When I do declaration in C++ ,
static const int SECS = 60 * MINUTE; const static int SECS = 60 * MINUTE; is there any difference between these two?
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2 Answers
is there any difference between these two?
No. Not at all. The order doesn't matter (in this case!).
Moreover, if you write this:
const int SECS = 60 * MINUTE; //at namespace level at namespace level, then it is equivalent to this:
static const int SECS = 60 * MINUTE; Because at namespace level const variables has internal linkage by default. So the static keyword doesn't do anything if const is already there — except for increasing the readability.
Now what if you want the variable to have external linkage and const at the same time, well then use extern:
//.h file extern const int SECS; //declaration //.cpp file extern const int SECS = 60 * MINUTE; //definition Hope that helps.
5 Comments
Tristan Brindle
It's worth pointing out that this is one of the differences between C99 and C++ --
const variables at file scope (namespace scope in C++) have internal linkage in C++ and external linkage in C99 -- see here
Sarfaraz Nawaz
@TristanBrindle: Thanks for sharing that knowledge. I didn't know that. I don't know C/C99 enough. I don't code in C/C99. Fortunately this question is tagged as
C++ only.
gx_
"The order doesn't matter in this case" maybe, nevertheless "
const static" will always seem wrong to me. static is a storage-class-specifier, it applies to the variable, whereas const is a cv-qualifier, it applies to the type (to the left, or immediate right). So to me the correct pattern is "static int const SECS" (or "static const int SECS"). (A maybe better example is static int const* p (or static const int* p) where p is declared static but not const, whereas the pointed-to int is declared const but not static.)Sarfaraz Nawaz
@gx_:
static is a muti-facted keyword: it defines storage; it defines linkage; it defines membership (with class and instances). But you have a point, nevertheless. :-)gx_
@Nawaz The point is not that
static defined as a "storage class specifier" by the grammar (like extern btw) or that it has different meanings depending on context (both are true). The point is that it should come first in a declaration, before any type or const. (Edit: ok I see you actually agreed.) Btw I just found this: stackoverflow.com/a/12006039const always applies to the type to its immediate left; if there is none, it applies to the next type on its right.
So the following three declarations
const static int SECS = 60 * MINUTE; // or static const int SECS = 60 * MINUTE; // or static int const SECS = 60 * MINUTE; are all equal. static applies to the whole declaration; and const applies to the int type.
The position of const would only make a difference if you had a "more complicated" type, like e.g. a reference or a pointer:
int a; const int * b = a; // 1. int * const c = a; // 2. In this case there is a difference between the place of const - for 1. it applies to the int (i.e. it is a pointer to a const int, i.e. you can't change the value), and for 2., it applies to the pointer (i.e. you can't modify where c points to).
