我只是注意到不能在枚举上使用标准数学运算符,如++或+=。

那么,在c++枚举中遍历所有值的最佳方法是什么?


当前回答

typedef enum{
    first = 2,
    second = 6,
    third = 17
}MyEnum;

static const int enumItems[] = {
    first,
    second,
    third
}

static const int EnumLength = sizeof(enumItems) / sizeof(int);

for(int i = 0; i < EnumLength; i++){
    //Do something with enumItems[i]
}

其他回答

对于MS编译器:

#define inc_enum(i) ((decltype(i)) ((int)i + 1))

enum enumtype { one, two, three, count};
for(enumtype i = one; i < count; i = inc_enum(i))
{ 
    dostuff(i); 
}

注意:这比简单的模板化自定义迭代器答案要少得多。

你可以通过使用typeof而不是decltype来让它与GCC一起工作,但我现在没有那个编译器来确保它能编译。

枚举就不行。也许枚举不是最适合您的情况。

一个常见的约定是将最后一个枚举值命名为MAX,并使用它来控制一个int类型的循环。

将变量强制转换为int&类型可以在保持类型可读的同时进行递增。

#include <iostream>

enum MyEnum
{
    ONE,
    TWO,
    THREE,
    FOUR,
};

int main()
{
    for (MyEnum v = MyEnum::ONE; v <= MyEnum::FOUR; ++(int&)v)
    {
        std::cout<<v<<std::endl;
    }

    return 0;
}
0
1
2
3

这些解决方案太复杂了,我喜欢这样:

enum NodePosition { Primary = 0, Secondary = 1, Tertiary = 2, Quaternary = 3};

const NodePosition NodePositionVector[] = { Primary, Secondary, Tertiary, Quaternary };

for (NodePosition pos : NodePositionVector) {
...
}
enum class A {
    a0=0, a3=3, a4=4
};
constexpr std::array<A, 3> ALL_A {A::a0, A::a3, A::a4}; // constexpr is important here

for(A a: ALL_A) {
  if(a==A::a0 || a==A::a4) std::cout << static_cast<int>(a);
}

constexpr std::array甚至可以迭代非顺序的枚举,而无需编译器实例化数组。这取决于编译器的优化启发式以及是否取数组的地址。

In my experiments, I found that g++ 9.1 with -O3 will optimize away the above array if there are 2 non-sequential values or quite a few sequential values (I tested up to 6). But it only does this if you have an if statement. (I tried a statement that compared an integer value greater than all the elements in a sequential array and it inlined the iteration despite none being excluded, but when I left out the if statement, the values were put in memory.) It also inlined 5 values from a non-sequential enum in [one case|https://godbolt.org/z/XuGtoc]. I suspect this odd behavior is due to deep heuristics having to do with caches and branch prediction.

这里有一个godbolt的简单测试迭代的链接,演示了数组并不总是被实例化。

这种技术的代价是写入enum元素两次,并保持两个列表同步。