source: doc/proposals/interned_string.h @ a4e1b09

#pragma once

// Copyright (c) 2015 University of Waterloo
//
// The contents of this file are covered under the licence agreement in 
// the file "LICENCE" distributed with this repository.

#include <functional>
#include <string>
#include <unordered_set>
#include <utility>

/// Keeps canonical copies of a std::string for quicker comparisons
class interned_string {
        /// Shared map of canonical string representations
        static std::unordered_set< std::string > canonical;

        /// Canonical representation of empty string
        static const std::string* empty_string() {
                static const std::string* mt = [](){
                        return &*canonical.emplace( "" ).first;
                }();
                return mt;
        }

        /// Canonicalize string
        template<typename S>
        static const std::string* intern( S&& s ) {
                return &*canonical.emplace( std::forward<S>(s) ).first;
        }

        /// Pointer to stored string
        const std::string* s;
        
public:
        interned_string() : s{empty_string()} {}
        interned_string(const char* cs) : s{intern(cs)} {}
        interned_string(const std::string& ss) : s{intern(ss)} {}

        operator const std::string& () const { return *s; }

        bool operator== (const interned_string& o) const { return s == o.s; }
        bool operator!= (const interned_string& o) const { return s != o.s; }
        bool operator< (const interned_string& o) const { return *s < *o.s; }
};

inline std::ostream& operator<< (std::ostream& out, const interned_string& s) {
        return out << (const std::string&)s;
}

namespace std {
        template<> struct hash<interned_string> {
                std::size_t operator() (const interned_string& s) const {
                        return std::hash<const std::string*>{}( &(const std::string&)s );
                }
        };
}