Clever Geek Handbook

Argument passing through auto in range based on

vector<MyClass> objects;
for (auto obj : objects)
{}

Will there obj

be a copy of every object or link?

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3 answers


Yes, you will receive a copy for each item you will not be with auto&

or auto&&

. This one is better understood with an example

struct MyClass {
    MyClass() {}
    MyClass(const MyClass&) {
        cout << "copy ctor invoked" << endl;
    }
};

int main(){
    vector<MyClass> objects;
    MyClass obj1;
    objects.push_back(obj1); // Vector filling
    cout << "------" << endl;
    for (auto obj : objects) // Auto loop
    {}
    cout << "------" << endl;
    for (auto& obj : objects)
    {}
}


Output:

copy ctor invoked // Vector filling
------
copy ctor invoked // Auto loop
------
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As written, you will receive a copy for each item. To avoid getting a copy, use

for (auto&& obj: object) {
}


You can also use one of the other obvious options:

  • Still a derived type, but explicitly use the link below auto&

    .
  • Be clear, but use the following link const

    : auto const&

    .
  • In fact, you can also use the type: MyClass&

    or MyClass const&

    .
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Simple answer: your intuition is correct and you get a copy.

The reason for this lies in the type inferred auto

. Scott Myers recently spoke at CppCon 2014 about this topic: https://www.youtube.com/watch?v=wQxj20X-tIU

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