FFI: How to bind to a cpp std::vector which contains another std::vector in nim?

[bertikrueger]

Hello everyone.

I am trying to write bindings for a C++ class which contains, among other 
stuff, a C++ std::vector that stores another std::vector inside. I want to 
access this std::vector from nim. I wrote the corresponding type declarations 
in nim. The nim compiler compiles everything without complaining but the cpp 
compiler from the cpp backend throws the error message: **error: ... was not 
declared in this scope**. I am using the CppVector from the nim-cppstl bindings 
to the C++ STL (`https://github.com/Clonkk/nim-cppstl/tree/master`).

I made a stripped down dummy example:
    
    
    {.emit: """
    
    #include <vector>
    
    struct IntVector {
            std::vector<int> numbers;
    };
    
    struct VectorFiller {
            // fields
            std::vector<IntVector> vectors;  // a vector of IntVectors (nested 
vector)
            
            // constructor
            VectorFiller() { }
            
            // getters
            std::vector<IntVector> getVectors() {    // get vector of vectors
                return this->vectors;
            }
            
            IntVector getFirstVector() {        // get first vector only
                return this->vectors[0];
            }
            
            // methods
            void fillVectors(int numVectors, int maxSize) { // fill vectors 
with garbage
                for(int i = 0; i < numVectors; i++)
                {
                    IntVector tmpVector;
                    
                    for(int j = 0; j < maxSize; j++)
                        tmpVector.numbers.push_back(i + j);
                    
                    this->vectors.push_back(tmpVector);
                }
            }
    };
    
    """.}
    
    import cppstl/std_vector
    
    
    # nim type declarations
    type
        IntVector* {.importcpp: "IntVector".} = object
            numbers*: CppVector[cint]
    
    type
        VectorFiller* {.importcpp: "VectorFiller".} = object
            vectors*: CppVector[IntVector]
    
    
    # constructor
    proc newVectorFiller*(): VectorFiller {.constructor, importcpp: 
"VectorFiller()", constructor.}
    
    # methods
    proc getVectors*(vectorFiller: VectorFiller): CppVector[IntVector] 
{.importcpp: "#.getVectors()".}
    proc getFirstVector*(vectorFiller: VectorFiller): IntVector {.importcpp: 
"#.getFirstVector()".}
    proc fillVectors*(vectorFiller: var VectorFiller; numVectors: cint; 
maxSize: cint) {.importcpp: "#.fillVectors(@)".}
    
    
    # main procedure
    proc main() =
        # create a new vectorFiller
        var vectorFiller: VectorFiller = newVectorFiller()
        
        # fill vector with more or less random garbage (for demo only)
        vectorFiller.fillVectors(cint(3), cint(4))
        
        # getting a single (non nested) vector works like a charm
        let firstVector: IntVector = vectorFiller.getFirstVector()
        echo "firstVector no. of elements: ", firstVector.numbers.len()
        echo "firstVector.numbers[2] = ", firstVector.numbers[2]
        
        
        # let's try using the vectorFillers object attributes directly
        # -> does not work -> cpp compiler error:
        # error: 'IntVector' was not declared in this scope
        let vectors: CppVector[IntVector] = vectorFiller.vectors
        
        
        # let's try the getters
        # again does not work, same error:
        # error: 'IntVector' was not declared in this scope
        let vectors_via_getter: CppVector[IntVector] = vectorFiller.getVectors()
    
    
    # start main proc
    main()
    
    Run

What i am doing wrong here?

Thank you very much in advance.