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

大多数解决方案基于(MIN, MAX)范围内的循环,但忽略了枚举中可能存在漏洞的事实。

我的建议是:

        for (int i = MYTYPE_MIN; i <= MYTYPE_MAX; i++) {
            if (MYTYPE_IsValid(i)) {
                MYTYPE value = (MYTYPE)i;
                // DoStuff(value)
            }   
        }   
        

其他回答

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]
}
#include <iostream>
#include <algorithm>

namespace MyEnum
{
  enum Type
  {
    a = 100,
    b = 220,
    c = -1
  };

  static const Type All[] = { a, b, c };
}

void fun( const MyEnum::Type e )
{
  std::cout << e << std::endl;
}

int main()
{
  // all
  for ( const auto e : MyEnum::All )
    fun( e );

  // some
  for ( const auto e : { MyEnum::a, MyEnum::b } )
    fun( e );

  // all
  std::for_each( std::begin( MyEnum::All ), std::end( MyEnum::All ), fun );

  return 0;
}

我经常这样做

    enum EMyEnum
    {
        E_First,
        E_Orange = E_First,
        E_Green,
        E_White,
        E_Blue,
        E_Last
    }

    for (EMyEnum i = E_First; i < E_Last; i = EMyEnum(i + 1))
    {}

或者如果不是连续的,但有规则步长(例如位标志)

    enum EAnimalCaps
    {
        E_None    = 0,
        E_First   = 0x1,
        E_CanFly  = E_First,
        E_CanWalk = 0x2
        E_CanSwim = 0x4,
        E_Last
    }
    
    class MyAnimal
    {
       EAnimalCaps m_Caps;
    }

    class Frog
    {
        Frog() : 
            m_Caps(EAnimalCaps(E_CanWalk | E_CanSwim))
        {}
    }

    for (EAnimalCaps= E_First; i < E_Last; i = EAnimalCaps(i << 1))
    {}

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。

这里有一些非常易读且易于理解的方法,适用于弱类型的C和c++常规枚举,以及强类型的c++枚举类。

我建议使用-Wall -Wextra -Werror编译下面所有的例子。这为您提供了额外的安全性,如果您忘记在开关情况下覆盖任何枚举值,编译器将抛出编译时错误!这迫使您保持枚举定义和开关用例同步,这是代码的额外安全措施。只要你:

覆盖开关案例中的所有枚举值,和 没有默认开关大小写。 使用-Wall -Wextra -Werror旗帜构建。

我建议您遵循所有这3点,因为这是一个很好的实践,可以创建更好的代码。

1. 对于标准的弱类型C或c++ enum:

C定义(这也适用于c++):

typedef enum my_error_type_e 
{
    MY_ERROR_TYPE_SOMETHING_1 = 0,
    MY_ERROR_TYPE_SOMETHING_2,
    MY_ERROR_TYPE_SOMETHING_3,
    MY_ERROR_TYPE_SOMETHING_4,
    MY_ERROR_TYPE_SOMETHING_5,
    /// Not a valid value; this is the number of members in this enum
    MY_ERROR_TYPE_count,
    // helpers for iterating over the enum
    MY_ERROR_TYPE_begin = 0,
    MY_ERROR_TYPE_end = MY_ERROR_TYPE_count,
} my_error_type_t;

c++定义:

enum my_error_type_t 
{
    MY_ERROR_TYPE_SOMETHING_1 = 0,
    MY_ERROR_TYPE_SOMETHING_2,
    MY_ERROR_TYPE_SOMETHING_3,
    MY_ERROR_TYPE_SOMETHING_4,
    MY_ERROR_TYPE_SOMETHING_5,
    /// Not a valid value; this is the number of members in this enum
    MY_ERROR_TYPE_count,
    // helpers for iterating over the enum
    MY_ERROR_TYPE_begin = 0,
    MY_ERROR_TYPE_end = MY_ERROR_TYPE_count,
};

C或c++对弱类型枚举的迭代:

注意:通过my_error_type++递增枚举是不允许的——甚至在c风格的enum上也不允许,所以我们必须这样做:my_error_type = (my_error_type_t)(my_error_type + 1)。注意,my_error_type+ 1是允许的,然而,因为这个弱enum在这里会自动隐式强制转换为int类型,这样就可以不手动强制转换为int类型:my_error_type = (my_error_type_t)((int)my_error_type + 1)。

for (my_error_type_t my_error_type = MY_ERROR_TYPE_begin; 
        my_error_type < MY_ERROR_TYPE_end;
        my_error_type = (my_error_type_t)(my_error_type + 1)) 
{
    switch (my_error_type) 
    {
        case MY_ERROR_TYPE_SOMETHING_1:
            break;
        case MY_ERROR_TYPE_SOMETHING_2:
            break;
        case MY_ERROR_TYPE_SOMETHING_3:
            break;
        case MY_ERROR_TYPE_SOMETHING_4:
            break;
        case MY_ERROR_TYPE_SOMETHING_5:
            break;
        case MY_ERROR_TYPE_count:
            // This case will never be reached.
            break;
    }
}

2. 对于限定作用域的强类型c++枚举类:

c++定义:

enum class my_error_type_t
{
    SOMETHING_1 = 0,
    SOMETHING_2,
    SOMETHING_3,
    SOMETHING_4,
    SOMETHING_5,
    /// Not a valid value; this is the number of members in this enum
    count,
    // helpers for iterating over the enum
    begin = 0,
    end = count,
};

这个强类型枚举的c++迭代:

注意,强制递增枚举类变量需要额外的(size_t)强制转换(或者(int)也可以接受)!这里我还选择使用c++风格的static_cast<my_error_type_t>强制转换,但是C风格的(my_error_type_t)强制转换(如上所示)也可以。

for (my_error_type_t my_error_type = my_error_type_t::begin; 
        my_error_type < my_error_type_t::end;
        my_error_type = static_cast<my_error_type_t>((size_t)my_error_type + 1)) 
{
    switch (my_error_type) 
    {
        case my_error_type_t::SOMETHING_1:
            break;
        case my_error_type_t::SOMETHING_2:
            break;
        case my_error_type_t::SOMETHING_3:
            break;
        case my_error_type_t::SOMETHING_4:
            break;
        case my_error_type_t::SOMETHING_5:
            break;
        case my_error_type_t::count:
            // This case will never be reached.
            break;
    }
}

Also notice the scoping. In the C++ strongly-typed enum class I used my_error_type_t:: to access each scoped enum class member. But, in the C-style weakly-typed regular enum, very similar scoping can be achieved, as I demonstrated, simply be prefixing each enum member name with MY_ERROR_TYPE_. So, the fact that the C++ strongly-typed enum class adds scoping doesn't really add much value--it's really just a personal preference in that regard. And the fact that the C++ strongly-typed enum class has extra type-safety also has pros and cons. It may help you in some cases but it definitely makes incrementing the enum and iterating over it a pain-in-the-butt, which, honestly, means it is doing its job. By making it harder to increment the scoped enum class variable as though it was an integer, the C++ strongly-typed enum class is doing exactly what it was designed to do. Whether or not you want that behavior is up to you. Personally, I frequently do not want that behavior, and so it is not uncommon for me to prefer to use C-style enums even in C++.

参见:

[我的回答]在c++11中有一种方法通过索引初始化一个向量吗? [我的问答]在c++中迭代枚举类的常用方法是什么? 我对c++中枚举类(强类型enum)和常规enum(弱类型enum)之间的一些差异的回答:如何自动将强类型enum转换为int? 我的一些关于-Wall -Wextra -Werror和其他构建选项的个人笔记,来自我的eRCaGuy_hello_world回购。