如何迭代枚举？

Question

I just noticed that you can not use standard math operators on an enum such as ++ or += .

So what is the best way to iterate through all of the values in a C++ enum ?

Answer 1

The typical way is as follows:

enum Foo {
  One,
  Two,
  Three,
  Last
};

for ( int fooInt = One; fooInt != Last; fooInt++ )
{
   Foo foo = static_cast<Foo>(fooInt);
   // ...
}

Please note, the enum Last is meant to be skipped by the iteration. Utilizing this "fake" Last enum, you don't have to update your terminating condition in the for loop to the last "real" enum each time you want to add a new enum. If you want to add more enums later, just add them before Last. The loop in this example will still work.

Of course, this breaks down if the enum values are specified:

enum Foo {
  One = 1,
  Two = 9,
  Three = 4,
  Last
};

This illustrates that an enum is not really meant to iterate through. The typical way to deal with an enum is to use it in a switch statement.

switch ( foo )
{
    case One:
        // ..
        break;
    case Two:  // intentional fall-through
    case Three:
        // ..
        break;
    case Four:
        // ..
        break;
     default:
        assert( ! "Invalid Foo enum value" );
        break;
}

If you really want to enumerate, stuff the enum values in a vector and iterate over that. This will properly deal with the specified enum values as well.

Answer 2

#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;
}

Answer 3

With c++11, there actually is an alternative: writing a templatized custom iterator.

let's assume your enum is

enum class foo {
  one,
  two,
  three
};

This generic code will do the trick, quite efficiently - place in a generic header, it'll serve you for any enum you may need to iterate over:

#include <type_traits>
template < typename C, C beginVal, C endVal>
class Iterator {
  typedef typename std::underlying_type<C>::type val_t;
  int val;
public:
  Iterator(const C & f) : val(static_cast<val_t>(f)) {}
  Iterator() : val(static_cast<val_t>(beginVal)) {}
  Iterator operator++() {
    ++val;
    return *this;
  }
  C operator*() { return static_cast<C>(val); }
  Iterator begin() { return *this; } //default ctor is good
  Iterator end() {
      static const Iterator endIter=++Iterator(endVal); // cache it
      return endIter;
  }
  bool operator!=(const Iterator& i) { return val != i.val; }
};

You'll need to specialize it

typedef Iterator<foo, foo::one, foo::three> fooIterator;

And then you can iterate using range-for

for (foo i : fooIterator() ) { //notice the parentheses!
   do_stuff(i);
}

The assumption that you don't have gaps in your enum is still true; there is no assumption on the number of bits actually needed to store the enum value (thanks to std::underlying_type)

Answer 4

too much complicated these solution, i do like that :

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

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

for (NodePosition pos : NodePositionVector) {
...
}

Answer 5

I often do it like that

    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))
    {}

or if not successive, but with regular step (eg bit flags)

    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))
    {}

Answer 6

If your enum starts with 0 and the increment is always 1.

enum enumType 
{ 
    A = 0,
    B,
    C,
    enumTypeEnd
};

for(int i=0; i<enumTypeEnd; i++)
{
   enumType eCurrent = (enumType) i;            
}

If not I guess the only why is to create something like a

vector<enumType> vEnums;

add the items, and use normal iterators....

Answer 7

Something that hasn't been covered in the other answers = if you're using strongly typed C++11 enums, you cannot use ++ or + int on them. In that case, a bit of a messier solution is required:

enum class myenumtype {
  MYENUM_FIRST,
  MYENUM_OTHER,
  MYENUM_LAST
}

for(myenumtype myenum = myenumtype::MYENUM_FIRST;
    myenum != myenumtype::MYENUM_LAST;
    myenum = static_cast<myenumtype>(static_cast<int>(myenum) + 1)) {

  do_whatever(myenum)

}

Answer 8

You can't with an enum. Maybe an enum isn't the best fit for your situation.

A common convention is to name the last enum value something like MAX and use that to control a loop using an int.

