Here’s a minimal complete solution that will do what you want.
You can add more operators etc to make the class more useful as you see fit.
#include <iostream>
#include <string>
#include <map>
// define some tags to create uniqueness
struct portal_tag {};
struct cake_tag {};
// a string-like identifier that is typed on a tag type
template<class Tag>
struct string_id
{
// needs to be default-constuctable because of use in map[] below
string_id(std::string s) : _value(std::move(s)) {}
string_id() : _value() {}
// provide access to the underlying string value
const std::string& value() const { return _value; }
private:
std::string _value;
// will only compare against same type of id.
friend bool operator < (const string_id& l, const string_id& r) {
return l._value < r._value;
}
};
// create some type aliases for ease of use
using PortalId = string_id<portal_tag>;
using CakeId = string_id<cake_tag>;
using namespace std;
// confirm that requirements are met
auto main() -> int
{
PortalId portal_id("2");
CakeId cake_id("is a lie");
std::map<CakeId, PortalId> p_to_cake; // OK
p_to_cake[cake_id] = portal_id; // OK
// p_to_cake[portal_id] = cake_id; // COMPILER ERROR
// portal_id = cake_id; // COMPILER ERROR
// portal_id = "1.0"; // COMPILER ERROR
portal_id = PortalId("42"); // OK
return 0;
}
here’s an updated version that also handles hash maps, streaming to ostream etc.
You will note that I have not provided an operator to convert to string
. This is deliberate. I am requiring that users of this class explicitly express the intent to use it as a string by providing an overload of to_string
.
#include <iostream>
#include <string>
#include <map>
#include <unordered_map>
// define some tags to create uniqueness
struct portal_tag {};
struct cake_tag {};
// a string-like identifier that is typed on a tag type
template<class Tag>
struct string_id
{
using tag_type = Tag;
// needs to be default-constuctable because of use in map[] below
string_id(std::string s) : _value(std::move(s)) {}
string_id() : _value() {}
// provide access to the underlying string value
const std::string& value() const { return _value; }
private:
std::string _value;
// will only compare against same type of id.
friend bool operator < (const string_id& l, const string_id& r) {
return l._value < r._value;
}
friend bool operator == (const string_id& l, const string_id& r) {
return l._value == r._value;
}
// and let's go ahead and provide expected free functions
friend
auto to_string(const string_id& r)
-> const std::string&
{
return r._value;
}
friend
auto operator << (std::ostream& os, const string_id& sid)
-> std::ostream&
{
return os << sid.value();
}
friend
std::size_t hash_code(const string_id& sid)
{
std::size_t seed = typeid(tag_type).hash_code();
seed ^= std::hash<std::string>()(sid._value);
return seed;
}
};
// let's make it hashable
namespace std {
template<class Tag>
struct hash<string_id<Tag>>
{
using argument_type = string_id<Tag>;
using result_type = std::size_t;
result_type operator()(const argument_type& arg) const {
return hash_code(arg);
}
};
}
// create some type aliases for ease of use
using PortalId = string_id<portal_tag>;
using CakeId = string_id<cake_tag>;
using namespace std;
// confirm that requirements are met
auto main() -> int
{
PortalId portal_id("2");
CakeId cake_id("is a lie");
std::map<CakeId, PortalId> p_to_cake; // OK
p_to_cake[cake_id] = portal_id; // OK
// p_to_cake[portal_id] = cake_id; // COMPILER ERROR
// portal_id = cake_id; // COMPILER ERROR
// portal_id = "1.0"; // COMPILER ERROR
portal_id = PortalId("42"); // OK
// extra checks
std::unordered_map<CakeId, PortalId> hashed_ptocake;
hashed_ptocake.emplace(CakeId("foo"), PortalId("bar"));
hashed_ptocake.emplace(CakeId("baz"), PortalId("bar2"));
for(const auto& entry : hashed_ptocake) {
cout << entry.first << " = " << entry.second << '\n';
// exercise string conversion
auto s = to_string(entry.first) + " maps to " + to_string(entry.second);
cout << s << '\n';
}
// if I really want to copy the values of dissimilar types I can express it:
const CakeId cake1("a cake ident");
auto convert = PortalId(to_string(cake1));
cout << "this portal is called '" << convert << "', just like the cake called '" << cake1 << "'\n";
return 0;
}