stringfmt.cpp

#include <iostream>
#include <type_traits>
#include <vector>


template<typename, typename = void>
struct is_std_container : std::false_type {};

template<typename T>
struct is_std_container<T,
	std::void_t<decltype(std::declval<T&>().begin()),
	            decltype(std::declval<T&>().end()),
	            typename T::value_type
	            >>
	: std::true_type {};

// std::negation in C++17
template<class B>
struct negation : std::integral_constant<bool, !bool(B::value)> { };


// Things that can use std::to_string, in general
template<typename T>
inline typename std::enable_if_t<std::is_arithmetic<T>::value, std::string>
to_s(T arg)
{
	return std::to_string(arg);
}

// Base case
template<typename T>
inline typename std::enable_if_t<std::is_convertible<T, std::string>::value, std::string>
to_s(T arg)
{
	return arg;
}

// Containers (but not strings, otherwise ambiguity
template<typename T>
typename std::enable_if_t<is_std_container<T>::value
                          && negation<std::is_convertible<T, std::string>>::value, std::string>
to_s(T c, const std::string &sep=", ")
{
	std::string s = "{" + to_s(*c.begin());
	for (auto &&it = c.begin() + 1; it < c.end(); ++it) {
		s += sep + to_s(*it);
	}
	return s + "}";
}

// Base case, trailing newline
template<typename T = void>
std::string concat()
{
	return "\n";
}

template<typename T, typename ...Ts>
std::string concat(T arg, Ts... args)
{
	return to_s(arg) + concat(args...);
}

int main()
{
	std::vector<int> v = {0, 1, 42};
	std::vector<std::vector<std::string>> v2 {{"a", "#w3"}, {"asd", "sdfg", "po"}};

	std::cout << concat("Hello there, I am ", 23, " years old and I have this vector: ", v);
	std::cout << concat("I also have this strange vector: ", v2);
}

Evil.