使用std :: pair的C ++ const正确性

Question

I am maintaining a container class with an interface similar to std::map / std::unordered_map .

The interface claims to store std::pair<const X,Y> (ie that's what value_type is). Internally, however, the implementation stores a sorted array of std::pair<X,Y> .

The current implementation uses reinterpret_cast to implement the iterators. My question is, is there a better alternative?

Moving to storing an array of std::pair<const X,Y> wouldn't be possible, as the implementation needs to copy elements around in the array to implement insertion and deletion. One of the ways it does this is using std::sort .

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Edit: Although I believe the reinterpret_cast invokes undefined behavior (or implementation defined?) I have yet to come across a compiler where this doesn't work - Am I worrying about nothing?

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Current implementation of iterator's dereference:

template <class K, class M>
std::pair<const K,M>& operator*() {
  std::pair<K,M>& result = ...;
  return *reinterpret_cast<std::pair<const K,M>*)(&result);
}

Answer 1

I believe you can't solve this by returning a std::pair . Instead, you're going to have to return a proxy object that looks like a standard pair, but if you update the second member it propagates through to the main container, while the first member is expose as const as you desire.

Answer 2

"Better alternative?" What is wrong with reinterpret_cast ? In this case, the cast is even well-defined since you're casting between objects with compatible (in fact, identical) representations.