常量引用 std::vector

Question

How can I have a reference to a specific element in a std::vector?

The easy way should be to store it as the index of the element (I am using a size_t variable).

The problem I have is the possibility of inserting an element before the current, making the stored value incorrect.

Answer 1

There isn't a very good way to keep track of a single element inside the vector by a pointer, reference, or index. In particular:

1. If you insert a new element into the vector , it may cause all outstanding references to the vector 's elements to be invalidated if an internal reallocation occurs. Insertions can happen either by calling insert , reserve , push_back , or assign (plus perhaps a few others). This could cause the reference to refer to an invalid object, resulting in undefined behavior if the reference is used.

2. If you remove an element from the vector , then the reference may no longer point at the same element. Accessing the reference may result in you referring to the wrong object.

If you really must hold a reference to an element in a vector , one option would be to have the vector store (smart) pointers to objects instead of the objects themselves. That way, you can store a copy of that pointer elsewhere and no matter what happens in the vector , the pointer should still be valid. This is really the Fundamental Theorem of Software Engineering in practice - adding another layer of indirection can solve most problems.

Answer 2

The only reliable solution is to have your vector be a vector of pointers and just remember your pointer.

Otherwise you can't maintain a reference because, as you mentioned, the vector might move elements around.

Answer 3

Use a vector that contains auto pointers, ie std::vector<std::unique_ptr<YOUR_ELEMENT_TYPE>> if you are using c++0x.