An enumeration names the finite alternatives represented by integral values. The practical mistake is placing unrelated enum constants in the global namespace: duplicate names become errors, while integer backing can let unrelated color sets mix ambiguously. section 4.2
Listing 4.3 Problematic global namespace enumerations
enum TrafficColor {
Green,
Yellow, #1
Red #2
};
enum PaintColor {
Gray,
White, #3
Black,
Paint_Red, #4
Blue,
Paint_Yellow,
Paint_Green
};
int main() {
int color = 2; #5
switch (color) {
case Red:
std::cout << "we have a red one\n";
break;
case Blue:
std::cout << "we have a blue one\n";
break;
case Yellow:
std::cout << "we have a yellow one\n";
break;
default:
std::cout << "we have a white one\n";
break;
// case White: #6
};
return 0;
}
A traffic-light and automobile-paint example shows how global names force weakened names such as Paint_Red and leave an integer color value unclear. section 4.2
Prefer scoped enumerations with enum class. Use values such as Traffic::Yellow and Paint::Blue, and qualify switch cases explicitly. Separate scopes preserve meaningful names and make the selected enum set visible; retain legacy code cautiously and fix it incrementally with testing. section 4.2
Listing 4.4 Separate namespace enumerations
enum class Traffic { #1
Green,
Yellow,
Red
};
enum class Paint { #2
Gray,
Black,
White,
Red,
Blue,
Yellow,
Green
};
int main() {
auto color = Traffic::Yellow; #3
switch (color) {
case Traffic::Red: #4
std::cout << "we have a red light\n";
break;
case Traffic::Yellow:
std::cout << "we have a yellow light\n";
break;
case Traffic::Green:
default:
std::cout << "we have a green light\n";
break;
};
Paint can = Paint::Blue;
switch (can) {
case Paint::Red:
std::cout << "we have a red cube\n";
break;
case Paint::Yellow:
std::cout << "we have a blue cube\n";
break;
case Paint::White:
[[fallthrough]];
default: #5
std::cout << "we have a white cube\n";
break;
};
return 0;
}
The corrected example separates Traffic and Paint, then switches over qualified enumerators without renaming colors to avoid collisions. section 4.2
