Which C 20 standard library containers have a .at function? For example, at least std::map, std::unordered_map and std::vector do. What others are there?

Is there some way to work this out without going through the 2000 page standard by hand?

CodePudding user response：

From comments, it seems you want something like:

template <typename Container, typename T>
typename Container::pointer my_at(Container&, const T&)
requires (requires(Container c, const T& key) { c.at(key); })
{
   // ...
}

Demo

CodePudding user response：

I don't think a pointer is the appropriate type to return for this operation, instead it should be an iterator. At which point the definition splits into two cases:

Random access containers:

iterator at_(size_type index) noexcept { return index < size() ? begin()   index : end(); }
const_iterator at_(size_type index) const noexcept { return index < size() ? begin()   index : end(); }

map and unordered_map:

template<typename T>
iterator at_(T && key) noexcept { return find(std::forward<T>(key)); }
template<typename T>
const_iterator at_(T && key) const noexcept { return find(std::forward<T>(key)); }

And then rather than testing against nullptr, you test against the container's end().

It's also a reasonable question as to whether (unordered_)map needs an alias for an existing member to match your nomenclature

CodePudding user response：

I ended up grepping libstdc include directory for \bat( and that has given me:

std::basic_string
std::basic_string_view
std::array
std::vector
std::vector<bool>
std::unordered_map
std::map
std::dequeue

I'm not sure if this is exhaustive.

There is also rope and vstring but I don't think these are standard.