Returning member vectors.

[shadowayex]

I have a couple classes that have member vectors, and I want to return these vectors (or a copy of them, I only need them for reading purposes) with a method call. I would prefer returning a copy, rather than the actual one to ensure the data doesn't get changed. How would I do this?

[jsmith]

You can return it by value or by const reference. Either achieves what you want, and the implementation is the same.

struct Foo {
    // Provide one or the other, not both.
    std::vector<int> get() const
        { return vec; }

    const std::vector<int>& get() const
         { return vec; }

  private:
    std::vector<int>  vec;
};

[Mathes]

return a copy, no handle!

look at this program:

#include <iostream>
#include <vector>

using namespace std;

class myClass
{
    vector<int> m_vector;
    
public:
    myClass()
    {
        for (size_t i = 0; i < 10; i++) {
            this->m_vector.push_back(i);
        }
    }

    const vector<int>& getVector() const
    {
        return this->m_vector;
    }
};

const myClass generate()
{
    myClass obj;
    return obj;
}

int main()
{
    myClass obj;
    const vector<int> *x = &obj.getVector();

    size_t i = 0;
    cout << "That is what the vector looks like (*x): ";
    for (; i < 10; i++) {
        cout << x->at(i) << " ";
    }
    cout << endl;

    const vector<int> *v = &(generate().getVector());

    cout << "The vector from *v: ";
    for (i = 0; i < 10; i++) {
        cout << v->at(i) << " ";
    }
    cout << endl;

    vector<int> *y = const_cast< vector<int>* >(x);
    y->at(0) = 1000;

    cout << "The vector from *x again: ";
    for (i = 0; i < 10; i++) {
        cout << x->at(i) << " ";
    }
    cout << endl;

    return 0;
}

that is the output

That is what the vector looks like (*x): 0 1 2 3 4 5 6 7 8 9 
The vector from *v: 0 0 2 3 4 5 6 7 8 9 
The vector from *x again: 1000 1 2 3 4 5 6 7 8 9 

what happend here?
@v:
we created a temporary object and get a pointer to its vector, when the object is destroyed, it is not guranteed that the reference to the vector is valid after that!
or think of a destructor of myClass that calls this->m_vector.clear(); and then your pointer points to an empty vector.

@y & x:
its possible to remove the const! so you can edit the internal vector, even though you returned a const reference.

sure, these problems maybe never occur in your program. but this is why you should not return a handle to members of a class.
always return a copy

[jsmith]

@Mathes:

Note that subverting C++'s type system through const_cast is undefined behavior according to the standard. You should already be suspicious of line 50, before even trying to compile the program.