如何围绕C风格的回调创建lambda-wrapper？

Question

I have such layers: C# main application -- CLI/C++ wrapper for C++ library -- C++ library.

CLI middle layer was written because of the limitations of C#-C++ interoperability. I used it also to polish some rough edges from C# point of view -- in other words, the middle layer allows to hide C/C++ types (like char* ) and do all necessary conversions.

One conversion I fail to make is passing a callback. I tried to do it as in Pass a c++ lambda to old c function pointer but I cannot pass a lambda as a callback.

C++ callback definition:

typedef void (*CppOnEngineCloseCallback)
    (void * customParam, const wchar_t * errorMessage);

I know customParam is not used, so I define such callback for middle layer (CLI/C++):

typedef void (*CLIOnEngineCloseCallback)
        ( String ^ errorMessage);

In the wrapper I create such lambda:

// lambda signature should resemble CppOnEngineCloseCallback
auto lambda = [&] (void * customParam, const wchar_t * errorMessage) {
    cli_callback(Marshal::PtrToStringUni( IntPtr(static_cast<void*>(
        const_cast<wchar_t*>(errorMessage)))));
};

My concern is if cli_callback will be properly kept in CLI environment (this is my first project) -- after all, I pass it in time A, and use it in time B.

Compiler has even stronger concern, because when I try to use the lambda:

InitEngine( // C++ function that requires callback
    &lambda);

it says it cannot convert lambda into CppOnEngineCloseCallback .

Is it doable in the first place? How?

Answer 1

You're using auto , hence you're hiding the actual type, and making it harder on yourself to manage the C++ strongly typed type safety.

Probably the lambda function isn't the right type, or at least not the type you're expecting.

CppOnEngineCloseCallback is probably a function pointer type (or std::funciton type).
So, to help yourself, try changing the lambda 's type to the expected type, and see what the compiler says.

CppOnEngineCloseCallback lambda = [&] (void * customParam, const wchar_t * errorMessage) {
    cli_callback(Marshal::PtrToStringUni( IntPtr(static_cast<void*>(
        const_cast<wchar_t*>(errorMessage)))));
};

The InitEngine() method expects a CppOnEngineCloseCallback , so you must make sure it gets it.

Besides that, you can only assign a lambda with empty brackets [] to a function pointer. In your case you're not using anything from the local scope, hence you don't really need [&] .

Lambda functions : operator ret(*)(params)()

This user-defined conversion function is only defined if the capture list of the lambda-expression is empty.

If you do want to use this kind of lambda expression, you'll have to use std::function instead of a function pointer:

typedef std::function<void(void *, const wchar_t *)> CppOnEngineCloseCallback;

Answer 2

In general, it is not possible, to cast a lambda to a function pointer. As you know, a lambda is an object of a compiler generated class, that only behaves like a function.

For your actual problem: you mentioned that customParam is not used.

A typical pattern is, that customParam is provided by the caller of the library and used in the callback function to identify some object handling the callback. Inside the callback, you then map to the function you actually want to call:

void MyNativeCallback(void * customParam, const wchar_t * errorMessage)
{
    static_cast<MyClass*>(customParam)->Callback(errorMessage);
}

So you could pass a pointer to your lambda as customParam to the library, and provide a helper function as above as callback. Anyways, getting the pointer types correctly will not be a simple task. I would prefer a small helper class instead.