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

Question

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?

standard C++ formats are much more verbose, sadly. You're going to want a library. — Mooing Duck
– Mooing Duck, Commented Sep 25, 2012 at 18:35

moooeeeep · Accepted Answer · 2012-09-25 18:44:46Z

5

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;
}

edited Sep 25, 2012 at 18:44

answered Sep 25, 2012 at 18:33

moooeeeep

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4 Comments

Seth Carnegie Over a year ago

Wow, what a terrible syntax...

owagh Over a year ago

this looks like it will work... although i'll hold out for a while hoping for a better solution with a cleaner syntax.

moooeeeep Over a year ago

You get used to it ... However, there are rare cases where you can get trouble with operator precedence because of this approach.

user1084944 Over a year ago

The use of % was probably inspired by python

Community · Accepted Answer · 2017-05-23 12:12:23Z

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);
    }
}

edited May 23, 2017 at 12:12

CommunityBot

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answered Sep 25, 2012 at 19:08

jxh

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3 Comments

moooeeeep Over a year ago

nice. Here's a link to play around: ideone.com/23xIY

moooeeeep Over a year ago

I just discovered a downside of this: when you pass in a format string that doesn't match the arguments supplied, you get rather unspecific errors, ranging from printed garbage to segmentation faults. boost::format provides a helpful error message instead. See this: ideone.com/63BK1

jxh Over a year ago

@moooeeeep: GCC provides an extension attribute that would warn about mismatched parameters to printf arguments (__attribute__((format(...)))).

Code-Apprentice · Accepted Answer · 2012-09-25 18:33:19Z

0

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.

answered Sep 25, 2012 at 18:33

Code-Apprentice

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