How to use C++ std::ostream with printf-like formatting?

Question

I am learning C++. cout is an instance of std::ostream class. How can I print a formatted string with it?

I can still use printf

Answer 1

I suggest using ostringstream instead of ostream see following example :

#include <vector>
#include <string>
#include <iostream>
#include "CppUnitTest.h"

#define _CRT_NO_VA_START_VALIDATION

std::string format(const std::string& format, ...)
{
    va_list args;
    va_start(args, format);
    size_t len = std::vsnprintf(NULL, 0, format.c_str(), args);
    va_end(args);
    std::vector<char> vec(len + 1);
    va_start(args, format);
    std::vsnprintf(&vec[0], len + 1, format.c_str(), args);
    va_end(args);
    return &vec[0];
}

example usage:

std::ostringstream ss;
ss << format("%s => %d", "Version", Version) << std::endl;
Logger::WriteMessage(ss.str().c_str()); // write to unit test output
std::cout << ss.str() << std::endl; // write to standard output

Answer 2

I wrote independently but came up with answer similar to user3283405

My solution uses vasprintf() to acheive formatting, and uses operator overloading of << of std::ostream to free the memory in right place.

Usage:

std::cout << putf(const char *format, ...); //Same format as C printf(3)

Code:

#define _GNU_SOURCE
#include <cstdarg>
#include <iostream>
#include <cstdio>

struct putf_r{
        char *s;
};

putf_r putf(const char *fmt, ...){
        va_list ap;
        va_start(ap, fmt);
        putf_r a;
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wformat-nonliteral"
        vasprintf(&a.s, fmt, ap);
#pragma GCC diagnostic pop
        va_end(ap);
        return a;
}

std::ostream& operator<<(std::ostream& os, putf_r a){
        os<<a.s;
        free(a.s);
        return os;
}

int main(){
        std::cout << putf("%3d\n", 23) << putf("%a\n", 256.);
}

Note that compiler doesn't check format inside putf(), so compiler flag -Wformat-nonliteral will not warn for suspicious code in putf() and you need to care uncontrolled format string problem by yourself.
Detailed info can be found on GitHub