Unordered_map with custom class as key

I am working on a compiler right now and I am trying to represent the scope class by inheriting from unordered_map, because it is essentially a hash table of symbols to declarations. I added a custom hash function to the symbol, but I am getting an error complaining there is no default constructor for initializing a std::pair. Here is the relevant code:

Symbol.hpp

#pragma once

#include <string>
#include <unordered_set>


class Symbol
{
    friend class Symbol_table;

    Symbol(std::string const* str) : m_str(str) { }
    /// Constructs the symbol from `str`.

public:
    Symbol() : m_str() { }

    std::string const& str() const { return *m_str; }
    /// Returns the spelling of the token.

    friend bool operator==(Symbol a, Symbol b) 
    {
        return a.m_str == b.m_str;
    }

    friend bool operator!=(Symbol a, Symbol b) 
    {
        return a.m_str != b.m_str;
    }

private:
    std::string const* m_str;  
};


class Symbol_table : std::unordered_set<std::string>
{
public:
    Symbol get(std::string const& str);
    /// Returns the unique symbol for `str`.

    Symbol get(char const* str);
    /// Returns the unique symbol for `str`.
};

inline Symbol
Symbol_table::get(std::string const& str)
{
    return &*emplace(str).first;
}

inline Symbol
Symbol_table::get(char const* str)
{
    return &*emplace(str).first;
}


namespace std
{
    template<>
    struct hash<::Symbol>
    {
        std::size_t operator()(::Symbol sym) const noexcept
        {
            std::hash<std::string const*> h;
            return h(&sym.str());
        }
    };
};

Scope.hpp

#pragma once

#include "decl.hpp"
#include "name.hpp"

#include <string>
#include <vector>
#include <unordered_map>

struct Scope : std::unordered_map<Symbol, Decl*>
{
    Decl* lookup(Symbol sym)
    {
        auto iter = find(sym);
        if (iter == end())
        {
            return nullptr;
        }
        return iter->second;
    }

    void declare(Decl* d)
    {
        assert(!already_declared(d));
        emplace(d->get_name()->get_str(), d);
    }

    bool already_declared(Decl* d)
    {
        return d->get_name() != nullptr;
    }
};

struct Scope_stack : std::vector<Scope>
{
    Decl* lookup(Symbol sym)
    {
        for (auto iter = rbegin(); iter != rend(); ++iter)
        {
            if (Decl * d = iter->lookup(sym))
            {
                return d;
            }
        }
        return nullptr;
    }
};

And here is the compilation error:

/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1805:31: error: no matching constructor for initialization of
      'std::__1::pair<const Symbol, Decl *>'
            ::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
                              ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There is a lot more below this error that I put in this pastebin.

emplace(d->get_name()->get_str(), d);

Here you try to construct a std::unordered_map<Symbol, Decl*>::value_type (i.e. a std::pair<Symbol, Decl*>) from a const std::string& and a Decl*.

The problem is that Symbol has no constructor taking a std::string, only a std::string const*.

This is the line in the error messages that gives the hint:

/Library/Developer/CommandLineTools/usr/include/c++/v1/utility:422:5: note: candidate constructor not viable: no known conversion from
      'std::__1::basic_string<char>' to 'const const Symbol' for 1st argument
    pair(_T1 const& __t1, _T2 const& __t2)