Extending enum types [duplicate] - c++

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Extending enums in C++?
I am using a library which defines its own set of errors as an enum type.
enum error_type {...}
The library also has a function which takes that enum type, and prints the error.
void set_status(error_type new_error)
If I want to define my own error, and give it to the set_status function, is it possible to extend the error_type enum somehow, or maybe override it?

Answer 1

There might be some sharp edges, but I think this should work for you:
#include<iostream>
// Helper struct
template<typename T, int N, typename... LIST>
struct whereInType {
static const int index = -1;
};
template<typename T, int N, typename HEAD, typename... TAIL>
struct whereInType<T,N,HEAD,TAIL...> {
static const int index = whereInType<T, N+1, TAIL...>::index;
};
template<typename T, int N, typename... TAIL>
struct whereInType<T,N,T,TAIL...> {
static const int index = N;
};
// The actual union type
template<typename... ENUMS>
class enum_union {
public:
template<typename ENUM>
constexpr enum_union(ENUM val) :
which_enum(whereInType<ENUM,0,ENUMS...>::index),
value(val) {}
constexpr operator long long(){
return static_cast<long long>(which_enum)<<32 | value;
}
template<typename ENUM, int IGNORE=0>
constexpr bool operator==(const ENUM& e) {
return *this == enum_union<ENUMS...>(e);
}
template<int IGNORE=0>
constexpr bool operator==(const enum_union<ENUMS...>& oth) {
return which_enum==oth.which_enum && value==oth.value;
}
template<typename T>
constexpr bool operator!=(const T& oth) {
return !(*this == oth);
}
private:
int which_enum;
int value;
};
// An example usage
enum normal_errors {
E_OUTOFMEMORY,
E_IOERROR
};
enum weird_errors {
E_OUTOFAARDVARKS,
E_DIVIDEBYCUCUMBER
};
typedef enum_union<normal_errors, weird_errors> any_error;
// Some tests
void print(any_error e) {
switch(e) {
case any_error(E_OUTOFMEMORY):
std::cout << "Out of Memory\n";
break;
case any_error(E_IOERROR):
std::cout << "I/O Error\n";
break;
case any_error(E_OUTOFAARDVARKS):
std::cout << "WE NEED AARDVARKS!!! NOW!!!!!\n";
break;
case any_error(E_DIVIDEBYCUCUMBER):
std::cout << "please reinstall universe\n";
break;
}
}
main(){
print(E_OUTOFMEMORY);
print(E_IOERROR);
print(E_OUTOFAARDVARKS);
print(E_DIVIDEBYCUCUMBER);
if (any_error(E_OUTOFMEMORY) == E_OUTOFAARDVARKS) {
std::cout<<"bad\n";
}else{
std::cout<<"good\n";
}
if (any_error(E_OUTOFMEMORY) != E_OUTOFAARDVARKS) {
std::cout<<"good\n";
}else{
std::cout<<"bad\n";
}
if (any_error(E_OUTOFMEMORY) == E_OUTOFMEMORY) {
std::cout<<"good\n";
}else{
std::cout<<"bad\n";
}
if (any_error(E_OUTOFMEMORY) != E_OUTOFMEMORY) {
std::cout<<"bad\n";
}else{
std::cout<<"good\n";
}
}