var, out, and const in Delphi function parameter decoration

(1 ) var modifier

Adding Var is an address pass that modifies the original variable

Var

s:string;

Begin

S: = ' Hello ';

Changesvar (s);

ShowMessage (S);

End

Changesvar definition

Procedure Tform1.changesvar (var a:string);

Begin

A: = A + ' world ';

End

The above will output Hello world, because it is the address, modify the original A

(2 ) without any modifiers

Var

s:string;

Begin

S: = ' Hello ';

Changes (s);

ShowMessage (S);

End

Changes definition

Procedure Tform1.changes (a:string);

Begin

A: = A + ' world ';

End

The above will output hello, because the method changes actually creates a new a, and the output is the original A, the value does not change

(3 ) out modifier

Var

s:string;

Begin

S: = ' Hello ';

Changesout (s);//At this time the value of S is ' Hello ' instead of ' Hello,world '!, the original value of s in the procedure Hello is discarded

ShowMessage (S);

End

Changesout definition

Procedure Tform1.changesout (out a:string);

Begin

A: = A + ' world ';

End

The above output world,out only accept the returned value, and any input to out will be ignored. The actual arguments that are passed to the procedure at the same time do not have to be initialized, such as calls to Changesout:

Var

tmp:string;

Begin

Changesout (TMP);//compilation can also be done by

End;

(4) Const modifier

Const-decorated parameters are not allowed to be modified after they are passed in

If you modify parameters in the process, you will get an error, such as:

Procedure xxxx. Testconst (const a:string);

Begin

A: = ' ss '; Attempt to modify const modified parameters will be an error

End;

var, out, and const in Delphi function parameter decoration