std::string formating like sprintf

Question

Hi guys, I have to format std::string with sprintf and send it into file stream. How can I do this?

Answer 1

You can't do it directly, because you don't have write access to the underlying buffer. You'll have to do it first in a c-string, then copy it into a std::string:

  char buff[100];
  sprintf(buff, "%s", "Hello");
  std::string buffAsStdStr = buff;

But I'm not sure why you wouldn't just use a string stream? I'm assuming you have specific reasons to not just do this:

  std::ostringstream stringStream;
  stringStream << "Hello";
  std::string copyOfStr = stringStream.str();

Answer 2

string doesn't have what you need, but std::stringstream does. Use a stringstream to create the string and then extract the string. Here is a comprehensive list on the things you can do. For example:

cout.setprecision(10); //stringstream is a stream like cout

will give you 10 decimal places of precision when printing a double or float.

Answer 3

If you only want a printf-like syntax (without calling printf yourself), have a look at Boost Format.

Answer 4

If you want to use a format string, try Boost.Format.

Answer 5

std::string str(100, ' ');
sprintf(&str[0], "%s %s", "Hello", "World");

Answer 6

A very late answer, but for those who, like me, do like the 'sprintf'-way: I've written and are using the following functions. If you like it, you can expand the %-options to more closely fit the sprintf ones; the ones in there currently are sufficient for my needs. You use stringf() and stringfappend() same as you would sprintf. Just remember that the parameters for ... must be POD types.

//=============================================================================
void DoFormatting(string& sF, const char* sformat, va_list marker)
{
    char *s, c, ch=0;
    int n, i=0, m;
    long l;
    float f;
    double d;
    string sf = sformat;
    stringstream ss;

    m = sf.length();
    while (i<m)
    {
        ch = sf.at(i);
        if (ch == '%')
        {
            i++;
            if (i<m) 
            {
                ch = sf.at(i);
                switch(ch)
                {
                    case 's': { s = va_arg(marker, char*);  ss << s;         } break;
                    case 'c': { n = va_arg(marker, int);    ss << (char)n;   } break;
                    case 'd': { n = va_arg(marker, int);    ss << (int)n;    } break;
                    case 'l': { l = va_arg(marker, long);   ss << (long)l;   } break;
                    case 'f': { d = va_arg(marker, double); ss << (float)d;  } break;
                    case 'e': { d = va_arg(marker, double); ss << (double)d; } break;
                    case '%': { ss << '%'; } break;
                    default: 
                    {
                        ss << "%" << ch << " <- unknown format ";
                        i = m; //get out of loop
                    }
                }
            }
        }
        else ss << ch;
        i++;
    }
    va_end(marker);             
    sF = ss.str();
}

//=============================================================================
void stringf(string& stgt,const char *sformat, ... )
{
    va_list marker;
    va_start(marker, sformat);
    DoFormatting(stgt, sformat, marker);
}

//=============================================================================
void stringfappend(string& stgt,const char *sformat, ... )
{
    string sF = "";
    va_list marker;
    va_start(marker, sformat);
    DoFormatting(sF, sformat, marker);
    stgt += sF;
}