std::string formatting like sprintf

Question

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

Answer 1

C++20 will include std::format which resembles sprintf in terms of API but is fully type-safe, works with user-defined types, and uses Python-like format string syntax. Here's how you will be able to format std::string and write it to a stream:

std::string s = "foo";
std::cout << std::format("Look, a string: {}", s);

or

std::string s = "foo";
puts(std::format("Look, a string: {}", s).c_str());

Alternatively, you could use the {fmt} library to format a string and write it to stdout or a file stream in one go:

fmt::print(f, "Look, a string: {}", s); // where f is a file stream

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As for sprintf or most of the other answers here, unfortunately they use varargs and are inherently unsafe unless you use something like GCC's format attribute which only works with literal format strings. You can see why these functions are unsafe on the following example:

std::string format_str = "%s";
string_format(format_str, format_str[0]);

where string_format is an implementation from the Erik Aronesty's answer. This code compiles, but it will most likely crash when you try to run it:

$ g++ -Wall -Wextra -pedantic test.cc 
$ ./a.out 
Segmentation fault: 11

Disclaimer: I'm the author of {fmt} and C++20 std::format.