Can My Code Mix Apples and Oranges?
In your C project, you use numerous "usings" to clarify the intended purpose of variables, primarily for std::string identifiers like PortalId or CakeId. Your current approach allows the following:
using PortalId = std::string; using CakeId = std::string; PortalId portal_id("2"); CakeId cake_id("is a lie"); portal_id = cake_id; // OK
But this flexibility is undesirable. You seek compile-time type checking to prevent mixing different object types while preserving the original object's functionality.
Can C Fulfill This Request?
Yes, C provides a solution for your dilemma. Using C 17 and some clever coding, it's possible to create a strongly typed identifier that prevents mixing types. The results look something like this:
SAFE_TYPEDEF(std::string, PortalId); SAFE_TYPEDEF(std::string, CakeId); int main() { 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; }</cakeid>
In this code, assignments between different types are not permitted, and adding incompatible types to containers will result in compiler errors.
What's the Secret?
The magic lies within the SAFE_TYPEDEF macro. Here's the updated version that handles all your requirements:
#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 std::string& { return r._value; } friend auto operator std::ostream& { return os ()(sid._value); return seed; } }; // let's make it hashable namespace std { template<class tag> struct hash<string_id>> { 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;</cake_tag></portal_tag></tag></string_id></class></class></unordered_map></map></string></iostream>
This code defines a string_id class that serves as a strongly typed identifier. It's tagged with either portal_tag or cake_tag to ensure uniqueness. The class also overloads operators like
With this implementation, you can enforce strong typing and prevent mixing types in your code. It covers all the scenarios you mentioned in your question and provides a stable solution for your requirements.
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