Question

In an attempt to wrap some unmanaged code in a managed .dll I’m trying to convert a Generic::List of data points into a std::vector. Here’s a snippet of what I’m trying to do:

namespace ManagedDLL
{
    public ref class CppClass
    {
        void ListToStdVec( const List<double>& input_list, std::vector<double>& output_vector )
        {
            // Copy the contents of the input list into the vector
            // ...
        }

        void ProcessData( List<double> sampleData )
        {
            std::vector<double> myVec;

            ListToStdVec( sampleData, myVec );

            // Now call the unmanaged code with the new vector
            // ...
        }
    }
}

Compiling this gives me:

error C3699: ‘&’ : cannot use this indirection on type ‘const System::Collections::Generic::List’

I’ve probably missed something fundamental here (I’m relatively new to .net’s way of doing things), but that looks like reasonably valid code to me.. ?

[Edit] I’ve tried both Andy and Dario’s suggestions and they work, but how do I then access the members of the input list? I’ve tried all sorts of combinations of dreferencing and nothing seems to compile:

void ListToStdVec( const List<double>% input_list, std::vector<double>& output_vector )
{
    int num_of_elements = input_list->Count;
}

void ListToStdVec( const List<double>^ input_list, std::vector<double>& output_vector )
{
    int num_of_elements = input_list.Count;
}

…both give me:

error C2662: ‘System::Collections::Generic::List::Count::get’ : cannot convert ‘this’ pointer from ‘const System::Collections::Generic::List’ to ‘System::Collections::Generic::List %’

…so how do you access the reference / pointer?

Answer 1

As List<T> is a managed .NET class, it’s passed by managed GC-Handle denoted by ^ and not by C++-reference.

Ex:

void ListToVec(List<double>^ input_list, std::vector<double>& out)

You don’t need additional const here. The notation List<T>^% creates a tracking reference (comparable to C++-pointers) rather than a call by reference.
Just access the members by list->... and list[...].