error in c, but not in c++

Question

const int t=5;
char buf[t+5];

When I compile this gives error in C but not in C++!!
Can anybody please explain me the reason?

Note: I know the const defaults to internal linkage in 'C++', where as in 'C' it defaults to external linkage. Does it has any relation to the above case??

Answer 1

I think it's because the compiler can't evaluate t+5 to a constant expression. It looks like it should be OK, but:

One important point about array declarations is that they don't permit the use of varying subscripts. The numbers given must be constant expressions which can be evaluated at compile time, not run time.

Source

Answer 2

Yes, this is related to C's external linking of t.

You've declared an externally linked integer t. If you link this file with another that defines t, then your buffer's size would have to be determined after the file's compile-time, which is of course impossible in C.

Answer 3

This isn't valid in C89 C, though it may be valid in C99

See this stack overflow question

Answer 4

In C the Size of an Array has to be an Constant Expression. Const Int is in C not an Constant Expression. It's meaning is more like "readonly". Use #define t 5 instead.

Answer 5

As others explained, C is kept more simple than C++ and doesn't allow const variables to appear in integer constant expressions. But in both C89 and C++ declared arrays must have compile-time constant sizes.

You can use enumerations for this

enum {
  BufSize = 5
};

char buf[BufSize + 5];

It doesn't have to do with internal linkage - external linkage variables are equally viable in integer constant expressions in C++. The internal linkage in C++ rather is a consequence, but not a neccessity, of allowing them to appear in constant expressions. The C++ Standard explains why they have internal linkage by default

Because const objects can be used as compile-time values in C++, this feature urges programmers to provide explicit initializer values for each const. This feature allows the user to put const objects in header files that are included in many compilation units