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.

Answer 1

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!