C++

Reference

<system_error>

error_category

abstract class
<system_error>

std::error_category

class error_category;
Error category
This type serves as a base class for specific category types.

Category types are used to identify the source of an error. They also define the relation between error_code and error_condition objects of its category, as well as the message set for error_code objects.

Objects of these types have no distinct values and are not-copyable and not-assignable, and thus can only be passed by reference. As such, only one object of each of these types shall exist, each uniquely identifying its own category: all error codes and conditions of a same category shall return a reference to same object.

Some standard objects are of a type derived from this class. A reference to them can be obtained by calling a specific global function for each:
headercategory function
<system_error>generic_category
system_category
<ios>
<iostream>		iostream_category
<future>future_category

Member functions (non-virtual)

(constructor)
Construct error_category (public member function)
relational operators
Relational operators (public member function)

Virtual member functions

(destructor)
Destroy error_category (public virtual member function)
name
Return category name (public pure virtual member function)
default_error_condition
Default error condition (public virtual member function)
equivalent
Check error code equivalence (public virtual member function)
message
Error message (public pure virtual member function)

Example

To create a custom error category, a program shall derive from the error_category base class and override the virtual functions as needed:
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// custom error_category example #include <iostream> // std::cout, std::endl #include <system_error> // std::is_error_condition_enum, std::error_category, // std::error_condition, std::error_code #include <string> // std::string // custom error conditions enum type: enum class custom_errc { success=0, client_error, server_error, other }; namespace std { template<> struct is_error_condition_enum<custom_errc> : public true_type {}; } // custom category: class custom_category_t : public std::error_category { public: virtual const char* name() const { return "custom"; } virtual std::error_condition default_error_condition (int ev) const { if ((ev>=200)&&(ev<300)) return std::error_condition(custom_errc::success); else if ((ev>=400)&&(ev<500)) return std::error_condition(custom_errc::client_error); else if ((ev>=500)&&(ev<600)) return std::error_condition(custom_errc::server_error); else return std::error_condition(custom_errc::other); } virtual bool equivalent (const std::error_code& code, int condition) const { return *this==code.category() && static_cast<int>(default_error_condition(code.value()).value())==condition; } virtual std::string message(int ev) const { switch (ev) { case 200: return "OK"; case 403: return "403 Forbidden"; case 404: return "404 Not Found"; case 500: return "500 Internal Server Error"; case 503: return "503 Service Unavailable"; default: return "Unknown error"; } } } custom_category; // make_error_code overload to generate custom conditions: std::error_condition make_error_condition (custom_errc e) { return std::error_condition(static_cast<int>(e), custom_category); } int main() { // let's generate a 404: std::error_code e (404, custom_category); if (e == custom_errc::success) std::cout << "Success: " << e.message(); else if (e == custom_errc::client_error) std::cout << "Client Error: " << e.message(); else if (e == custom_errc::server_error) std::cout << "Server Error: " << e.message(); else std::cout << "Unknown"; std::cout << std::endl; return 0; }

Output:
Client Error: 404 Not Found