willerz / ctprintf

C++17 constexpr printf

I am providing code in the repository to you under an open source license. Because this is my personal repository, the license you receive to my code is from me and not my employer (Facebook).

Warning

This project is C++ "stunt coding": an exercise in finding out if I could do this.

If you are looking to produce formatted output in a real program I recommend you use the idiomatic C++ mechanism or the idiomatic C mechanism.

What is this?

This is a suite of constexpr format function objects in standard C++17 and a printf-alike user-defined string literal suffix that's implemented with a nonstandard GCC/LLVM C++ extension.

What does printf-alike mean?

% is the special character that means to insert some formatting. The valid format characters are a bit different. Length modifiers (hh, h, l, ll) are never used because C++ allows us to detect the type of the argument.

The valid format charaters right now are:

Character	Description
%	A literal percent character. No argument.
d	Print an integer argument as decimal. Accepts any signed or unsigned integer type. Replaces d, i, u.
x	Print an integer argument as lowercase hexadecimal. Accepts any unsigned integer type. Replaces x.
X	Print an integer argument as uppercase hexadecimal. Accepts any unsigned integer type. Replaces X.
s	Print a string argument. Accepts std::basic_string_view of the same character type as the format string and chacter pointers to NULL-terminated (c-style) strings. Replaces s.

Is it functionally complete?

No. Notably there is no support for rendering floating point numbers, padding or aligning fields, or setting the formatted precision.

Are there any other limitations?

You can probably hit your environment's limits for number of superclasses, number of template parameters, maximum length of a function name, etc.

Usability

If you get anything wrong (wrong argument type, extra arguments, missing arguments, etc) you can expect screens of nested template errors, none of which really tell you what you did wrong.

Performance

In realistic benchmarks it's comparable to C++'s ofstream and slower than C's fprintf. In unrealistic benchmarks it's faster than both, but that's not relevant.