Difference between revisions of "Global variables"

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{{Global variables}}
 
{{Global variables}}
  
A global [[Variable]] is a variabe that is declared in the main section of a program or the interface part of a unit. Globals declared in a program cannot be accessed from within a unit. Globals declared in a unit can be accessed from within a program or another unit.
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A [[Variable|variable]] is global if it is exported from a module.
 +
This usually refers to variables declared in a [[Var|<syntaxhighlight lang="pascal" inline>var</syntaxhighlight> section]]
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* prior any other nested [[Block|block]] in a [[Program|<syntaxhighlight lang="pascal" inline>program</syntaxhighlight>]], or
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* in the <syntaxhighlight lang="pascal" inline>interface</syntaxhighlight> part of a [[Unit|unit]].
  
  program GlobalVariables;
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Global variables can be accessed from all other modules that import the exporting modules.
  var
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Note, however, that a <syntaxhighlight lang="pascal" inline>program</syntaxhighlight> can not be imported.
    g: integer;
 
  begin
 
  end.
 
  
'''g''' is a global variable.
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<syntaxhighlight lang="pascal">
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program globalVariableDemo(input, output, stdErr);
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var
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x: integer;
  
 +
procedure doMagic;
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begin
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// here, x is global to doMagic
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end;
  
== External variable ==
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procedure foo;
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var
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// shadow the global x
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x: integer;
 +
begin
 +
// here, x is local,
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// as the top-scope x can not be accessed
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end;
  
External variable is a global variable has been declared in [[Unit|unit]] [[Interface|interface]] section.
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// MAIN //
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begin
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// here, x is local
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end.
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</syntaxhighlight>
  
== Read More ==
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== remarks ==
* [[Var]]
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If speed matters, global variables are/were used for frequently invoked routines, since allocating [[Local variables|local variables]] on the stack takes time.
* [[Local_variables]]
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This, however, is considered bad style.
 +
Nest your variables as deep as possible, but as high as necessary.
  
[[Category:Pascal]]
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A [[Resourcestring|<syntaxhighlight lang="delphi" inline>resourceString</syntaxhighlight> variable]] is always global.
 +
 
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The [[FPC]] supports thread variables, too.
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They are sort of half-way between global and local variables.
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A [[ThreadVar|<syntaxhighlight lang="delphi" inline>threadVar</syntaxhighlight>iable]] is local to a thread.
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== see also ==
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* [[Scope|Tutorial: Scope]]
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* [[Singleton Pattern|singleton pattern]]
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* [https://en.wikipedia.org/wiki/Global_variable Article “Global variable” on the English Wikipedia]
 +
 
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[[Category:Code]]

Latest revision as of 13:44, 16 October 2019

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A variable is global if it is exported from a module. This usually refers to variables declared in a var section

Global variables can be accessed from all other modules that import the exporting modules. Note, however, that a program can not be imported.

program globalVariableDemo(input, output, stdErr);
var
	x: integer;

procedure doMagic;
begin
	// here, x is global to doMagic
end;

procedure foo;
var
	// shadow the global x
	x: integer;
begin
	// here, x is local,
	// as the top-scope x can not be accessed
end;

// MAIN //
begin
	// here, x is local
end.

remarks

If speed matters, global variables are/were used for frequently invoked routines, since allocating local variables on the stack takes time. This, however, is considered bad style. Nest your variables as deep as possible, but as high as necessary.

A resourceString variable is always global.

The FPC supports thread variables, too. They are sort of half-way between global and local variables. A threadVariable is local to a thread.

see also