const std::map operator []

[Cleiton Santoia]

What about include a 'const operator []' in std::map that returns a std::optional instead a reference to T ?

std::optional< std::reference_wrapper< T > > operator[] (const Key& key) const { ... }

Maybe this is a case for committee consider std::optional for references

std::optional< T& > operator[] (const Key& key) const { ... } 

BR

Cleiton

[Nicol Bolas]

On Saturday, November 4, 2017 at 7:18:59 PM UTC-4, Cleiton Santoia wrote:

What about include a 'const operator []' in std::map that returns a std::optional instead a reference to T ?

std::optional< std::reference_wrapper< T > > operator[] (const Key& key) const { ... }

First, it would have to be a `const T`.

Second, ignoring the question of `optional<reference_wrapper<const T>>` vs. `const T*`, to have the `const` and non-`const` overloads of a function return such disparate types is not reasonable. They're supposed to be functions that do the same thing.

Yes, it's unfortunate that `operator[]` potentially modifies the object. It would perhaps have been better if it were more like `find` or something. But that ship has sailed. Creating an interface that behaves in such a different way from the other overload is not a good solution to this problem.

[Zach Laine]

On Tue, Feb 20, 2018 at 7:17 AM, <frederik...@gmail.com> wrote:

I've considered doing similar things a few times. I usually opt for throwing an exception if the key doesn't match an item however.

That's not a very exceptional circumstance, nor is it an error.  An optional is a much better convention is such code.

Zach

[olafv...@gmail.com]

Op zondag 5 november 2017 00:18:59 UTC+1 schreef Cleiton Santoia:

A wrapper returning second_type* is so much easier. See my find_ptr and friends.

const typename T::value_type::second_type* find_ptr(const T& c, const U& v)

https://github.com/OlafvdSpek/xbt/blob/master/misc/xbt/find_ptr.h