std::format - what exactly happens at compile-time?

[Andi Anderson (nuuSolutions)]

[jovibor]

Compilation.

[Andi Anderson (nuuSolutions)]

some very smart people here. Thanks!. Are you a Microsoft MVP?

[Stephan T. Lavavej (StephanTLavavej)]

A less hilarious but more relevant answer:

<format> performs compile-time checking of format strings, but doesn't do any other work at compile-time that could somehow accelerate run-time processing. The checking is driven by the behavior of consteval, which (unlike plain constexpr) forces compile-time evaluation even when the top-level call happens at runtime. For part of the involved logic, see here:

STL/stl/inc/format

Lines 3490 to 3495 in 1c59a20

	// Special compile time version of _Parse_format_specs. This version is parameterized on
	// the type of the argument associated with the format specifier, since we don't really
	// care about avoiding code bloat for code that never runs at runtime, and we can't form
	// the erased basic_format_args structure at compile time.
	template <class _Ty, class _ParseContext>
	consteval _ParseContext::iterator _Compile_time_parse_format_specs(_ParseContext& _Pc) {