Lambda in a C callback

heap-lambda.cc

#include <functional>
#include <iostream>

// using a lambda in a C callback

extern "C" void  
register_callback (void (*callback)(void *), void * context)
{
    // for simplicity we just call it
    callback (context);
}

extern "C" void 
invoke_function (void* ptr) 
{
    (*static_cast<std::function<void()>*>(ptr))();
}

class example
{
public:
    example (int val) : a(val)
    {
        register_callback (invoke_function,
            new std::function<void()>([=](){ 
                std::cout << "value is " << a;
            })
        );  
    }
private:
    int a;
};
 
int main()
{
    example ex(42);
    return 0;
}