在结构中使用枚举值

Question

I have the following class:

class MyClass 
{
    public:

        enum myEnum
        {
            a = 0b0,
            b = 0b1,
        };

        union myUnion
        {
            uint32_t all;

            struct myStruct
            {
                uint32_t start     : 0b1;
                uint32_t enumValue : myEnum::a;
            } bits;
        };

    ...

};

I am simply unsure how to use the value of myEnum::a within the struct . I've tried a number of ways of referencing the value to no avail. How would I achieve this?

Here's the error: Error: Name followed by "::" must be a class or namespace name in "MAX1300BEUG/MAX1300.h", Line: 50, Col: 37

Thanks, Adam

Answer 1

Enums don't have a scope. Therefore applying the scope resolution operator :: to myEnum is wrong.

The enum values are declared in the enclosing namespace, so within myStruct , which is within the same scope as myEnum , you can refer to its values directly using unqualified lookup: a , b .

Outside of MyClass , it can be resolved with MyClass::a .

---

PS The value of a is 0. A zero width bitfield may not be named. There is some sort of disconnect between what you're trying to do, and what you're trying to achieve by doing it.

Answer 2

myEnum::a is defined as 0 . Zero width bit fields are not allowed in this context.

First, fix your typos (there are several of them, as you can see in the comment section of your question). Second, don't try to create a zero width bit field in this context.

Answer 3

There is much going on here, so I will try to address these items one at a time.

Enum Value Scope Resolution - in C++, values of an enumeration are declared in the same scope as the enumeration (unlike C#, wherein the enumeration is a namespace for its values). The fully qualified scope of your enum values are ::MyClass::a and ::MyClass::b . All prefixes components in the fully qualified scope that are shared with the referencing code's scope may be omitted as long as omission does not result in the desired value being hidden by another value of the same name. So in the example above, you may refer to the values as a or b .

The other problem is that MyClass::myStruct::enumValue is being declared as a zero-length bit-field because MyClass::a has the numeric value of 0.