c definition and declaration

[himanshumodi]

plz tell me that in c..
whether
int a;
is declaration or definition??
will memory be allocated at the time it is written...

[JosAH]

It is a tentative definition; I normally use the declaration 'extern int a' in a header
file and use one tentative definition in exactly one .c file: 'int a'.

kind regards,

Jos

[scottdavie]

memory will be allocated. Although you have set no number, a number will be stored in a. If you

cout << a << endl;

after int a; then a number is still displayed.

[weaknessforcats]

Nothing tentative about it. Memory is allocated. That makes it a definition. It's not initialized to any particular value but that is irrelevant.

As a definition, it is not supposed to be in a header file.

[JosAH]

Nothing tentative about it.

Yes it is; see the C99 Standard for this (I can't quote it here because it is all
over the place).

kind regards,

Jos

[weaknessforcats]

Well, there you go, I gave a C++ answer again on a C topic.

[scottdavie]

They should only be defined in header files if they are part of a function or a class. Otherwise I dont believe they should be.

[weaknessforcats]

I repeat, definitions do not belong in header files.

Every time you include that header you get another definition. Multiple definitions will cause problems at link time.

[scottdavie]

I repeat, definitions do not belong in header files.

Every time you include that header you get another definition. Multiple definitions will cause problems at link time.

Using #ifndef would prevent this problem

[edwardrsmith]

But that doesn't mean that it is good style. By including definitions in a header file you are also slowing down compile time. The entire reason that there are header files and libraries is that way it is unnecessary to compile the entire library every time it is used. This would be incredibly tome consuming and a pain. Instead a header file contains the decleration of the function (and often the documentation for the function as well) while the library contains the compiled but unlinked definitions.

If you search for header files in google you will get a lot of good examples of this.

Edward

[JosAH]

Using #ifndef would prevent this problem

No it wouldn't; different compilation units can include the same header file
(with a definition in it) and there is no #ifndef that can prevent another definition
in another compilation unit.

kind regards,

Jos

[edwardrsmith]

But that doesn't mean that it is good style. By including definitions in a header file you are also slowing down compile time. The entire reason that there are header files and libraries is that way it is unnecessary to compile the entire library every time it is used. This would be incredibly tome consuming and a pain. Instead a header file contains the decleration of the function (and often the documentation for the function as well) while the library contains the compiled but unlinked definitions.

If you search for header files in google you will get a lot of good examples of this.

Edward

EDIT: I am not saying that it is unnecessary to use ifndef but instead am saying that ifndef is only part of the solution.

Sorry: hit the wrong button and didn't notice.

[edwardrsmith]

I think he was talking about using ifndef combined with define in the format of

#ifndef test_h
#define test_h
 
// Code
 
#endif

This would prevent it from being defined twice as it would only be compiled once.

Edward

[JosAH]

I think he was talking about using ifndef combined with define in the format of

#ifndef test_h
#define test_h
 
// Code
 
#endif

This would prevent it from being defined twice as it would only be compiled once.

Edward

Sure but it can't prevent multiple definitions over multiple translation units.
Multiple defintions in one translation unit are signalled by the compiler.

kind regards,

Jos