Answer 9

Assuming that enum is numbered sequentially is error prone. Moreover, you may want to iterate over selected enumerators only. If that subset is small, looping over it explicitly might be an elegant choice:

enum Item { Man, Wolf, Goat, Cabbage }; // or enum class

for (auto item : {Wolf, Goat, Cabbage}) { // or Item::Wolf, ...
    // ...
}

Answer 10

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);
}

A constexpr std::array can iterate even non-sequential enums without the array being instantiated by the compiler. This depends on things like the compiler's optimization heuristics and whether you take the array's address.

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.

Here is a link to a simple test iteration on godbolt that demonstrates the array does not always get instantiated.

The price of this technique is writing the enum elements twice and keeping the two lists in sync.

Answer 11

You can try and define the following macro:

#define for_range(_type, _param, _A1, _B1) for (bool _ok = true; _ok;)\
for (_type _start = _A1, _finish = _B1; _ok;)\
    for (int _step = 2*(((int)_finish)>(int)_start)-1;_ok;)\
         for (_type _param = _start; _ok ; \
 (_param != _finish ? \
           _param = static_cast<_type>(((int)_param)+_step) : _ok = false))

Now you can use it:

enum Count { zero, one, two, three }; 

    for_range (Count, c, zero, three)
    {
        cout << "forward: " << c << endl;
    }

It can be used to iterate backwards and forwards through unsigned, integers, enums and chars:

for_range (unsigned, i, 10,0)
{
    cout << "backwards i: " << i << endl;
}


for_range (char, c, 'z','a')
{
    cout << c << endl;
}

Despite its awkward definition it is optimized very well. I looked at disassembler in VC++. The code is extremely efficient. Don't be put off but the three for statements: the compiler will produce only one loop after optimization! You can even define enclosed loops:

unsigned p[4][5];

for_range (Count, i, zero,three)
    for_range(unsigned int, j, 4, 0)
    {   
        p[i][j] = static_cast<unsigned>(i)+j;
    }

You obviously cannot iterate through enumerated types with gaps.

Answer 12

您还可以为枚举类型重载递增/递减运算符。

Answer 13

Here's another solution which only works for contiguous enums. It gives the expected iteration, except for ugliness in the increment, which is where it belongs, since that's what's broken in C++.

enum Bar {
    One = 1,
    Two,
    Three,
    End_Bar // Marker for end of enum; 
};

for (Bar foo = One; foo < End_Bar; foo = Bar(foo + 1))
{
    // ...
}

Answer 14

If you do not like to pollute you enum with a final COUNT item (because maybe if you also use the enum in a switch then then the compiler will warn you of a missing case COUNT:), you can do this:

enum Colour {Red, Green, Blue};
const Colour LastColour = Blue;

Colour co(0);
while (true) {
  // do stuff with co
  // ...
  if (co == LastColour) break;
  co = Colour(co+1);
}

Answer 15

In Bjarne Stroustrup's C++ programming language book, you can read that he's proposing to overload the operator++ for your specific enum . enum are user-defined types and overloading operator exists in the language for these specific situations.

You'll be able to code the following:

#include <iostream>
enum class Colors{red, green, blue};
Colors& operator++(Colors &c, int)
{
     switch(c)
     {
           case Colors::red:
               return c=Colors::green;
           case Colors::green:
               return c=Colors::blue;
           case Colors::blue:
               return c=Colors::red; // managing overflow
           default:
               throw std::exception(); // or do anything else to manage the error...
     }
}

int main()
{
    Colors c = Colors::red;
    // casting in int just for convenience of output. 
    std::cout << (int)c++ << std::endl;
    std::cout << (int)c++ << std::endl;
    std::cout << (int)c++ << std::endl;
    std::cout << (int)c++ << std::endl;
    std::cout << (int)c++ << std::endl;
    return 0;
}

test code: http://cpp.sh/357gb

Mind that I'm using enum class . Code works fine with enum also. But I prefer enum class since they are strong typed and can prevent us to make mistake at compile time.

Answer 16

There is already discussion about std::initializer_list (C++11) in the comments. I am mentioning example to iterate over the enum.

or std::initializer_list and a simpler syntax:

enum E {
    E1 = 4,
    E2 = 8,
    // ..
    En
};

constexpr std::initializer_list<E> all_E = {E1, E2, /*..*/ En};

and then

for (auto e : all_E) {
    // Do job with e
}

Reference Link

Answer 17

Here are some very readable and easy-to-understand approaches, for both weakly-typed C and C++ regular enum s, and strongly-typed C++ enum class es.

