Calling a C callback function from C++ via a lambda

Question

I have this code in a plain C static library:

extern "C" {
    typedef void (__cdecl* VisitChildren)(Option*);
    void __cdecl DoVisitChildren(Children* List, VisitChildren Visitor);
}

And I'm trying to use it from some C++ code (unit tests) using a lambda.

...
DoVisitChildren(children, [&] (Option* option) {
...
});

I'm getting the compiler error C2664 ... cannot convert parameter 2 from 'unittests::UnitTest1::TestBuild::<lambda_b286d160b9bab3e09ab93cd59fc49f0b>' to 'VisitChildren'

If I remove the capture '&' it compiles and works, but I need to capture some bits and bobs.

Is this possible?

Answer 1

A closure created by a lambda expression can be implicitly converted to a function pointer, but only if it does not capture any variables. Also, it will be converted to a pointer to an extern "C++" function, not an extern "C" function, and technically those are incompatible types.

So no, you can't do this.

A hacky workaround is to store your actual closure in a global variable and pass a callback which invokes it. This will only work if your program is single-threaded and the DoVisitChildren call does not store the callback for later use.

std::function<void(Option*)> callback;
extern "C" void __cdecl invoke_callback(Option* o) { callback(o); }

// ...

callback = [&] (Option* option) { /* ... */ };
DoVisitChildren(children, invoke_callback);