#include <cassert>
#include <cstdarg>

#include <sstream>
#include <string>

using namespace std;

string& stringprintf( string &s, const char *fmt, ... )
{
    assert( s.empty() );

    va_list ap;
    va_start(ap, fmt);
    const char *next_c; // next character
    while ( *fmt != '\0' )
    {
        switch ( *fmt )
        {
            case '%':
            {
                next_c = fmt + 1;
                switch ( *next_c )
                {
                    case '\0':
                        s += *fmt;
                        break;
                    case '%':
                        s += '%';
                        ++fmt;
                        break;
                    case 's':
                        s += va_arg(ap, char*);
                        ++fmt;
                        break;
                    case 'd':
                    {
                        ostringstream oss;
                        oss << va_arg(ap, int);
                        s += oss.str();
                        ++fmt;
                        break;
                    }
                    case 'f':
                    {
                        ostringstream oss;
                        oss << va_arg(ap, double);
                        s += oss.str();
                        ++fmt;
                        break;
                    }
                    default:
                        assert( !"format unsupported." );
                        break;
                }

                break;
            }
            default:
                s += *fmt;
                break;
        }

        ++fmt;
    }
    va_end(ap);

    return s;
}

-----------------------------------------------------

测试一下

int main()
{
    string s;
    stringprintf(s, "int: %d, double: %f, str: %s", 18, 18.55, "test");
    cout << s << endl;
}