How to call C++ function from C?

I would do it in the following way:

(If working with MSVC, ignore the GCC compilation commands)

Suppose that I have a C++ class named AAA, defined in files aaa.h, aaa.cpp, and that the class AAA has a method named sayHi(const char *name), that I want to enable for C code.

The C++ code of class AAA - Pure C++, I don't modify it:

aaa.h

#ifndef AAA_H
#define AAA_H

class AAA {
    public:
        AAA();
        void sayHi(const char *name);
};

#endif

aaa.cpp

#include <iostream>

#include "aaa.h"

AAA::AAA() {
}

void AAA::sayHi(const char *name) {
    std::cout << "Hi " << name << std::endl;
}

Compiling this class as regularly done for C++. This code "does not know" that it is going to be used by C code. Using the command:

g++ -fpic -shared aaa.cpp -o libaaa.so

---

Now, also in C++, creating a C connector:

Defining it in files aaa_c_connector.h, aaa_c_connector.cpp. This connector is going to define a C function, named AAA_sayHi(cosnt char *name), that will use an instance of AAA and will call its method:

aaa_c_connector.h

#ifndef AAA_C_CONNECTOR_H 
#define AAA_C_CONNECTOR_H 

#ifdef __cplusplus
extern "C" {
#endif
 
void AAA_sayHi(const char *name);

#ifdef __cplusplus
}
#endif


#endif

aaa_c_connector.cpp

#include <cstdlib>

#include "aaa_c_connector.h"
#include "aaa.h"

#ifdef __cplusplus
extern "C" {
#endif

// Inside this "extern C" block, I can implement functions in C++, which will externally 
//   appear as C functions (which means that the function IDs will be their names, unlike
//   the regular C++ behavior, which allows defining multiple functions with the same name
//   (overloading) and hence uses function signature hashing to enforce unique IDs),


static AAA *AAA_instance = NULL;

void lazyAAA() {
    if (AAA_instance == NULL) {
        AAA_instance = new AAA();
    }
}

void AAA_sayHi(const char *name) {
    lazyAAA();
    AAA_instance->sayHi(name);
}

#ifdef __cplusplus
}
#endif

Compiling it, again, using a regular C++ compilation command:

g++ -fpic -shared aaa_c_connector.cpp -L. -laaa -o libaaa_c_connector.so

---

Now I have a shared library (libaaa_c_connector.so), that implements the C function AAA_sayHi(const char *name). I can now create a C main file and compile it all together:

main.c

#include "aaa_c_connector.h"

int main() {
    AAA_sayHi("David");
    AAA_sayHi("James");

    return 0;
}

Compiling it using a C compilation command:

gcc main.c -L. -laaa_c_connector -o c_aaa

I will need to set LD_LIBRARY_PATH to contain $PWD, and if I run the executable ./c_aaa, I will get the output I expect:

Hi David
Hi James

EDIT:

On some linux distributions, -laaa and -lstdc++ may also be required for the last compilation command. Thanks to @AlaaM. for the attention

You need to create a C API for exposing the functionality of your C++ code. Basically, you will need to write C++ code that is declared extern "C" and that has a pure C API (not using classes, for example) that wraps the C++ library. Then you use the pure C wrapper library that you've created.

Your C API can optionally follow an object-oriented style, even though C is not object-oriented. Ex:

// *.h file
// ...
#ifdef __cplusplus
#define EXTERNC extern "C"
#else
#define EXTERNC
#endif

typedef void* mylibrary_mytype_t;

EXTERNC mylibrary_mytype_t mylibrary_mytype_init();
EXTERNC void mylibrary_mytype_destroy(mylibrary_mytype_t mytype);
EXTERNC void mylibrary_mytype_doit(mylibrary_mytype_t self, int param);

#undef EXTERNC
// ...

// *.cpp file
mylibrary_mytype_t mylibrary_mytype_init() {
  return new MyType;
}

void mylibrary_mytype_destroy(mylibrary_mytype_t untyped_ptr) {
   MyType* typed_ptr = static_cast<MyType*>(untyped_ptr);
   delete typed_ptr;
}

void mylibrary_mytype_doit(mylibrary_mytype_t untyped_self, int param) {
   MyType* typed_self = static_cast<MyType*>(untyped_self);
   typed_self->doIt(param);
}