Method returning container for use in range-based for loop

Question

I have a class that contains some standard container that I want to return in a method, like so (just an example):

class IntArray
{
public:
    IntArray(const vector<int>& vals) : vals(vals) {}

    const vector<int>& getValues() const { return vals; }
    vector<int>& getValues() { return vals; }

private:
    vector<int> vals;
};

I returned the vector by reference to avoid making a copy of it (I would rather not want to rely on RVO). I don't want to do it using OutputIterators, because I really want to keep it short with C++11 range-based for loops like so:

for (int val : arr.getValues()) {
    // Something
}

But say I want to change the type of the member variable to list<int>, then I would have to change the method's return type, too, which might lead to incompatibilities. I also don't want to implement begin() and end() methods because there might be more than one such container per class.

What would be the preferred way of doing this?

All classes supported by std::begin and std::end, or which have a begin and end member functions, will be supported by the ranged-based for loop. Of course this includes all standard containers. — Some programmer dude
– Some programmer dude, Commented Aug 18, 2014 at 5:24
Why are you using this wrapper to begin with? Why not just return a sequence<T> where sequence<T> wraps the vector<> or list<> and provides the .begin() and .end() methods? — Michael Aaron Safyan
– Michael Aaron Safyan, Commented Aug 18, 2014 at 5:24
@Michael: IntArray is just an example here. In reality, I'm using things like meshes that might contain both submeshes and textures, for example. And introducing a class that just wraps these two containers seems a bit heavy to me. — Alemarius Nexus
– Alemarius Nexus, Commented Aug 18, 2014 at 5:32

Steve Lorimer · Accepted Answer · 2014-08-18 05:26:31Z

3

You could create a typedef and refer to that in your function signatures:

class IntArray
{
public:
    typedef std::vector<int> Vec;

    IntArray(const Vec& vals) : vals(vals) {}

    const Vec& getValues() const { return vals; }
    Vec& getValues() { return vals; }

private:
    Vec vals;
};

Now if you want to change the storage type, you just change the typedef:

typedef std::list<int> Vec;

answered Aug 18, 2014 at 5:26

Steve Lorimer

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Comments

Tom · Accepted Answer · 2014-08-18 06:17:08Z

0

You can generalise this with your own template:

template<typename CONTAINER>
class IntArray
{
    typedef CONTAINER<int> Vec;
public:
    IntArray(const Vec& vals) : vals(vals) {}
    const Vec& getValues() const { return vals; }
    Vec& getValues() { return vals; }

private:
    Vec vals;
};

Now you can specify the container type at will:

std::list<int> vals { 0, 1, 2, 3, 4 };
IntArray<std::list> arr(vals);
for(int val: arr) {
    // Something
}

answered Aug 18, 2014 at 6:17

Tom

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