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

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In C++ 20 we are able let the compiler automatically generate the implementation for operator== for us like this (and all the other default comparasions too, but I'm just interested in operator== here):

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

Is there a way to archieve the same (automatically generate operator==) but in C++17?

Since libraries are an okay solution I had a look at boost/operators. Would the following be the equivalent as the above?

#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?
};
c++17 comparator boost c++
1 Answers

Only for aggregates (a slightly widened set around POD (trivial+standard layout) types).

Using PFR you can 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

Example using 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";
}

Prints

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

Using strictly c++14 (specifying vector<Point> and set<Point> template arguments).

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