How to create a string with format specifier?

Question

I want to create a public api that takes a string as an argument and places this string at the loaction where I have placed a format specifer in another string in that Api.

e.g. string PrintMyMessage( const string& currentValAsString)
     {
          string s1("Current value is %s",currentValAsString);
          return s1;
     }

Currently I am getting following build Error.

1>d:\extra\creatingstrwithspecifier\creatingstrwithspecifier\main.cxx(8): error C2664: 'std::basic_string<_Elem,_Traits,_Ax>::basic_string(const std::basic_string<_Elem,_Traits,_Ax> &,unsigned int,unsigned int)' : cannot convert parameter 2 from 'const std::string' to 'unsigned int'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Ax=std::allocator<char>
1>          ]
1>          No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

I just want to know what could be a better way to accomplish this task.

other possible duplicate: http://stackoverflow.com/q/2342162/1025391 — moooeeeep, Apr 24 '15 at 08:42

score 2 · Accepted Answer · edited May 23 '17 at 11:46

As discussed also in this rather similar question, you could use the Boost Format Library. For example:

std::string PrintMyMessage( const string& currentValAsString )
{
  boost::format fmt = boost::format("Current value is %s") % currentValAsString; 
  // note that you could directly use cout:
  //std::cout << boost::format("Current value is %s") % currentValAsString;
  return fmt.str();
}

In the answers to the other question you can also find other approaches, e.g. using stringstreams, snprintf, or string concatenation.

Here's a complete generic example:

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

std::string greet(std::string const & someone) {
    boost::format fmt = boost::format("Hello %s!") % someone;
    return fmt.str();
}

int main() {
    std::cout << greet("World") << "\n";
}

Or, if you can't or don't want to use Boost:

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

std::string greet(std::string const & someone) {
    const char fmt[] = "Hello %s!";
    std::vector<char> buf(sizeof(fmt)+someone.length());
    std::snprintf(&buf[0], buf.size(), fmt, someone.c_str());
    return &buf[0];
}

int main() {
    std::cout << greet("World") << "\n";
}

Both examples produce the following output:

$ g++ test.cc && ./a.out
Hello World!

[All in all, the format class translates a format-string (with eventually printf-like directives) into operations on an internal stream, and finally returns the result of the formatting, as a string, or directly into an output stream.](http://www.boost.org/doc/libs/1_49_0/libs/format/doc/format.html#how_it_works) — moooeeeep, Jun 11 '12 at 11:50
@Jonny I added more examples. I assume your build environment didn't include the necessary Boost library. Also have a look at my other example if you can't or don't want to use Boost. — moooeeeep, Apr 23 '15 at 17:01

PermanentGuest · Answer 2 · 2012-06-11T10:07:23.407

0

string PrintMyMessage( const string& currentValAsString)
{
    char buff[100];
    sprintf(buff, "Current value is %s", currentValAsString.c_str());

    return buff;
}

edited Jun 11 '12 at 10:07

answered Jun 11 '12 at 10:01

PermanentGuest

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Aladdin · Answer 3 · 2012-06-11T10:10:43.550

0

string PrintMyMessage(const char* fmt, ...)
{
    char buf[1024];
    sprintf(buf, "Current value is ");

    va_list ap;
    va_start(ap, fmt);
    vsprintf(buf + strlen(buf), fmt, ap);

    return buf;
}

string str = PrintMyMessage("Port is %d, IP is :%s", 80, "192.168.0.1");

edited Jun 11 '12 at 10:10

answered Jun 11 '12 at 10:04

Aladdin

171
8

You should initialise your buffer with `={0}` and there is a vsnprintf template with for ensuring the buffer is not overflown. – Dennis Jun 11 '12 at 10:33
You are right, actually, 1024 size buffer may be a potential bug:) – Aladdin Jun 13 '12 at 02:21

score -2 · Answer 4 · answered Jun 11 '12 at 10:04

-2

You can simply write this:

return  "Current value is " + currentValAsString;

answered Jun 11 '12 at 10:04

Nawaz

353,942
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How to create a string with format specifier?

4 Answers4