adjust associative containers to support heterogeneous lookup

[DHowett-MSFT]

Heterogeneous lookup has been supported since C++14. To quote the linked article,

C++14 allows the lookup to be done via an arbitrary type, so long as the comparison operator can compare that type with the actual key type. This would allow a map from std::string to some value to compare against a const char* or any other type for which an operator< overload is available. It is also useful for indexing composite objects in a std::set by the value of a single member without forcing the user of find to create a dummy object (for example creating an entire struct Person to find a person by name).

Now, it's magic in that you enable it by specifying std::less<> as the third template argument to an associative container; that is:

std::map<std::string, std::string             > map1;
std::map<std::string, std::string, std::less<>> map2;

map2 supports heterogeneous lookup, whereas map1 does not.

[fcharlie]

If you want to use a hash map that supports heterogeneous lookups, there are references below:

• absl::flat_hash_map
• parallel-hashmap

[dlong11]

Are you thinking something like this for stirng keys?

struct StringComparison
{
using is_transparent = void;
bool operator()(std::string_view a, std:string_view b) const
{
return a < b;
}
...
};

std::map<std::string, std::string,, StringComparison> m = <map....>`

[fcharlie]

@DHowett-MSFT Bartek has written an article that may be helpful to you:
Heterogeneous Lookup in Ordered Containers, C++14 Feature