CODEWITHSUNDEEP
c++c++11
Flash
Flash

Reputation: 16703

Returning a vector of objects by value

I am wondering if the following is a safe way of returning a deep vector of objects.

class MyClass {
    //...
}

std::vector<MyClass> get_list(a,b,c) {
    // obj is created on the stack
    MyClass obj(a,b,c);
    std::vector<MyClass> objects();
    objects.push_back(obj);
    // objects[0] contains a pointer to a stack variable?
    return objects
}

In particular, how does the returned vector not contain a reference to a stack memory location?

Upvotes: 0

Views: 239

Answers (3)

Benjamin Lindley
Benjamin Lindley

Reputation: 103693

Whether it's safe or not depends upon MyClass. But assuming it's a well behaved class with value semantics, then yes, it's perfectly safe. std::vector stores its objects by value. When you do this:

objects.push_back(obj);

A copy of obj is put into the vector (assuming you actually had a vector, which you don't). And when you return the vector it is moved (though this move can be and usually is optimized away) out to the calling function.

Upvotes: 3

for_stack
for_stack

Reputation: 22886

NO.

objects.push_back(obj);

The local variable, i.e. obj, is COPIED into the vector. So the vector won't contain a reference to a local variable.

By the way, you CANNOT define a vector like this: 

std::vector<MyClass> objects();

In fact, this is a function declaration: a function named objects, which has no parameter, and returns a std::vector.

Instead, you should do it like this:

std::vector<MyClass> objects;

Upvotes: 2

Tim
Tim

Reputation: 1527

In this situation, objects.push_back(obj); calls the T const& version of std::vector::push_back which will copy the parameter.

Upvotes: 0

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