如果地图的键是指向字符串的指针，是否可以使用unordered :: map :: find？

Question

I have this map:

using namespace std; // just for readability

unordered_map<shared_ptr<string>, whatever_type> map;

Then a string comes from the outside. Can I use find somehow?

map.find(someInputString); // won't work, keys are wrapped by pointers

Is using a loop the only option?

PS I am interested if there is some special signature of the find method, or another method to do this, to avoid a loop.

Answer 1

I was able to make it work with regular map. Here is the code:

#include <map>
#include <string>
#include <memory>

struct PtrCompare {
    using is_transparent = bool;
    auto operator() (const std::shared_ptr<std::string>& lhs, const std::shared_ptr<std::string>& rhs) const {
        return *lhs < *rhs;
    }
    auto operator() (const std::shared_ptr<std::string>& lhs, const std::string& rhs) const {
        return *lhs < rhs;
    }
    auto operator() (const std::string& lhs, const std::shared_ptr<std::string>& rhs) const {
        return lhs < *rhs;
    }
};

std::map<std::shared_ptr<std::string>, int, PtrCompare>  my_hash;

auto check(const std::string& x) {
    return my_hash.find(x);
}

It uses the feature of transparent comparators, which is available on C++14. However, transparent comparators for unordered maps are only available with C++20, and thus, my solution doesn't work with unordered maps.

The way to make it work with non-transparent comparator for unordered maps would be to still use custom hasher and comparator, which would compare arguments after dereferencing them, but create a shared pointer from a string you want to compare with, and find using this shared pointer. To me this solution is super ugly, so I am not suggesting this.