Right way to initialize a static library (C++)? - c++

Question

I have a static C library that I want to port to C++, in the C library I got some global variables that store some common data used by the functions for example:
// global variable in the C library
int global_number_of_cpu_cores;
init_global_vars()
{
global_number_of_cpu_cores = get_info();
}
void lib_function()
{
// use global_number_of_cpu_cores
}
when using the library, it must be first initialized by the init function but in C++, the object's constructors are executed before the main function, so I cannot code:
class class_lib
{
class_lib()
{
// use global_number_of_cpu_cores but this is uninitialized!
}
}
Also, you can initialize global variables with functions:
int program_var = lib_function(); // lib_function uses global_number_of_cpu_cores but this is unintialized!
what is a decent/elegant way to solve this when designing a C++ library?
how do well-designed C++ libraries like Boost, Qt, etc solve this? any idea?

Answer 1

What you can do - simply create a C wrapper interface for C++ I.e. something like following:
In header file, e.g. foo.h
#ifndef __FOO_H_INCLUDED__
#define __FOO_H_INCLUDED__
#ifdef __cplusplus
extern "C" {
#endif
void print_logical_cpus()
#ifdef __cplusplus
}
#endif / * extern "C" */
#endif / * __FOO_H_INCLUDED__ */
in the implementation file e.g. foo.cpp
#include "foo.h"
#include <cstdio>
#include <thread>
struct GlobalSettings {
GlobalSettings():
logical_cpus(std::thread::hardware_concurrency())
{}
std::size_t logical_cpus;
};
static GlobalSettings __gsettings;
extern "C" {
void print_logical_cpus()
{
std::printf("Logical cpus %z", __gsettings.logical_cpus);
}
}
If you still need to port on raw C, there are compiler specific tricks to run functions before and after main.
GCC uses attribute((constructor)) and attribute((destructor)))
MS VC++ uses __declspec(allocate(".CRT$XLC"))