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An implementation of printf using c++17's variadic templates. It was inspired by some of the "safe printf" examples that I've seen. But none of them attempted to actually implement the printf fully with all of its quirks. This code is (an attempt) to do that.

An advantage of this, is that we can support ANY type being passed to a format, even complex objects. For example, when CXX17_PRINTF_EXTENSIONS is enabled "%?" means "call to_string on the argument and print the result" This uses ADL, so it will find the version that you specify in your namespaces, or use std::to_string as a fallback. If no to_string is found, it uses the internal one which asserts.

NOTE: floating point is not implemented, as it is complex to do correctly, but is on the TODO list.

Usage is similar to snprintf, but more robust. Instead of a buffer/size pair being passed as a parameter, you pass a context object which has 3 functions and a data member:

// writes a single character to the output of your choosing
void write(char ch);

// writes a string of characters to the output of your choosing
void write(const char *p, size_t n);

// called when formatting is complete, useful for ensuring NUL termination
void done();

// increment this every time we write is called, this is what printf will
// return.
size_t written = 0;

A simple example of the usage of the library is as follows:

char buf[256];
cxx17::buffer_writer ctx(buf, sizeof(buf));
cxx17::Printf(ctx, "[hello %*s %d]", 10, "world", 123);

// buffer now contains "[hello      world 123]"

The context uses duck typing, so any object that meets the critera will suffice, but there are several examples in Formatters.h

Additionally, while the context based interface is very flexible and can accomidate essentially any destination stream or buffer. This implementation also includes more familiar interfaces:

  • int cxx17::sprintf(std::ostream &os, const char *format, const Ts &... ts);
  • int cxx17::sprintf(char *str, size_t size, const char *format, const Ts &... ts);
  • int cxx17::printf(const char *format, const Ts &... ts);

All of which work in the expected ways without the need to manually manage the concept of "contexts".

Performance so far, when optimizations are at -O3, slightly edges out glibc's printf. Here is the included test program's output on my machine:

hello      world, A, -123, 00001234 0x7ffff03cb7c8 0000004294967292 ffffffff 0000000000000100
hello      world, A, -123, 00001234 0x7ffff03cb7c8 0000004294967292 ffffffff 0000000000000100
cxx17 Took:  366344 µs to execute.
printf Took: 384387 µs to execute.

When profile guided optimizations are used, it performs even better. I am sure however, that there is room for some optimizations too :-)