format.md

std::format is a new way to format text which

• separates formatting from outputting
• is more type-safe
• allows reordering

C++17	C++20
std::printf("%d baskets of %s", n, desc); //OR std::cout << n << " baskets of " << desc;	std::cout << std::format("{} baskets of {}", n, desc);

Ordering

whoops!	Apples! 5 baskets left
std::printf("%s! %d baskets left", n, desc);	std::cout << std::format("{1}! {0} baskets left", n, desc);

Formatting

Detailed formatting is similar to printf, but actually based on Python!

The general form is

fill-and-align sign # 0 width precision L type

(with each part optional).

	format	result
	std::string s; // fill-and-align s = std::format("{:6}", 123); //default s = std::format("{:<6}", 123); //left s = std::format("{:^6}", 123); //center s = std::format("{:>6}", 123); //right s = std::format("{:6}", "abc"); //default s = std::format("{:<6}", "abc"); //left s = std::format("{:^6}", "abc"); //center s = std::format("{:>6}", "abc"); //right s = std::format("{:$^6}", "abc"); //fill with // sign s = std::format("{}", 123); //default s = std::format("{}", -123); //default s = std::format("{:-}", 123); //same as def s = std::format("{:-}", -123); //same as def s = std::format("{:+}", 123); //always sign s = std::format("{:+}", -123); //always sign s = std::format("{: }", 123); //space if pos s = std::format("{: }", -123); //space if pos	// results... // fill-and-align assert(s == " 123"); // numbers > assert(s == "123 "); assert(s == " 123 "); // slightly < assert(s == " 123"); assert(s == "abc "); // strings < assert(s == "abc "); assert(s == " abc "); // slightly < assert(s == " abc"); assert(s == "$abc$$"); // fill with $ // sign /* {} */ assert(s == "123"); /* {} */ assert(s == "-123"); /*{:-}*/ assert(s == "123"); /*{:-}*/ assert(s == "-123"); /*{:+}*/ assert(s == "+123"); /*{:+}*/ assert(s == "-123"); /*{: }*/ assert(s == " 123"); /*{: }*/ assert(s == "-123");

Chrono

C++17	C++20
???	std::cout << std::format("The time is {}", std::system_clock::now());