在标头中声明符号常量而不进行初始化？

Question

It is common practice to define symbolic constants in a header file:

#define T_FOO 1
#define T_BAR 2

Ugly.

static const int T_FOO = 1;
static const int T_BAR = 2;

Better, since not preprocessor.

enum
{
    T_FOO = 1,
    T_BAR
} T_Type;

Better still, since T_Type carries information of purpose, and the compiler can do additional checks (eg if all cases are handled in a switch ).

There's probably half a dozen more variants. One thing though... they all disclose numerical values to the client. I'd like to keep those values hidden, simply because they shouldn't matter. But the one way I could think of...

typedef int T_Type;

// defined elsewhere
extern const T_Type T_FOO;
extern const T_Type T_BAR;

... does not work for eg case statements (as T_FOO and T_BAR are constants, but not a compile-time constant expressions).

Is there a way to have it all?

• Declaring symbolic constants in a header without disclosing numerical values,
• but useable as constant expressions eg in switch statements?

My level of understanding says "no", but I know that I don't know everything. ;-)

Answer 1

To be usable in as switch statement labels the values have to be seen by the compiler earlier in the source of this translation unit.

So essentially, no , you can't declare symbolic constants without disclosing their values, and use them as labels in a switch .

However, you can use an if - else construction.

Answer 2

您可以将映射到T_Type的方法/函数指针保存在某个地方，但是，是的，这仅仅是出于问题的解决，因此不值得首先创建-硬编码逻辑只能与硬编码值一起使用。

Answer 3

Your typedef declaration was wrong. What about this one?

typedef int T_Type;

// defined elsewhere

extern const T_Type T_FOO;
extern const T_Type T_BAR;

// elsewhere defined as, say
const T_Type T_FOO = 1; 
const T_Type T_BAR = 2;