1、 TStrings

TStrings is the base class for objects that represent a list of strings.

Unit

Classes

Description

Derive a class from TStrings to store and manipulate a list of strings. TStrings contains abstract methods and should not be directly instantiated.

Descendants of TStrings can represent several individual strings, such as the individual lines that appear in a list box. Some objects use descendants of TStrings to represent one long body of text so that it can be manipulated in smaller chunks.

TStrings introduces many properties and methods to

Add or delete strings at specified positions in the list.

Rearrange the strings in the list.

Access the string at a particular location.

Read the strings from or write the strings to a file or stream.

Associate an object with each string in the list.

TStrings.BeginUpdate

Enables the TStrings object to track when the list of strings is changing.

procedure BeginUpdate;

Description

BeginUpdate is called automatically by any property or method that changes the list of strings. Once the changes are complete, the property or method calls EndUpdate. Call BeginUpdate before directly modifying the strings in the list, and EndUpdate after. When implementing properties or methods that change the list in descendants of TStrings, call BeginUpdate before the changes are made, and EndUpdate when the changes are complete.

TStrings simply keeps track of when the list of strings is being changed. Some descendants of TStrings use this information to perform certain actions, such as telling a control to repaint, when updates are complete.

2、 Copy function

Returns a substring of a string or a segment of a dynamic array.

Unit

System

Category

string handling routines

function Copy(S; Index, Count: Integer): string;

function Copy(S; Index, Count: Integer): array;

Description

S is an expression of a string or dynamic-array type. Index and Count are integer-type expressions. Copy returns a substring or sub array containing Count characters or elements starting at S[Index].

If Index is larger than the length of S, Copy returns an empty string or array.

If Count specifies more characters or array elements than are available, only the characters or elements from S[Index] to the end of S are returned.

Note: When S is a dynamic array, Copy can only be used as a parameter in a call to a procedure or function that expects an array parameter. That is, it acts like the Slice function when working with dynamic arrays.

3、 Val procedure

Converts a string to a numeric representation.

Unit

System

Category

string handling routines

procedure Val(S; var V; var Code: Integer);

Description

Val converts the string value S to its numeric representation, as if it were read from a text file with Read.

S is a string-type expression; it must be a sequence of characters that form a signed real number.

V is an integer-type or real-type variable. If V is an integer-type variable, S must form a whole number.

Code is a variable of type Integer.

If the string is invalid, the index of the offending character is stored in Code; otherwise, Code is set to zero. For a null-terminated string, the error position returned in Code is one larger than the actual zero-based index of the character in error.

Val performs range checking differently depending upon the setting of the $R compiler directive and the type of the parameter V.

Setting Result

{$R+} An out-of-range value always generates a run-time error.

{$R-} The values for out-of-range vary depending upon the data type of V.

4、 Class methods

A class method is a method (other than a constructor) that operates on classes instead of objects. The definition of a class method must begin with the reserved word class. For example,

type

TFigure = class

public

class function Supports(Operation: string): Boolean; virtual;

class procedure GetInfo(var Info: TFigureInfo); virtual;

...

end;

The defining declaration of a class method must also begin with class. For example,

class procedure TFigure.GetInfo(var Info: TFigureInfo);

begin

...

end;

In the defining declaration of a class method, the identifier Self represents the class where the method is called (which could be a descendant of the class in which it is defined). If the method is called in the class C, then Self is of the type class of C. Thus you cannot use Self to access fields, properties, and normal (object) methods, but you can use it to call constructors and other class methods.

A class method can be called through a class reference or an object reference. When it is called through an object reference, the class of the object becomes the value of Self.