Variadic template string and string_view concat for C++17

concat.cpp

#include <string>

// concat(a,b,c) is the equivalent to std::string(a) + std::string(b) + std::string(c)
template<class T>
std::string concat(T t) {
    return std::string(t);
}

template<class T, class... Types>
std::string concat(T t, Types&&... others) {
    return concat(t) + concat(others...);
}

// The concat2 implementation is 250% faster because it avoids creating many new string instances that are directly deleted.
// concat2(a,b,c) is the equivalent of:
// std::string output;
// output.append(a);
// output.append(b);
// output.append(c);
// The benchmark: https://quick-bench.com/q/nC45fivQ3TIRXMysQ3hJwC5oSsw

template<class T>
void append_to_str(std::string& inout, T t) {
    inout.append(t);
}

template<class T, class... Types>
void append_to_str(std::string& inout, T t, Types&&... others) {
    append_to_str(inout, t);
    append_to_str(inout, others...);
}

template<class T>
std::string concat2(T t) {
    return std::string(t);
}

template<class T, class... Types>
std::string concat2(T t, Types&&... others) {
    std::string output;
    output.append(t);
    append_to_str(output, others...);
    return output;
}


constexpr std::string_view hello = "Hello";
constexpr const char* world = "world";
const std::string cpp = "C++!";

int main() {
    std::string res = concat(hello, " ", world, " ", cpp);
    std::string res2 = concat2(hello, " ", world, " ", cpp);
    printf("%s", res.c_str());
    printf("%s", res2.c_str());
}