I recommend compiling all examples below with -Wall -Wextra -Werror . This gives you the added safety that if you forget to cover any enum value in the switch case your compiler will throw a compile-time error ! This forces you to keep your enum definition and switch cases in-sync, which is an extra safety measure for your code. This tip works so long as you:

1. Cover all enum values in your switch case, and
2. Do NOT have a default switch case.
3. Build with the -Wall -Wextra -Werror flags.

I recommend you follow all 3 of those points, as it is a good practice and creates better code.

1. For a standard, weakly-typed C or C++ enum :

C definition (this is also valid 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++ definition:

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 or C++ iteration over this weakly-typed enum:

Note: incrementing an enum by doing my_error_type++ is not allowed--not even on C-style enums, so we must do this instead: my_error_type = (my_error_type_t)(my_error_type + 1) . Notice that my_error_type + 1 is allowed, however, since this weak enum is automatically implicitly cast to an int here to make this addition possible withOUT having to manually cast it to an int like this: 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. For a scoped , strongly-typed C++ enum class :

C++ definition:

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,
};

C++ iteration over this strongly-typed enum:

Notice the extra (size_t) cast (or (int) would be acceptable too) required to forcefully increment the enum class variable! I also chose to use the C++-style static_cast<my_error_type_t> cast here, but a C-style (my_error_type_t) cast, as done above, would have been fine as well.

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++.

See also:

1. [my answer] Is there a way to initialize a vector by index in c++11?
2. [my Q&A] What are commonly-used ways to iterate over an enum class in C++?
3. My answer on some of the differences between enum class es ( strongly-typed enums) and regular enum s ( weakly-typed enums) in C++: How to automatically convert strongly typed enum into int?
4. Some of my personal notes on the -Wall -Wextra -Werror and other build options , from my eRCaGuy_hello_world repo.

Answer 18

For MS compilers:

#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); 
}

Note: this is a lot less code than the simple templatized custom iterator answer.

You can get this to work with GCC by using typeof instead of decltype , but I don't have that compiler handy at the moment to make sure it compiles.

Answer 19

Upsides: enums can have any values you like in any order you like and it's still easy to iterate over them. Names and values are defined once, in the first #define.

Downsides: if you use this at work, you need a whole paragraph to explain it to your coworkers. And, it's annoying to have to declare memory to give your loop something to iterate over, but I don't know of a workaround that doesn't confine you to enums with adjacent values (and if the enum will always have adjacent values, the enum might not be buying you all that much anyway.)

//create a, b, c, d as 0, 5, 6, 7
#define LIST x(a) x(b,=5) x(c) x(d)
#define x(n, ...) n __VA_ARGS__,
enum MyEnum {LIST}; //define the enum
#undef x //needed
#define x(n,...) n ,
MyEnum myWalkableEnum[] {LIST}; //define an iterable list of enum values
#undef x //neatness

int main()
{
  std::cout << d;
  for (auto z : myWalkableEnum)
    std::cout << z;
}
//outputs 70567

The trick of declaring a list with an undefined macro wrapper, and then defining the wrapper differently in various situations, has a lot of applications other than this one.

Answer 20

Casting the variable to an int& lets you increment while keeping the type readable.

#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

Answer 21

(Taking Marski's answer as a big hint...)

Since the body of an enum definition is the same as an initialiser list it is possible to do this without repeating the item list if we use a simple macro to write out the values:

#define ITEM_LIST_MACRO Wolf, Goat, Cabbage

enum Item { ITEM_LIST_MACRO }; // Define the enum

// Now iterate through it
for (auto item : { ITEM_LIST_MACRO }) {
}

Pros: Simple, no repetition, and no need to maintain annoying first/last sentinel values. (Indeed I think this might be the only solution suggested so far that does not require the user to remember to update an 'end' marker when a new item is added to the list.)

Cons: Doesn't work with scoped enums (enum class) because the initialiser list would need the scope (Item::Wolf etc). Also doesn't work if you want to specify the values of the enum members rather than having them defaulted.

