Assigning values after defining an enumeration

Question

I'm a newbie to C++ and especially C++11, so since I've now got to use it, a few questions about 'enum' and 'enum class' came up:

Can I assign values after the enumeration has been declared?

enum MyEnum;
MyEnum::HELLO = 0;
MyEnum::WORLD = 1;

Can I assign values to a number? (ex.: Myenum::0 = 2)

enum MyEnum;
MyEnum::0 = 16;
MyEnum::1 = 24;
MyEnum::3 = 64;

How does enum class work when using a struct or class as the underlying type?

Would the entries in the enumeration be valid instances of the struct/class?

class Test {
    private int v = 0;
    Test(int v) {
        this->v = v;
    }
};

enum class MyEnum : Test {
    Test0 = new Test(0),
    Test1 = new Test(1),
};

I found these links when I searched for the topic:

Which, as you can see, left a few questions.

Here's another one, more general: http://stackoverflow.com/questions/14041711/whats-an-enum-class-and-why-should-i-care/14042083#14042083 — Potatoswatter, Jan 06 '13 at 13:52

score 4 · Accepted Answer · edited May 23 '17 at 12:12

4

No. You cannot do any of those.

However, you may provide the definition the enum after its declaration as:

enum MyEnum; //declaration

enum MyEnum  //definition
{
  HELLO = 0,
  WORLD = 1;
};

Can I assign values to a number?

No. That doesn't make sense. A number already has a value. It is constant.

Please get an introductory book on programming in C++. Here are few recommendations :

The Definitive C++ Book Guide and List

edited May 23 '17 at 12:12

Community

1
1

answered Jan 06 '13 at 13:43

Nawaz

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Ok, so that clears up most of the questions I have. the only one remaining is: Why doesn't GCC pick this up as being wrong? All of the above examples compile fine with my copy of the GCC compiler (using -w -Wfatal-errors -Wextra -Wall -Wmain -std=c++11), I'm not sure if they however run fine. – Xaymar Jan 06 '13 at 14:28
None of them will compile. Post your code here. http://www.ideone.com and give me the link. I wanna see it myself. – Nawaz Jan 06 '13 at 14:31
My Error, I just forgot to include the file (so used to that being done automatically from BlitzMax and BlIde). Now it doesn't compile with an error :). – Xaymar Jan 06 '13 at 14:43

score 2 · Answer 2 · answered Jan 06 '13 at 13:46

First of all, you cannot ever assign to a value in C++. The following is illegal:

42 = 23;

And for pretty much the same reason you cannot assign the value of an enum either.

Furthermore, you cannot even use the values of an enum after its declaration (i.e. you cannot write auto x = enum_name::name;), only after definition. You can only use the enum’s name.

Can I assign values to a number?

I’m not sure what this is supposed to mean but apart from what I’ve said before that syntax is illegal anyway. That is, you cannot access the second enum value by writing enum_name::1.

How does enum class work when using a struct or class as the underlying type?

You cannot use custom types as underlying types, only built-in integral types.

The 'Can I assign a value to a number' was meant, that the 'key' for the enumeration is numeric (e.g. '4'), so that it would be accessible via MyEnum::4. But I guess that is not possible, so I'll have to find another way of listing entries by their Id. — Xaymar, Jan 06 '13 at 14:37
@Xaymar Yes, enumeration keys must be valid identifiers so they cannot be numeric. — Konrad Rudolph, Jan 06 '13 at 14:38

Assigning values after defining an enumeration

2 Answers2