constants not constant?

[geneing]

How come this is illegal?

const int N = 10;
double a[ N ];

I get "error: variable-size type declared outside of any function".
What's the reason behind this syntax rule?

 

[Chris McDonald]

How come this is illegal?

const int N = 10;
double a[ N ];

I get "error: variable-size type declared outside of any function".
What's the reason behind this syntax rule?

N is a variable whose value is set at run-time.
Hence the size of 'a' cannot be known at compile time.

 

[Peter Nilsson]

How come this is illegal?

const int N = 10;
double a[ N ];

I get "error: variable-size type declared outside of any
function". What's the reason behind this syntax rule?

In C, const does not mean constant, it means read only.

C99 supports variable length arrays, but only with automatic
(local to function) storage.

 

[Daniel Rudy]

How come this is illegal?

const int N = 10;
double a[ N ];

I get "error: variable-size type declared outside of any function".
What's the reason behind this syntax rule?

A const declaration creates a read-only variable. If you want a true
constant who's value is known at compile time, then do this:

#define N 10

 

[Eric Sosman]

How come this is illegal?

const int N = 10;
double a[ N ];

I get "error: variable-size type declared outside of any function".
What's the reason behind this syntax rule?

There is no syntax problem here; no "syntax rule" is violated.
This is a semantic violation, namely, the syntactically correct but
non-constant `N' is used in a context where a constant expression
is required. (As others have explained, `const' is only 62.5%
`constant'.)

 

[geneing]

A const declaration creates a read-only variable. If you want a true

constant who's value is known at compile time, then do this:

#define N 10

Wasn't the point of having "const" (in part) to be able to replace
#define?

 

[E. Robert Tisdale]

How come this is illegal?

const int N = 10;
double a[N];

I get "error: variable-size type declared outside of any function".
What's the reason behind this syntax rule?

This is a flaw in the design of the C computer programming language
which was fixed in the C++ computer programming language
but could not be fixed in C
because so much legacy code depended upon it.

 

[Artie Gold]

Wasn't the point of having "const" (in part) to be able to replace
#define?

Yes. In *C++*. Not C.

HTH,
--ag

 

[CBFalconer]

Wasn't the point of having "const" (in part) to be able to replace
#define?

Not in the C language. If you want C++ that is another newsgroup.

 

[Peter Nilsson]

How come this is illegal?

const int N = 10;
double a[N];

I get "error: variable-size type declared outside of any
function". What's the reason behind this syntax rule?

This is a flaw in the design of the C computer programming
language which was fixed in the C++ computer programming
language but could not be fixed in C because so much legacy
code depended upon it.

C took the const keyword from C++. Thus, C++ did not 'fix'
existing C usage of const, and there was no 'legacy code'
to speak of.

 

[Daniel Rudy]

At about the time of 5/10/2005 6:50 PM, (e-mail address removed) stated the
following:

Wasn't the point of having "const" (in part) to be able to replace
#define?

AFAIK, that applies only in C++. In C, you still have to use #define
for true compile time constants.

 

[Lawrence Kirby]

How come this is illegal?

const int N = 10;
double a[N];

I get "error: variable-size type declared outside of any function".
What's the reason behind this syntax rule?

This is a flaw in the design of the C computer programming language
which was fixed in the C++ computer programming language
but could not be fixed in C
because so much legacy code depended upon it.

How would "fixing" this in C break legacy code?

Lawrence