How do I send a format string to iostream?

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, Sep 25 '12 at 18:35

moooeeeep · Accepted Answer · 2012-09-25T18:44:46.237

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 '12 at 18:44

answered Sep 25 '12 at 18:33

moooeeeep

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Wow, what a terrible syntax... – Seth Carnegie Sep 25 '12 at 18:37
this looks like it will work... although i'll hold out for a while hoping for a better solution with a cleaner syntax. – owagh Sep 25 '12 at 18:38
You get used to it ... However, there are rare cases where you can get trouble with operator precedence because of this approach. – moooeeeep Sep 25 '12 at 18:42
The use of `%` was probably inspired by python – Sep 25 '12 at 19:19

score 2 · Answer 2 · edited May 23 '17 at 12:12

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 '17 at 12:12

Community

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answered Sep 25 '12 at 19:08

jxh

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nice. Here's a link to play around: http://ideone.com/23xIY – moooeeeep Sep 25 '12 at 19:28
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: http://ideone.com/63BK1 – moooeeeep Oct 01 '12 at 11:05
@moooeeeep: GCC provides an extension attribute that would warn about mismatched parameters to printf arguments (`__attribute__((format(...)))`). – jxh Oct 01 '12 at 15:21

score 0 · Answer 3 · answered Sep 25 '12 at 18:33

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 '12 at 18:33

Code-Apprentice

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How do I send a format string to iostream?

3 Answers3