#include <string>
#include <cstdio>
#include <vector>
#include <iostream>

[[gnu::format(printf, 1, 2)]]
auto format(const char * fmt, ...) -> std::string {
    va_list args1;
    va_start(args1, fmt);
    va_list args2;
    va_copy(args2, args1);
    std::vector<char> buf(1 + std::vsnprintf(nullptr, 0, fmt, args1));
    va_end(args1);
    std::vsnprintf(buf.data(), buf.size(), fmt, args2);
    va_end(args2);
    return buf.data();
}

int main() {
    int i = 123;
    // warning: format specifies type 'double' but the argument has type 'int' [-Wformat]
    auto str = format("%f\n", i);
    //                 ~~     ^
    //                 %d
    std::cout << str;
}