How do I set constants at run-time in a c++ header file, imported through Cython?

Question

I have some C++ code that currently relies on hard-coded constants, which are imported into multiple other cpp files, and I would like my python (pyx) file to set the constants once at runtime.

So, cython.pyx imports files a.cpp , b.cpp , and c.cpp , and constants.hpp

Files a.cpp , b.cpp , and c.cpp all import constants.hpp .

I would like instead to have one universal constants file, eg new_constants.yml , which python imports and sends through to the cpp files. This also means (I think) that I won't have to re-compile the c code every time I want to tweak the constants.

I'm used to scripting languages (python, js), so working with old C++ code is throwing me off a bit, and I'm sure parts of this question sound like I'm retarded, so, thanks for being patient with me.

These are just some weird dependencies, and I can't wrap my mind around unspooling it.

Answer 1

C++ literally inserts #include 'd files into the code at compile time (technically before compile time - during the preprocessor run), so there is no way to change those values at runtime.

if you have the following

foo.h

const int value = 42;

and foo.cpp

#include "foo.h"
int foo(){ return value; }

When you compile foo.cpp, the preprocessor will substitute the exact contents of foo.h to replace #include "foo.h" in the cpp file and then the compiler will see

const int value = 42;
int foo(){ return value; }

and nothing else

The original source code for a c++ program is completely discarded once compilation is complete and is never used again.

You can see what the compiler sees using the -E flag to gcc which will make it output the pre-processed source.