The global namespace contains declarations not enclosed in another namespace. Placing unrelated enumeration constants there creates collisions, weakens names, and lets integer values hide which meaning a variable carries. section 4.2
Mistake: global namespace enums
TrafficColor and PaintColor place every enumerator in one namespace. Names such as Yellow can collide, while int color = 2 loses whether the value means a traffic color or paint color. Because the enums behave as integers, arbitrary or out-of-range values can also pass through unnoticed. 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;
}
Inspect the problematic enumeration design, including duplicate color meanings, unqualified case labels, and an uninformative integer assignment. section 4.2
Prefer enum class to give each enumeration its own scope. Qualified values such as Traffic::Yellow and Paint::Blue preserve intent and prevent unrelated color sets from mixing accidentally. 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;
}
Compare separately scoped Traffic and Paint enumerations with qualified switch cases and type-specific variables. section 4.2
