Convert a number to a string with specified length in C++

Question

I have some numbers of different length (like 1, 999, 76492, so on) and I want to convert them all to strings with a common length (for example, if the length is 6, then tho

Answer 1

This is an old thread, but as fmt might make it into the standard, here is an additional solution:

#include 

int n = 999;

const auto str = fmt::format("{:0>{}}", n, 6);

Note that the fmt::format("{:0>6}", n) works equally well when the desired width is known at compile time. Another option is abseil:

#include 

int n = 999;

const auto str = absl::StrFormat("%0*d", 6, n);

Again, abs::StrFormat("%06d", n) is possible. boost format is another tool for this problem:

#include 

int n = 999;

const auto str = boost::str(boost::format("%06d") % n);

Unfortunately, variable width specifier as arguments chained with the % operator are unsupported, this requires a format string setup (e.g. const std::string fmt = "%0" + std::to_string(6) + "d";).

In terms of performance, abseil and fmt claim to be very attractive and faster than boost. In any case, all three solutions should be more efficient than std::stringstream approaches, and other than the std::*printf family, they do not sacrifice type safety.