Why static const members cannot appear in a constant expression like 'switch'

Question

I have the following declaration of some static const members

.h

class MyClass : public MyBase
{
public:
    static const unsigned char sInvalid;
    static const unsigned char sOutside;
    static const unsigned char sInside;
    //(41 more ...)
}

.cpp

const unsigned char MyClass::sInvalid = 0;
const unsigned char MyClass::sOutside = 1;
const unsigned char MyClass::sInside = 2;
//and so on

At some point I want to use those value in a switch like :

unsigned char value;
...
switch(value) {
    case MyClass::sInvalid : /*Do some ;*/ break;
    case MyClass::sOutside : /*Do some ;*/ break;
    ...
}

But I get the following compiler error: error: 'MyClass::sInvalid' cannot appear in a constant-expression.

I have read other switch-cannot-appear-constant-stuff and didn't find an answer for me since I don't get why those static const unsigned char are not constant-expression.

I am using gcc 4.5.

Answer 1

The problems you see are due to the fact that this

static const unsigned char sInvalid;

cannot be a compile time constant expression, since the compiler doesn't know its value. Initialize them in the header like this:

class MyClass : public MyBase
{
public:
    static const unsigned char sInvalid = 0;
    ...

and it will work.

Answer 2

The values are indeed const, but they're not compile-time constants.

A switch condition is resolved at compile-time, not run-time. You can initialize sInvalid to whatever value, as long as it's only once, and the switch would never know about it until run-time.

It seems like you're better of using enums instead of static constants. Besides the fact that it would work, it seems more appropriate design-wise.