CODEWITHSUNDEEP
c++constantscompile-time-constantconstant-expression
DevSolar
DevSolar

Reputation: 70263

Declaring symbolic constants in header without initializing?

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 (e.g. 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 e.g. 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?

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

Upvotes: 1

Views: 457

Answers (3)

Dr. Debasish Jana
Dr. Debasish Jana

Reputation: 7118

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;

Upvotes: 0

bigblackdot
bigblackdot

Reputation: 138

You can hold method/function pointers mapped to T_Type somewhere, but yep, it's all just hacks out of problem which don't worth to be created in first place - hardcoded logic can only work with hardcoded values.

Upvotes: 0

Cheers and hth. - Alf
Cheers and hth. - Alf

Reputation: 145249

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.

Upvotes: 2

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