 1}

严格使用 c++14（指定 vector<Point> 和 set<Point> 模板参数）。

可以在 C++17 中模拟 C++20 的 'operator==(const T&) = default' 吗？

Can C++20's 'operator==(const T&) = default' be mimiqued in C++17?

c++

boost

c++17

comparator