Items are not adding to vector c++

Question

 class Way {
private:
    std::vector<Node> nodesCollection; 
public:
    Way();
    Way(const Way& orig);
    virtual ~Way();

    void SetNodesCollection(std::vector<Node> nodesCollection);
    std::vector<Node> GetNodesCollection() const;
};

I added vector as a property and I'm accessing that vector from following class and adding items.

Way wayNode;

 for (; WayIter!=wayNodes.end(); ++WayIter)
{
    const JSONNode& arrayNode = *WayIter;
    if (arrayNode.find("ref") != arrayNode.end() )
    {
        std::string id = arrayNode.find("ref")->as_string();

        if(nodesMap.find(id) != nodesMap.end())
        {
            wayNode.GetNodesCollection().push_back(nodesMap.find(id)->second);
        }
    }
}

but items are not adding to "nodesCollection". its count is zero. that adding object is not null.

I need some help to sort out this. I'm new to c++.

Thanks in advance.

what is wayNodes? It has appeared mysteriously in your code snippet... — 4pie0, Mar 05 '14 at 16:11
its a JSONNode , that "nodesMap.find(id)->second" is not null. — JanithOCoder, Mar 05 '14 at 16:14
Node or rather some container of Nodes? Please pay more attention to what you show and what you say. Note: wayNodes. s. — 4pie0, Mar 05 '14 at 16:15

score 1 · Accepted Answer · edited May 23 '17 at 11:49

1

You need to read about pass by value vs pass by reference. GetNodesCollection is returning a brand new copy of the vector. Then you add your nodes to this new copy, instead of to the original.

Well, maybe it is more return by value vs return by reference. Anyway the point is that you are getting a brand new copy instead of a reference to the original vector.

One way to fix would be to change the return type to a reference (you'd have to make the same change where GetNodesCollection is defined)

std::vector<Node>& GetNodesCollection();

edited May 23 '17 at 11:49

Community

1
1

answered Mar 05 '14 at 16:06

Graham Griffiths

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the method is declared as part of class Way in the first listing? – Graham Griffiths Mar 05 '14 at 16:16
method implementation std::vector& Way::GetNodesCollection() const { return nodesCollection; } but its getting an error – JanithOCoder Mar 05 '14 at 16:25
yes, so when you push_back now it will actually write to the vector within your Way object, so it shoudl not stay empty – Graham Griffiths Mar 05 '14 at 16:28
ah - you will have to remove the 'const' from the implementation (since you want to modify a member of the object) – Graham Griffiths Mar 05 '14 at 16:30

Items are not adding to vector c++

1 Answers1