C++/CLI from typedef std::function to managed delegate

Question

I have a big class from extern library and I need to use a unmanaged callback in managed code. Unmanaged code simplified:

typedef std::function<void(const std::string &, Float)> ProgressCallback;
class MeshGenerator{
public:   
ProgressCallback progress;
/// <summary>
/// set callback
/// </summary>
/// <param name="_callback"></param>
inline void SetCallback(ProgressCallback _callback){ this-> progress =_callback; }   
};

Now my manage Code simplified:

public delegate void CallbackDelegate(String^ cap, float data);
public ref class MeshWrapper
{

private:
MeshGenerator *gen;
public:
[MarshalAsAttribute(UnmanagedType::FunctionPtr)]
CallbackDelegate^ _Delegate;
inline MeshWrapper(){ 
  this->gen = new MeshGenerator();
  _Delegate = gcnew CallbackDelegate(this, &MeshWrapper::Progress);
  auto ptr = Marshal::GetFunctionPointerForDelegate(_Delegate).ToPointer();
  ProgressCallback *p = static_cast<ProgressCallback*>(ptr);
  this->gen->SetCallback(*p);
}   
inline void Progress(String^ cap, float s){ System::Console::WriteLine(cap + s);}
};

this code compile, but in MeshWrapper constructor the static cast ("static_cast<ProgressCallback*>(ptr);") give me an empty delegate.

I'm probably making some conceptual mistakes. Thanks for your help.

callback to inline ? ... also std::string is not compatible with dotnet string — Selvin
– Selvin, Commented May 10, 2024 at 9:10
@Selvin thanks for reply, but i cannot modify unmanaged class... i need to find the way to use ProgressCallback in managed code.. — The Overrider
– The Overrider, Commented May 10, 2024 at 9:13
It was some time since I coded C++, but you should be able to create a unmanaged helper class, that has a gcroot to your meshWrapper. That way you should not need to rely on any automated conversions. — JonasH
– JonasH, Commented May 10, 2024 at 10:01

Botje · Accepted Answer · 2024-05-10 09:28:15Z

1

You cannot simply cast a function pointer to a std::function pointer, you need to construct a std::function object:

auto pc = ProgressCallback(reinterpret_cast<void(*)(const std::string &, Float)>(ptr));
this->gen->SetCallback(pc);

Alternatively, pass a lambda that performs the cast in-line and maybe fixes up some of the arguments:

this->gen->SetCallback([=](const std::string & s, Float f) {
  auto fn = reinterpret_cast<void(*)(const std::string &, float)>(ptr);
  fn(msclr::interop::marshal_as<String^>(s), f);
});

edited May 10, 2024 at 9:28

answered May 10, 2024 at 9:21

Botje

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The Overrider Over a year ago

thank you, I'm close to solve the problem, the first method work, but when my Managed Progress method is called the string^ cap has wrong value probably because I need conversion from std::string to String^... the second doesn't compile with this message: "C++ local lambda not allowed in a member function of a managed class" ...

The Overrider · Accepted Answer · 2024-05-13 07:22:59Z

0

Thanks to @Botje for help, now I solved my problem like this:

public delegate void CallbackDelegate(std::string& cap, float data);

I changed my Callback delegate in managed Code with std::string&.

The reinterpret_cast is the same of @Botje.

In managed, Progress method changes like this:

void MeshWrapper::Progress(std::string&  cap, float s) {
 // Transform std string in String^!!!!!!
 String^ newSystemString = gcnew String(cap.c_str());   
 // Code that uses newSystemString here :)  
};

Thank you.

answered May 13, 2024 at 7:22

The Overrider

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