Iterate using an enum

Question

I have std::map which its key is an enum like this:

enum class my_enum{a=1,b,c,d};
std::map<my_enum,my_class> my_map;

I want to iterate over the map so I used this for:

for (auto current_type = (id_type)0;
     (int)current_type < 4;
     current_type = (my_enum)((int)current_type + 1)){
   //do things
}

I think it is rubbish. Do you have a better suggestion?

EDIT:

I am totally aware of using of iterators. I know they are prefereable. However, for some reason, I have to stick to the regular index-based for.

Have you consulted some [reference documentation](http://en.cppreference.com/w/cpp/container/map) for iteration facilities offered by `std::map` (`begin()`, `end()` etc.)? Have you tried applying that? What obstacles have you encountered? — Angew is no longer proud of SO, Feb 08 '16 at 12:47
You can use iterators with a "regular `for`." It's quite unclear what your limitations are. Can you explain in a bit more detail? — Angew is no longer proud of SO, Feb 08 '16 at 12:49
Not necessarily the best way, but unscoped `enum` would at least avoid the horrible casts... which, if you must keep them, should be `static_cast(src)` anyway as this is C++ after all. — underscore_d, Feb 08 '16 at 12:49
@Angew You are right again, but just please suppose I can not. — Humam Helfawi, Feb 08 '16 at 12:50
What is a "regular" `for` loop? As far as I am concerned `for (auto it = my_map.begin(), it != my_map.end(), ++it){}` is a "regular" for loop — NathanOliver, Feb 08 '16 at 12:50
`for (const auto& p : my_map)`, is that a regular for? It isn't clear at all why you need to iterate using the enumeration values. And if you do, then what is the connection to that problem with a map? — juanchopanza, Feb 08 '16 at 12:52
It is problem in the design. where the enum index(integer value) has its meaning. and I do not want to fix a lot of things which are out of my scope — Humam Helfawi, Feb 08 '16 at 12:52
This question has been downvoted and has been voted to close because the stupidity of the idea that I am trying to do. However, we all know that in the real life we have to adopt a rubbish code and work from a point where we do not have an option of fixing what was before it. I think downvotes should be about the clearness of the question not because we hate the way that the OP handle his/her problem :) and Many thanks for your time helping me in this :) — Humam Helfawi, Feb 08 '16 at 13:11
Sorry, but the question it totally unclear, probably because of the irrelevant map thing. How do you know why people down-voted anyway? — juanchopanza, Feb 08 '16 at 13:13
@juanchopanza maybe you are right. However, I could get a satisfied answer from someone.. — Humam Helfawi, Feb 08 '16 at 13:14
it is not duplicated. that question handel the case of enums not enums class. and there is only one anser about it and it is exactly as my bad approch — Humam Helfawi, Feb 08 '16 at 13:16

Cheers and hth. - Alf · Answer 1 · 2016-02-08T13:38:20.553

3

To iterate over a map m:

for( auto& item : m ) ...

To iterate over your enum class values:

#include <initializer_list>

enum class my_enum{a=1,b,c,d};

auto main() -> int
{
    using E = my_enum;
    for( my_enum const id : {E::a, E::b, E::c, E::d} ) {}
}

or

enum class my_enum{a=1,b,c,d};

auto main() -> int
{
    for( int i = int( my_enum::a ); i <= int( my_enum::d ); ++i )
    {
        auto const id = my_enum( i );
        // ...
    }
}

edited Feb 08 '16 at 13:38

answered Feb 08 '16 at 12:50

Cheers and hth. - Alf

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If you have a range class at hand, yes. Otherwise just an `int` loop control variable and cast it in the loop body. Added example to the answer. – Cheers and hth. - Alf Feb 08 '16 at 12:53
@HumamHelfawi if your map contains all 4 enum values, then you can just use range-for loop. – Revolver_Ocelot Feb 08 '16 at 12:56

score 1 · Accepted Answer · answered Feb 08 '16 at 13:01

1

Define your types like this:

enum class my_enum{a=1, b, c, d, END};
std::map<my_enum,my_class> my_map;

my_enum& operator++( my_enum &val ) {
  using IntType = typename std::underlying_type<my_enum>::type
  val = static_cast<my_enum>( static_cast<IntType>(val) + 1 );
  return val;
}

And then you can use them like this:

for (my_enum e = my_enum::a; e < my_enum::END; ++e) {
   my_class c = my_map[e];
   ....
}

(Although, you might prefer to use something other that operator[] if you don't want to create null-values for entries that don't exist.)

answered Feb 08 '16 at 13:01

ams

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What do you do if some header defines a macro named `END`? – Cheers and hth. - Alf Feb 08 '16 at 13:17
@Cheersandhth.-Alf Add a header that defines a macro named `class`, and run away. – ams Feb 08 '16 at 13:20
@ams: Just by conforming to the usual C++ convention and avoiding the all uppercase name, you can avoid that problem. No advantage in the Java-ism. Only problems, and code that shouts. – Cheers and hth. - Alf Feb 08 '16 at 13:23

Iterate using an enum

2 Answers2