Construct a string as in the printf function

There are various implementations of a function format

that looks something like this:

std::string format(const std::string& fmt, ...);

      

        
        
        
      

    

so your example:

std::string date = format("%d.%d.%d", year, month, day);

      

        
        
        
      

    

One possible implementation is shown below.

Boost has a library format that works a little differently. You are supposed to like cin

it cout

and their ilk:

cout << boost::format("%1%.%2%.%3%") % year % month % day;

      

        
        
        
      

    

Or, if you just want the line:

boost::format fmt("%1%.%2%.%3%");
fmt % year % month % day;
std::string date = fmt.str();

      

        
        
        
      

    

Please note that the flags are %

not what you are used to.

Finally, if you want to use a C ( char*

) string instead of C ++ string

, you can use the function asprintf

:

char* date; 
if(asprintf(&date, "%d.%d.%d", year, month, day) == -1)
{ /* couldn't make the string; format was bad or out of memory. */ }

      

        
        
        
      

    

You can even use vasprintf

to have your own function format

return a C ++ string:

std::string format(const char* fmt, ...)
{
    char* result = 0;
    va_list ap;
    va_start(ap, fmt);
    if(vasprintf(*result, fmt, ap) == -1)
        throw std::bad_alloc();
    va_end(ap);
    std::string str_result(result);
    free(result);
    return str_result;
}

      

        
        
        
      

    

It's not very efficient, but it works. There could also be a way to call vsnprintf

twice, the first one without a buffer to get the formatted length of the string, then allocate a string object with the required capacity, then call a second time to get the string. This avoids allocating memory twice, but you need to make two passes through the formatted string.