Defining variables in headers
Author
Hi,
There's this rule of thumb that I've been using for several years now, and the funny thing is, I can't even remember why I use this rule any more, and that scares me because for all I know, maybe this rule isnt even valid. :)
So, here's my rule... I never define a variable in a header... So for example if I have a constant I want, then I have to do this:
(In blah.h:)
extern const int width;
(In blah.cpp:)
const int width = 50;
Now the thing is I'm working on a project, and there everyone simply does this:
(In blah.h:)
const int width = 50;
So my question is, is there any downside to simply defining variables in the .h file, or have I been making trouble for myself all this time? :)
Thanks,
roos
Well if you ask me, your way is less likely to come up with conflicts between modules, so is therefore the better way.
Think about it like this; You include a header in 2 different C files (So the include guards wont kick in), and that variable will be 'compiled in' to both.
Then, when you get to the linking step, the linker will discover a conflict between the 2 files and wont link.
With your method, it's always going to connect them up at the linking stage, so you wont get the conflict. :)
God I'm tired, hope that made sense.
Think about it like this; You include a header in 2 different C files (So the include guards wont kick in), and that variable will be 'compiled in' to both.
Then, when you get to the linking step, the linker will discover a conflict between the 2 files and wont link.
With your method, it's always going to connect them up at the linking stage, so you wont get the conflict. :)
God I'm tired, hope that made sense.
Global variables are evil in some peoples eyes. My friend was docked marks for each and every global he had. They make your .exe file larger from my understanding. The way you do it seems to be the way most professional programmers do it too. If you're working on a project with more then 2 programmers you really should have a programmers development guide that lays out formatting, indents, etc in it and include this.
Hope this helped in someway.
Hope this helped in someway.
Author
Thanks guys. I made a quick test of this.. Actually it looks like what you said is right PlayfulPuppy, although if it's a CONST variable, then no conflict occurs
roos
roos
For a const, may as well put it in the header.
Anything that's not const does not belong there, because there needs to be actual storage to hold it then, and you'll have trouble if the header is used in multiple translation units - include guards can only protect you within the same translation unit, and then you get 'multiply defined symbols' (several object files containing what is supposed to be the same variable with the same name, and the linker doesn't know which to use).
Anything that's not const does not belong there, because there needs to be actual storage to hold it then, and you'll have trouble if the header is used in multiple translation units - include guards can only protect you within the same translation unit, and then you get 'multiply defined symbols' (several object files containing what is supposed to be the same variable with the same name, and the linker doesn't know which to use).
Some of it depends on where and how you define your variables in the header. For example, we will do something like this.
[src]
class Box
{
public:
static const int kMaxWidgets = 7;
Box()
~Box()
Widget get(int index)
private:
Widget mWidgets[kMaxWidgets];
};
[/src]
in this case, the static const int acts the same as a #define, but is more Standard C++ compliant.
[src]
class Box
{
public:
static const int kMaxWidgets = 7;
Box()
~Box()
Widget get(int index)
private:
Widget mWidgets[kMaxWidgets];
};
[/src]
in this case, the static const int acts the same as a #define, but is more Standard C++ compliant.
The right tags are [-code-] and [-/code-] (without the -).
You're defining the variable in a class and you have to make sure, that you are not redefining your class, so it's basically the same as with the global variables.
If you want to make a variable global for only one compilation unit, you can put it in an anonymous namespace, for example:
If you put this into a header file every compilation unit that includes this header has its own counter global variable.
You're defining the variable in a class and you have to make sure, that you are not redefining your class, so it's basically the same as with the global variables.
If you want to make a variable global for only one compilation unit, you can put it in an anonymous namespace, for example:
namespace{ int counter = 0;}If you put this into a header file every compilation unit that includes this header has its own counter global variable.
lukeyes: Your src should be source, then it will work correctly. Or the code tags like ext said.
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