How do I send a format string to iostream? [duplicate]

Question

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Closed 8 years ago.

Possible Duplicate:
std::string formatting like sprintf

Can I use the c++ iostream classes with a format string like printf?

Basically, I want to be able to do something like :-

snprintf (inchars, len, "%4f %6.2f %3d \n", float1, float2, int1);

easily using stringstreams. Is there an easy way to do this?

Answer 1

Yes, there's the Boost Format Library (which is stringstreams internally).

Example:

#include <boost/format.hpp>
#include <iostream>

int main() {
  std::cout << boost::format("%s %s!\n") % "Hello" % "World";
  return 0;
}

Answer 2

You can write a wrapper function that returns something that you can deliver into an ostringstream.

This function combines some of the solutions presented in the link moooeeeep pointed out in comments:

std::string string_format(const char *fmt, ...) {
    std::vector<char> str(100);
    va_list ap;
    while (1) {
        va_start(ap, fmt);
        int n = vsnprintf(&str[0], str.size(), fmt, ap);
        va_end(ap);
        if (n > -1 && n < str.size()) {
            str.resize(n);
            return &str[0];
        }
        str.resize(str.size() * 2);
    }
}

Answer 3

This kind of formatting takes quite a bit more effort using standard C++ streams. In particular, you have to use stream manipulators which can specify the number of digits to show after the decimal place.