Global variables - Revision history

Vegard: clarify

2020-02-10T10:34:26Z

clarify

← Older revision		Revision as of 10:34, 10 February 2020
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	but here, the argument may not be a temporary expression.		but here, the argument may not be a temporary expression.

	~~Also note~~ that any such globals declared in the parameter list must be declared after all the other parameters, since they are default-initialized to the global!		Note that any such globals declared in the parameter list must be declared after all the other parameters, since they are (local) references default-initialized as pointing to the global variable!

	[[Category:Programming]]		[[Category:Programming]]

Vegard: add note

2020-02-10T10:29:02Z

add note

← Older revision		Revision as of 10:29, 10 February 2020
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	</source>		</source>
	but here, the argument may not be a temporary expression.		but here, the argument may not be a temporary expression.

			Also note that any such globals declared in the parameter list must be declared after all the other parameters, since they are default-initialized to the global!

	[[Category:Programming]]		[[Category:Programming]]

Vegard: new page

2020-02-10T09:52:03Z

new page

New page

Global variables are discouraged by a lot of people for various different reasons [https://softwareengineering.stackexchange.com/questions/148108/why-is-global-state-so-evil]. One objection is that it becomes harder to reason about (and test) functions because they have a "hidden" input (i.e. the global state).

I think global variables have their place, and are honestly not that different from, say, instance variables used by methods. One way to simply make it clear that global state is used in the code would be to provide a couple of wrappers for defining and declaring the use of global variables, respectively:

Definition:

<source lang="C++">
#define DEFINE_GLOBAL(type, name) __thread typeof(type) global_##name
#define USE_GLOBAL(name) typeof(global_##name) &name = global_##name
</source>

Usage:

<source lang="C++">
#include <cstdio>

DEFINE_GLOBAL(int, x);

void foo()
{
USE_GLOBAL(x);

printf("x = %d\n", x);
}

#if 0 // doesn't work, since x is undeclared
void bar()
{
printf("x = %d\n", x);
}
#endif

// complex type declarations also work:
DEFINE_GLOBAL(void (*)(int), fn);
</source>

If you want to be able to override the global for a function (e.g. for testing using mock data, without actually modifying the global), one could even put the declaration in the parameter list:

<source lang="C++">
void baz(USE_GLOBAL(x))
{
printf("x = %d\n", x);
}
</source>

This function can be called either as <source lang="C++">baz()</source> in which case it will use the actual global variable, or
<source lang="C++">
int value = 12;
baz(value);
</source>
but here, the argument may not be a temporary expression.

[[Category:Programming]]
[[Category:C++]]