How can I pass a function by reference for a report callback in C++/CLI?

Question

I have some code which handles data files and reports an error when it runs into trouble, but I'm having trouble working out how to give my class a callback function. Here's a quick example of the sort of thing I'm trying to achieve:

public delegate void Reporter( System::String^ stringToReport );

/// <summary>
/// Simple file handler with callback options on error
/// </summary>
public ref class MyFileHandler
{
private:
    Reporter^ m_reporter;

    void ReportError(String^ error)
    {
        if( m_reporter )
        {
            m_reporter( error );
        }
    }

public:
    MyFileHandler()
    {

    }

    void SetErrorReporter( Reporter^ reporter )
    {
        m_reporter = reporter;
    }

    bool FailingOperation()
    {
        return false;
    }

    bool GetData()
    {
        bool succeeded = false;

        // Do some operation that fails
        succeeded = FailingOperation();

        if( succeeded == false )
        {
            ReportError( "Gah, something bad happened!" );
        }

    }
};


public ref class MyFileLoader
{
private:
    MyFileHandler m_mfh;

    void ErrorHandler(String^ errorMsg)
    {
        System::Windows::Forms::MessageBox::Show( errorMsg );
    }

public:
    MyFileLoader()
    {
        m_mfh.SetErrorReporter( &CallbackFunctionTests::MyFileLoader::ErrorHandler );
    }
};

...which has a problem:

1>CallbackTest.h(131) : error C3374: can't take address of 'CallbackFunctionTests::MyFileLoader::ErrorHandler' unless creating delegate instance

So I get the impression I'm mis-using or misunderstanding things. Maybe there's a better way of achieving this?

Answer 1

You need to create the delegate explicitly – and you need to pass the associated instance – in your case, this – to it:

m_mfh.SetErrorReporter(
    gcnew Reporter(this, &CallbackFunctionTests::MyFileLoader::ErrorHandler));

Answer 2

In case anyone's interested, here's the final (working) code:

public delegate void Reporter( System::String^ stringToReport );

/// <summary>
/// Simple file handler with callback options on error
/// </summary>
public ref class MyFileHandler
{
private:
    Reporter^ m_reporter;

    void ReportError(String^ error)
    {
        if( m_reporter )
        {
            m_reporter( error );
        }
    }

public:
    MyFileHandler()
    {

    }

    void SetErrorReporter( Reporter^ reporter )
    {
        m_reporter = reporter;
    }

    bool FailingOperation()
    {
        return false;
    }

    bool GetData()
    {
        bool succeeded = false;

        // Do some operation that fails
        succeeded = FailingOperation();

        if( succeeded == false )
        {
            ReportError( "Gah, something bad happened!" );
        }

        return succeeded;
    }
};


public ref class MyFileLoader
{
private:
    MyFileHandler m_mfh;

    void ErrorHandler(String^ errorMsg)
    {
        System::Windows::Forms::MessageBox::Show( errorMsg );
    }

public:
    MyFileLoader()
    {
        m_mfh.SetErrorReporter( gcnew Reporter(this, &CallbackFunctionTests::MyFileLoader::ErrorHandler) );
    }
};