example of C to C++ interface

c_interfaces.cpp

#include <memory>
#include "header.hpp"

std::unique_ptr<my_class<int>> obj;

extern "C"
void init_data(int x) {
    obj = std::make_unique<my_class<int>>(x);
}

extern "C"
int value() {
    return (*obj)();
}

c_interfaces.h

void init_data(int x);
int value();

header.hpp

template <typename T>
class my_class {
    T m_data;
public:
    my_class(T && x) : m_data{x} {}
    my_class(T const& x) : m_data{x} {}

    T const& operator()() const {return m_data;}
    T & operator()() {return m_data;}
};

main.c

#include "stdio.h"
#include "c_interfaces.h"

int main() {
    init_data(42);
    printf("%d\n", value());
}