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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?

Flash
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3 Answers3

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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.

Community
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Benjamin Lindley
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  • Thanks. In my case `MyClass` will contain some very large members data matrices as members. If things get copied, does that mean constructing `MyClass` via `MyClass(large_matrix)` is also a problem? When dealing with large objects what is the standard practice? Sorry, new to C++. – Flash Jul 09 '16 at 06:32
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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;
for_stack
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In this situation, objects.push_back(obj); calls the T const& version of std::vector::push_back which will copy the parameter.

Tim
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