C ++中的向量和结构

Question

I am trying to make a data tree that holds multiple data types and vectors. What I have so far is shown below:

struct VertStruct{
    double X, Y, Z;
};
struct FaceStruct{
    int F1, F2, F3, F4;
};
struct FaceNormalStruct{
    double X, Y, Z;
};
struct LODStruct{
    std::vector<VertStruct> Verts;
    std::vector<FaceStruct> Faces;
    std::vector<FaceNormalStruct> FaceNormals;
};
struct ChunkStruct{
    std::vector<LODStruct> LOD;
};

int main(){

    std::vector<ChunkStruct> Chunk;
    Chunk.resize(100);

    for(int i = 0; i < 100; i++)
    {
        Chunk[i].LOD.resize(5);

        for(int j = 0; j < 5; j++)
        {
            Chunk[i].LOD[j].Verts.resize(36);
            Chunk[i].LOD[j].Faces.resize(25);
            Chunk[i].LOD[j].FaceNormals.resize(25);
        }
    }
return 1;
}

Now this compiles fine and is exactly what I want, however, if I try to set a value to something like:

int Number = 42;
Chunk[5].LOD[4].Verts[3] = Number;

Then I get the following error:

 main.cpp|126|error: no match for 'operator=' in 'Chunk.std::vector<_Tp, _Alloc>::operator[] [with _Tp = ChunkStruct, _Alloc = std::allocator<ChunkStruct>](5u)->ChunkStruct::LOD.std::vector<_Tp, _Alloc>::operator[] [with _Tp = LODStruct, _Alloc = std::allocator<LODStruct>](4u)->LODStruct::Verts.std::vector<_Tp, _Alloc>::operator[] [with _Tp = VertStruct, _Alloc = std::allocator<VertStruct>](3u) = Number'|

So am I missing something or is what I am attempting to do not possible?

Answer 1

Verts[3] is of type VertStruct and Number is an int so the assignment is not possible (with the posted code). You could specify one of the members of VertStruct as the target of the assignment:

Chunk[5].LOD[4].Verts[3].X = Number;

If you wanted to be able to assign an int to a VertStruct you provide an operator=(int) (as mentioned already by Luchian) but it seems to me it would be quite ambiguous at the call site to what member(s) the int value is being assigned to:

struct VertStruct
{
    double X, Y, Z;
    VertStruct& operator=(int d)
    {
        X = d; // Or 'Y' or 'Z' or all.
        return *this;
    }
};

Answer 2

This

Chunk[5].LOD[4].Verts[3]

is a VertStruct , and you cannot assign an integer to it.

Answer 3

You are missing something, and it's not impossible (everything's possible if you can imagine it. That's what being a scientist is all about) :)

Chunk[5] returns a ChunkStruct .

Chunk[5].LOD returns a std::vector<LODStruct> .

Chunk[5].LOD[4] returns a LODStruct .

Chunk[5].LOD[4].Verts returns a std::vector<VertStruct> .

Finally, Chunk[5].LOD[4].Verts[3] returns a VertStruct . And you try to assign an int to it. Obviously, not gonna work with your code.

To get this to work (this exact syntax), you should overload operator = (int x) . Otherwise, assign the int to whichever member you want.

Answer 4

You are assigning one number which is an int into VertStruct , which is a struct and has 3 fields (X, Y, Z). If you wanted to set X coordinate, do:

Chunk[5].LOD[5].Verts[3].X = Number;