use shared object by dlopen|LoadLibrary

dlopen_LoadLibrary.sh

# origin link: https://gist.github.com/roachsinai/faf8ac80860065a7e78adc0b25abb445

# compilation process: https://en.wikibooks.org/wiki/C%2B%2B_Programming/Programming_Languages/C%2B%2B/Code/Compiler/Linker
# Shared library concept: http://ehuss.com/shared/

# 1. Creating the shared library
# Command reference: https://www.oreilly.com/library/view/c-cookbook/0596007612/ch01s05.html
g++ -dynamiclib -fPIC shared.cpp -o shared.so # MacOS
g++ -fPIC -shared shared.cpp -o shared.so # Linux

# 2. Creating the main executable file
g++ main.cpp -o main

# 3. Running main executable
./main

main.cpp

#include <iostream>
#include <dlfcn.h> // https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlfcn.h.html
#include "shared.h"

int main() {
    
    // declaration (template) of add function
    // function typedef: http://www.iso-9899.info/wiki/Typedef_Function_Type
    typedef int add_t(int, int);
    
    // load shared library
    // https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlopen.html
    void *handler = dlopen("./shared.so", RTLD_LAZY);
  
    // extract `add` symbol (pointer) and convert it to function pointer
    // https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlsym.html
    add_t *add = (add_t*) dlsym( handler, "add" );
    
    // execute function
    std::cout << (*add)(5, 2) << std::endl;
}

main_windows.cpp

#include <windows.h>
#include <iostream>
#include <thread>
#include <chrono>

/* Define a function pointer for our imported
 * function.
 * This reads as "introduce the new type add_t as the type:
 *                pointer to a function returning an int and
 *                taking no arguments.
 *
 * Make sure to use matching calling convention (__cdecl, __stdcall, ...)
 * with the exported function. __stdcall is the convention used by the WinAPI
 */
extern "C" typedef void(__stdcall* add_t)(int a, int b);

int main()
{
#ifdef _DEBUG
    //HINSTANCE hGetProcIDDLL = LoadLibrary(L"F:/office/VSProject/PoseNcnn/x64/Debug/PoseLib.dll");
#else
    HINSTANCE hGetProcIDDLL = LoadLibrary(L"./shared.dll");
#endif

    if (!hGetProcIDDLL) {
        std::cout << "could not load the dynamic library" << std::endl;
        return EXIT_FAILURE;
    }
    else
    {
        std::cout << "load the dynamic library success" << std::endl;
    }

    char data[1000];

    // resolve function address here
    add_t func = (add_t)GetProcAddress(hGetProcIDDLL, "ValidationLicense");

    if (!func) {
        std::cout << "could not locate the function" << std::endl;
        return EXIT_FAILURE;
    }
    else
    {
        std::cout << func(5, 2) << "\n";
    }

    //std::this_thread::sleep_for(std::chrono::milliseconds(200000));

    return EXIT_SUCCESS;
}

shared.cpp

#include <iostream>
#include "shared.h"

// function definition
int add(int a, int b) {
    return a + b;
}

shared.h

#pragma once

// extern: https://docs.microsoft.com/en-us/cpp/cpp/extern-cpp?view=vs-2019
// why extern "C" : https://stackoverflow.com/a/42183390/2790983
extern "C" {
    // function declaration
    int add(int, int);
}