| Цитата(Delphi) | Labels
Labels are used in built-in assembly statements as they are in the Delphi language--by writing the label and a colon before a statement. There is no limit to a label's length. As in Delphi, labels must be declared in a label declaration part in the block containing the asm statement. The one exception to this rule is local labels.
Local labels are labels that start with an at-sign (@). They consist of an at-sign followed by one or more letters, digits, underscores, or at-signs. Use of local labels is restricted to asm statements, and the scope of a local label extends from the asm reserved word to the end of the asm statement that contains it. A local label doesn't have to be declared. |
| Цитата(Delphi) | The @ operator
The @ operator returns the address of a variable, or of a function, procedure, or method; that is, @ constructs a pointer to its operand. For more information about pointers, see Pointers and pointer types. The following rules apply to @.
If X is a variable, @X returns the address of X. (Special rules apply when X is a procedural variable; see Procedural types in statements and expressions.) The type of @X is Pointer if the default {$T-} compiler directive is in effect. In the {$T+} state, @X is of type ^T, where T is the type of X (This distinction is important for assignment compatibility, see "Assignment-compatibility" on page 5-36).
If F is a routine (a function or procedure), @F returns F's entry point. The type of @F is always Pointer.
When @ is applied to a method defined in a class, the method identifier must be qualified with the class name. For example,
points to the DoSomething method of TMyClass. For more information about classes and methods, see Classes and objects.
Note
When using the @ operator, it is not possible to take the address of an interface method as the address is not known at compile time and cannot be extracted at runtime. |
| Цитата(Delphi) | Procedural types in statements and expressions
When a procedural variable is on the left side of an assignment statement, the compiler expects a procedural value on the right. The assignment makes the variable on the left a pointer to the function or procedure indicated on the right. In other contexts, however, using a procedural variable results in a call to the referenced procedure or function. You can even use a procedural variable to pass parameters:
| Код | var
F: function(X: Integer): Integer;
I: Integer;
function SomeFunction(X: Integer): Integer;
...
F := SomeFunction; // assign SomeFunction to F
I := F(4); // call function; assign result to I |
In assignment statements, the type of the variable on the left determines the interpretation of procedure or method pointers on the right. For example,
| Код | var
F, G: function: Integer;
I: Integer;
function SomeFunction: Integer;
...
F := SomeFunction; // assign SomeFunction to F
G := F; // copy F to G
I := G; // call function; assign result to I |
The first statement assigns a procedural value to F. The second statement copies that value to another variable. The third statement makes a call to the referenced function and assigns the result to I. Because I is an integer variable, not a procedural one, the last assignment actually calls the function (which returns an integer).
In some situations it is less clear how a procedural variable should be interpreted. Consider the statement
| Код | if F = MyFunction then ...; |
In this case, the occurrence of F results in a function call; the compiler calls the function pointed to by F, then calls the function MyFunction, then compares the results. The rule is that whenever a procedural variable occurs within an expression, it represents a call to the referenced procedure or function. In a case where F references a procedure (which doesn't return a value), or where F references a function that requires parameters, the previous statement causes a compilation error. To compare the procedural value of F with MyFunction, use
| Код | if @F = @MyFunction then ...; |
@F converts F into an untyped pointer variable that contains an address, and @MyFunction returns the address of MyFunction.
To get the memory address of a procedural variable (rather than the address stored in it), use @@. For example, @@F returns the address of F.
The @ operator can also be used to assign an untyped pointer value to a procedural variable. For example,
| Код | var StrComp: function(Str1, Str2: PChar): Integer;
...
@StrComp := GetProcAddress(KernelHandle, 'lstrcmpi'); |
calls the GetProcAddress function and points StrComp to the result.
Any procedural variable can hold the value nil, which means that it points to nothing. But attempting to call a nil-valued procedural variable is an error. To test whether a procedural variable is assigned, use the standard function Assigned:
| Код | if Assigned(OnClick) then OnClick(X); |
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| Цитата(Symbols) | | Код | Symbol Value Class Type
@Result Result variable offset Memory reference Size of type |
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