vector returning empty c++

Question

I have a problem where some vector returns are empty. Here is the basic setup:

//Foo.h
struct Foo{
    int n;
    double vals[2];
};

//MyClass.h
class MyClass{
private:
    std::vector<Foo> classVector;
public:
    std::vector<Foo> getPair(int i1, int i2);
};

//MyClass.cpp
std::vector<Foo> MyClass::getPair(int i1, int i2)
{
    std::vector<Foo> toReturn(2);
    toReturn[0] = classVector[i1 - 1];
    toReturn[1] = classVector[i2 - 1];
    return toReturn;
}

classVector is a vector that is already initialized and has data in it. I'm basically just trying to obtain two values from that vector and return them using this function. However, when I look at toReturn in debug mode once return is called, the size is reset to zero and the elements that had been copied over (I checked before the return call) are no longer in the vector.

I have a feeling it has to do with a shallow copy being passed on return. I tried creating my own copy constructor for Foo, but I deleted it when it didn't solve the problem.

`return std::vector(classVector.begin(), classVector.begin() + 2);` — Borgleader, Dec 03 '14 at 21:39
I'll try make_pair. The other suggestion won't work because the elements aren't necessarily in sequence. — ewpcasey, Dec 03 '14 at 21:41
But what's your problem ? You return by value, so the local vector toReturn will be destroyed when you return, and its content will have been copied ! — Christophe, Dec 03 '14 at 21:47
As always post mcve, your problem is that you are misinterpreting what you see in debuger, rather than shallow copy, copy ctor etc. — Slava, Dec 03 '14 at 21:52
@Slava Are there any better C++ IDEs than VS? Their error messages are really unhelpful. — ewpcasey, Dec 03 '14 at 22:00

score 1 · Accepted Answer · edited May 23 '17 at 11:50

The compiler will optimize your code so that the vector toReturn gets moved to the caller's vector.

So if you have:

void someFunc(MyClass& my)
{
    vector<Foo> answer = my.getPair(3,4);
}

After return is called in getPair, the contents of 'toReturn' will now live in 'answer' and 'toReturn' will be empty. As the function is about to return, no further use of 'toReturn' is possible, so there is no need for it to keep its contents. It is more efficient for the 'answer' vector to just grab the guts of the 'toReturn' vector instead of creating new elements in 'answer' and copying each of them from 'toReturn'.

For an explanation of move semantics, see here: What are move semantics?

vector returning empty c++

1 Answers1