C++ associative containers - why doesn't the standard defines methods to exchange and replaces keys?

I need to replace specific key values, while the rest of the value_type is left untouched. What I actually need to do, is copy the value, erase the entry and insert it with changed key value again. This is absolutely bad. I need to copy the whole value_type twice, and deallocate/allocate again.

Why the sta开发者_如何学运维ndard doesn't define methods like this:

// returns count of exchanged keys
size_type exchange_key(key_type const& x, key_type const& y);
// returns count of replaced keys
size_type replace_key(key_type const& old_key, key_type const& new_key);

Is there anything I'm missing?

I don't why it was not added in the first place, and i understand that it is too bad. I guess they just added what they felt was absolutely necessary.

I think i have read somewhere that Boost.MultiIndex provided this ability.

Associative containers are implemented in a way that does not allow to change the 'key' in an efficient manner. To make this explicit it does not provide convienence methods to replace a key. The associative container would also have to remove and insert again under the covers.

I think this is an abstraction problem. The standard doesn't say exactly how the containers are to be implemented, it only specifies the maximum complexity of some of the operations and leaves the rest to the implementation.

If the standard were to add a replace_key function, it would also have to specify that this should have a different complexity than the existing erase-insert combination. How can it do that without leaking implementation details? If the function isn't guaranteed to be faster on all implementations, it is pretty useless.

When you say that it would obviously be faster, you make assumptions about implementation details that the standard tries to avoid.

Now, you can, with .extract(key) (since C++17). https://en.cppreference.com/w/cpp/container/map/extract

This is because changing a key could affect the structure of an associative containers. Notably, std::map, which is a typically Red-Black tree, the tree structure mostly will be changed once you modify a key (e.g., rotating sub trees). In some data structures, even such dynamic changes are disallowed. So, it is challenging to expose such operation as a standard in an associative container.

Regarding the overhead you concerned, once you have value_type as a pointer or reference, the overhead of deleting/inserting a pair isn't too bad.

Well, honestly behind the screens it would result into an insert and delete operation anyhow, with the sole difference that the value-part will not be copied. While this seems to be your biggest concern, unless your object is very heavy on copying, in a large container, the update operation to re-stabilize the ordered container will be heavier anyhow.

Now this would require some important changes however going further than the associative containers, the two most important I can see are:

1. The std::pair class needs an update, because you must be able to update the key without creating a new pair (as this would also copy the value object).
2. You need an update function that removes a pair from the tree, calls the new logic from 1., and reinserts it.

I think the main problem lies with the first one, as std::pair is at the moment actually a very simple wrapper, and your proposal would delete that principle and add some (unnecessary) complexity to it. Also note that call 2 does not actually add new functionality but wraps a system the developer could manage himself easily through references and the like. If they would add all this wrapping to std, it would become a really huge bloated piece of library.

If you want this principle, you should look for more complex libraries (probably boost has some). Or you should simply use a reference/shared_ptr as your value_type.