Basically, the idea is to overload operator << for anything iterable, like vectors, lists, and custom classes properly defining begin() and the iterator scheme.
Initially, I wrote the following prototype
template<template<class, class ...> class Container, class T, class ... Whatever>
std::ostream& operator<<(std::ostream& out, const Container<T, Whatever...>& container) { stuff }
The problem is that I obviously get clashes with anything matching the template definition and already overloading <<, such as std::string. So, if I write cout << string { "Hello" }, it is ambiguous. I understand why.
So, the idea is to activate the above overload if and only if an operator<< isn't already defined. I chose to discard this case with std::enable_if in the following way:
template<class> struct sfinae_true : std::true_type {};
template<class ToPrint> static auto test_insertion(int) -> sfinae_true<decltype(std::cout << std::declval<ToPrint>())>;
template<class ToPrint> static auto test_insertion(long) -> std::false_type;
template<class ToPrint> struct is_printable : decltype(test_insertion<ToPrint>(0)) {};
template<class ToPrint> using NotPrintable = std::enable_if_t<! is_printable<ToPrint>::value>;
//overload not availlable if operator<< is already defined (avoids ambiguity)
template<template<class, class ...> class Container, class T, class ... Whatever, NotPrintable<Container<T, Whatever...>>* = nullptr>
std::ostream& operator<<(std::ostream& out, const Container<T, Whatever...>& container)
{
//stuff
}
The problem is, somehow, in C++17 at least, I can't print twice. That is,
vector<int> v1 = {1,2,3,4,5};
vector<int> v2 = {6,7,8};
cout << v1 << endl;
cout << v2 << endl;
the compiler tells me there is no operator<< for v2... Incredible right ? I have no idea why, and consequently how to fix the problem.
Remark: I know, I could simplyfy the prototype to just template<class Container> std::ostream& operator<<(std::ostrea& out, const Container& c), but let's say "I don't want to".
Complete sample code to copy / paste:
#include <iostream>
#include <type_traits>
#include <vector>
#include <string>
template<class> struct sfinae_true : std::true_type {};
template<class ToPrint> static auto test_insertion(int) -> sfinae_true<decltype(std::cout << std::declval<ToPrint>())>;
template<class ToPrint> static auto test_insertion(long) -> std::false_type;
template<class ToPrint> struct is_printable : decltype(test_insertion<ToPrint>(0)) {};
template<class ToPrint> using NotPrintable = std::enable_if_t<! is_printable<ToPrint>::value>;
//overload not availlable if operator<< is already defined (avoids ambiguity)
template<template<class, class ...> class Container, class T, class ... Whatever, NotPrintable<Container<T, Whatever...>>* = nullptr>
std::ostream& operator<<(std::ostream& out, const Container<T, Whatever...>& container)
{
out << "{ ";
auto it = container.begin();
auto it_end = container.end();
if(it != it_end)
{
out << *it;
++it;
}
for(; it != it_end; ++it)
out << " , " << (*it);
out << " }";
return out;
}
using namespace std;
int main()
{
vector<int> v1 = {1,2,3,4,5};
vector<int> v2;
cout << v1 << endl; //prints {1, 2, 3? 4, 5}
cout << v2 << endl; //compile error : no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘std::vector<int>’)
cout << string {"Hello"} << endl; //works fine
}
The error message is
error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘std::vector<int>’)
cout << v2 << endl;
I'm compiling with g++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, with the command g++ -o sample sample.cpp, where sample.cpp is the file containing the above code.