Using * Width & Precision Specifiers With boost::format

Question

I am trying to use width and precision specifiers with boost::format, like this:

#include 
#include 

int         


        

Answer 1

Well, this isn't a drop-in, but one way to do it would be to construct the format string dynamically:

#include 
#include 

int main()
{
    int n = 5;
    const std::string f("%" + 
                        boost::lexical_cast(n * 2) + "." +
                        boost::lexical_cast(n * 2) + "s");
    std::string s = (boost::format(f) % "Hello").str();
}

Of course, if you did this frequently, you could refactor construction of the format string into a function.

You could also use boost::format() to generate the format string; it's shorter, but potentially less readable, especially for long format strings:

const std::string f = (boost::format("%%%d.%ds") % (n*2) % (n*2)).str();
std::string s = (boost::format(f) % "Hello").str();

(credit to Ferruccio for posting the second idea in the comments)