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

bertikrueger (orginal) [2023-07-09T15:48:50+02:00] view original

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()

What i am doing wrong here?

Thank you very much in advance.

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10330 :: FFI: How to bind to a cpp std::vector which contains another std::vector in nim?