Question

I have some trouble understanding the output from the following code:

int main(void)
{
    vector<TreeNode> tree;
    for(int i = 0; i < 50; i++)
    {
        TreeNode node;
        tree.push_back(node);
    }

    for(int i = 0; i < 50; i++)
    {
        Image image;
        image.Path = "TestPath";
        image.Votes = 0;

        TreeNode node = tree.at(i);
        node.images.push_back(image);
        node.images.push_back(image);
    }

    TreeNode node = tree.at(0);
    cout << "TEST" << endl;
    cout << node.images.size() << endl;
}

Output:

TEST
0

Why is the output 0 and what should I do to fix it? I would expect the size to be 2 and not 0.
It has probably something to do with how structs work. My two defined structs is as follow:

typedef struct
{
    string Path;
    int Votes;
} Image;

typedef struct
{
    int treeIndex;
    float centroid;
    vector<float*> features;
    vector<Image> images;
} TreeNode;

Note that it is not an option for me to use classes instead of structs.

Answer 1

vector::at returns a reference to an object.

Your line

TreeNode node = tree.at(i);

is creating a copy of the TreeNode in node, so when you add images to it, it’s adding them to a new object that’s not in your vector.

If you instead use

TreeNode& node = tree.at(i);

It will add the images correctly.