Q: Using an extern const size_t?

[Jakob Bieling]

Hi,

I have the following scenario:

--- file1.cpp ---

size_t const array_size = 10;

--- end file1.cpp ---

--- file2.cpp ---

class test
{
int m_nums [array_size];
};

--- end file2.cpp ---

My compiler, VC++7.1, complains about not having a constant expression
for the array. Why can the array size not be 'calculated' at compile-time?
Or is it just a bug in VC++?

Thanks!

 

[Jakob Bieling]

I forgot a line:

--- file1.cpp ---

size_t const array_size = 10;

--- end file1.cpp ---

--- file2.cpp ---

extern size_t const array_size;

class test
{
int m_nums [array_size];
};

--- end file2.cpp ---

Sorry about that

 

[John Harrison]

Hi,

I have the following scenario:

--- file1.cpp ---

size_t const array_size = 10;

--- end file1.cpp ---

--- file2.cpp ---

I guessing that you missed out

extern const size_t array_size;

here.

class test
{
int m_nums [array_size];
};

--- end file2.cpp ---

My compiler, VC++7.1, complains about not having a constant expression
for the array. Why can the array size not be 'calculated' at compile-time?
Or is it just a bug in VC++?

Put

size_t const array_size = 10;

in a header file and include that header file in file1.cpp and file2.cpp.
This is OK because in C++ constants have internal linkage by default so you
won't get an error about array_size being declared twice.

john

 

[Jakob Bieling]

I guessing that you missed out

extern const size_t array_size;

here.

Yes, see my reply to myself

class test
{
int m_nums [array_size];
};

--- end file2.cpp ---

My compiler, VC++7.1, complains about not having a constant expression
for the array. Why can the array size not be 'calculated' at compile-time?
Or is it just a bug in VC++?

Put

size_t const array_size = 10;

in a header file and include that header file in file1.cpp and file2.cpp.
This is OK because in C++ constants have internal linkage by default so you
won't get an error about array_size being declared twice.

Right, but why does the above example not work? Theoretically, it
should, or what am I missing?

 

[Artie Gold]

I guessing that you missed out

extern const size_t array_size;

here.

Yes, see my reply to myself

class test
{
int m_nums [array_size];
};

--- end file2.cpp ---

My compiler, VC++7.1, complains about not having a constant

expression

for the array. Why can the array size not be 'calculated' at

compile-time?

Or is it just a bug in VC++?

Put

size_t const array_size = 10;

in a header file and include that header file in file1.cpp and file2.cpp.
This is OK because in C++ constants have internal linkage by default so
you

won't get an error about array_size being declared twice.

Right, but why does the above example not work? Theoretically, it
should, or what am I missing?

You are missing the fact that the value of `array_size' will not be
known until link time (as it is defined in a different translation unit).

HTH,
--ag

 

[Jakob Bieling]

--- file1.cpp ---

size_t const array_size = 10;

--- end file1.cpp ---

--- file2.cpp ---
extern const size_t array_size;
class test
{
int m_nums [array_size];
};

--- end file2.cpp ---

Right, but why does the above example not work? Theoretically, it
should, or what am I missing?

You are missing the fact that the value of `array_size' will not be
known until link time (as it is defined in a different translation unit).

Oh .. makes sense now, yes!

Thanks!