Answer 22

If you knew that the enum values were sequential, for example the Qt:Key enum, you could:

Qt::Key shortcut_key = Qt::Key_0;
for (int idx = 0; etc...) {
    ....
    if (shortcut_key <= Qt::Key_9) {
        fileMenu->addAction("abc", this, SLOT(onNewTab()),
                            QKeySequence(Qt::CTRL + shortcut_key));
        shortcut_key = (Qt::Key) (shortcut_key + 1);
    }
}

It works as expected.

Answer 23

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]
}

Answer 24

Extending @Eponymous's answer: It's great, but doesn't provide a general syntax. Here's what I came up with:

// Common/EnumTools.h
#pragma once

#include <array>

namespace Common {

// Here we forward-declare metafunction for mapping enums to their values.
// Since C++<23 doesn't have reflection, you have to populate it yourself :-(
// Usage: After declaring enum class E, add this overload in the namespace of E:
// inline constexpr auto allValuesArray(const E&, Commob::EnumAllValuesTag) { return std::array{E::foo, E::bar}; }
// Then `AllValues<NS::E>` will call `allValuesArray(NS::E{}, EnumAllValuesTag)` which will resolve
// by ADL.
// Just be sure to keep it sync'd with your enum!

// Here's what you want to use in, e.g., loops: "for (auto val : Common::AllValues<MyEnum>) {"

struct EnumAllValuesTag {}; // So your allValuesArray function is clearly associated with this header.

template <typename Enum>
static inline constexpr auto AllValues = allValuesArray(Enum{}, EnumAllValuesTag{});
// ^ Just "constexpr auto" or "constexpr std::array<Enum, allValuesArray(Enum{}, EnumAllValuesTag{}).size()>" didn't work on all compilers I'm using, but this did.

} // namespace Common

then in your namespace:

#include "Common/EnumTools.h"

namespace MyNamespace {

enum class MyEnum {
    foo,
    bar = 4,
    baz = 42,
};

// Making this not have to be in the `Common` namespace took some thinking,
// but is a critical feature since otherwise there's no hope in keeping it sync'd with the enum.
inline constexpr auto allValuesArray(const MyEnum&, Common::EnumAllValuesTag) {
    return std::array{ MyEnum::foo, MyEnum::bar, MyEnum::baz };
}

} // namespace MyNamespace

then wherever you need to use it:

for (const auto& e : Common::AllValues<MyNamespace::MyEnum>) { ... }

so even if you've typedef'd:

namespace YourNS {
using E = MyNamespace::MyEnum;
} // namespace YourNS

for (const auto& e : Common::AllValues<YourNS::E>) { ... }

I can't think of anything much better, short of the actual language feature everyone looking at this page want.

Future work:

1. You should be able to add a constexpr function (and so a metafunction) that filters Common::AllValues<E> to provide a Common::AllDistinctValues<E> for the case of enums with repeated numerical values like enum { foo = 0, bar = 0 }; .
2. I bet there's a way to use the compiler's switch -covers-all- enum -values to write allValuesArray such that it errors if the enum has added a value.

Answer 25

Using a lambda, I found this the best (modern) way of looping over enums. This highly improves the abstraction. Can even make it a template, so it's applicable to any enum. This code neither gives you issues with clang(-tidy).

#include <functional>

/// @brief Loop over all enum values where the last enum value is the invalid one
void forEachAction(std::function<void(Enum)> &&doThis) {
    for (int value = 0; value = static_cast<int>(Enum::LastValue); ++value ) {
        doThis(static_cast<Enum>(value ));
    }
}

...

forEachAction([this](Enum value) {
    ...  // what you want to execute for every enum
});

Answer 26

Most solution are based on loops over the (MIN, MAX) range but overlook the fact that might be holes in the enum.

My suggestions is:

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

Answer 27

C++ 没有自省功能，因此您无法在运行时确定这种事情。

Answer 28

Just make an array of ints and loop over the array, but make the last element say -1 and use it for exit condition.

If enum is:

enum MyEnumType{Hay=12,Grass=42,Beer=39};

then create array:

int Array[] = {Hay,Grass,Beer,-1};

for (int h = 0; Array[h] != -1; h++){
  doStuff( (MyEnumType) Array[h] );
}

This does not break down no matter the ints in the representation as long as -1 check does not collide with one of the elements of course.