Example C++ concepts with gcc 6.2 or greater

main.cpp

//compile this file with gcc 6.2 or greater

#include <iostream>
#include <string>
#include <locale>
#include <vector>
#include <cassert>
#include <list>
using namespace std::literals;
using namespace std;
// Declaration of the concept "EqualityComparable", which is satisfied by
// any type T such that for values a and b of type T,
// the expression a==b compiles and its result is convertible to bool

template<typename T>
concept bool EqualityComparable = requires(T a, T b) {
    { a == b } -> bool;
};

template<typename T>
concept bool Stringable = requires(T a){  // forces you to implement to_string method
	{a.to_string()} -> string;
};

template<typename T>
concept bool HasStringFunc = requires(T a){
	{ to_string(a)} -> string;  // has a to_string function which returns a string
	 
};

struct Person {

	double height;
	int weight;

	Person(double a, int b){
		height=a;
		weight=b;
	}

	string to_string(){
		
		return "Weight: "+ std::to_string(height) + " Height: "+ std::to_string(weight);
	}



};



namespace std {

string to_string(list<int> l){
	//better ways to do this but just as example
	string s = "";
	for(int a : l ){
		s+=(" "+to_string(a)+" ");
	}

	return s;

}

}

string to_string(std::vector<int> v){
	string s = "";
	for(int a : v ){
		s+=(" "+to_string(a)+" ");
	}

	return s;
}


void pretty_print(Stringable a){
	//we know that if the compiler made it this far we are good to go.
	cout<<a.to_string()<<endl;
}

void pretty_print(HasStringFunc a){
	cout<<to_string(a)<<endl;
}



 // declaration of a constrained function template
// template<typename T>
// void f(T&&) requires EqualityComparable<T>; // long form of the same
 
int main() {
 assert(__cpp_concepts >= 201500); // check compiled with -fconcepts
 assert(__cplusplus >= 201500); //check compiled with --std=c++1z
 std::list<int> l {1,2,3};
 Person jonathan(5.7,130);
 std::vector<int> v { 34,23,34,56,78}; // we can make it possible to pretty print a
 									// vector we just need to implement -> string to_string(vector<int> a);
 									// if we want to pretty print
 pretty_print(jonathan); // uses concept overload pretty_print(Stringable a)
 pretty_print(3); //uses concept overload pretty_print(HasStringFunc a) 
 pretty_print(l);
 // pretty_print(v);
 // this will result in an error , first you get the old template garbage you are used to
 //but you can ignore that because at the end highlighted in blue you will see
 // main.cpp:41:6: note:   constraints not satisfied
 // void pretty_print(Stringable a){
 // ^~~~~~~~~~~~
 // main.cpp:18:14: note: within ‘template<class T> concept const bool Stringable<T> [with T = std::vector<std::__cxx11::basic_string<char> >]’
 // concept bool Stringable = requires(T a){

}