删除向量中指针的方法 [C++]

Question

std::vector<Circle*> circles

for(int i=0; i<circles.size(); i++)    // 1
    delete circles[i];

for(auto & circle : circles)           // 2
    delete circle;

for(Circle * circle : circles)         // 3
    delete circle;

 for(Circle *& circle : circles){      // 4
    delete circle;

If I write it the first way, CLion IDE suggests me to use the second way so I guess they're the same. I'm not really sure why there's a reference next to auto and if any of methods 3 or 4 are correct as well? I would guess 4 is also same as 1 and 2.

Answer 1

For completeness, there's one more way:

for(auto circle : circles)
    delete circle;

Now, all these ways in this example are equivalent and will do the job of deleting circles elements equally well.

Having a reference to a pointer doesn't matter (the reference is "unwrapped" in expressions - in technical terms an lvalue-to-rvalue conversion), in all cases the actual pointer value is passed to delete .

The IDE seems to be suggesting the most idiomatic way of writing a range-based for loop:

for(auto & elem : collection)

This is usually the safest form; this way if collection contains large objects, they will be iterated by reference, without unnecessary copying. It doesn't matter in our case as a trivial type like a pointer can be easily copied.

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PS As others have noted, in modern C++ we try to avoid new and delete and use value semantics and smart pointers instead. So something simple like std::vector<Circle> circles could be all that's needed, or if pointers are required, std::vector<std::unique_ptr<Circle>> circles . No manual cleanup necessary in both cases.