Question

I’m interested (for various reasons) format using sprintf with the result in a std::string object. The most syntactically direct way I came up with is:

char* buf;
sprintf(buf, ...);
std::string s(buf);

Any other ideas?

Answer 1

Don’t use the printf line of functions for formatting in C++. Use the language feature of streams (stringstreams in this case) which are type safe and don’t require the user to provide special format characters.

If you really really want to do that, you could pre-allocate enough space in your string and then resize it down although I’m not sure if that’s more efficient than using a temporary char buffer array:

std::string foo;
foo.resize(max_length);
int num_bytes = snprintf(&foo[0], max_length, ...);
if(num_bytes < max_length)
{
    foo.resize(num_bytes);
}