ch01.htm

delphi自学的好教材！特别适合刚刚起步学习delphi的人员！同样对使用者具有参考价值！

		<TD ALIGN="LEFT"><I>Data Type</I></TD>
		<TD ALIGN="LEFT"><I>Size in Bytes</I></TD>
		<TD ALIGN="LEFT"><I>Possible Range of Values</I></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">ShortInt</TD>
		<TD ALIGN="LEFT">1</TD>
		<TD ALIGN="LEFT">-128 to 127</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Byte</TD>
		<TD ALIGN="LEFT">1</TD>
		<TD ALIGN="LEFT">0 to 255</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Char</TD>
		<TD ALIGN="LEFT">1</TD>
		<TD ALIGN="LEFT">0 to 255 (same as Byte)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">WideChar</TD>
		<TD ALIGN="LEFT">2</TD>
		<TD ALIGN="LEFT">0 to 65,535 (same as Word)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">SmallInt</TD>
		<TD ALIGN="LEFT">2</TD>
		<TD ALIGN="LEFT">-32,768 to 32,767</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Word</TD>
		<TD ALIGN="LEFT">2</TD>
		<TD ALIGN="LEFT">0 to 65,535</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">LongInt</TD>
		<TD ALIGN="LEFT">4</TD>
		<TD ALIGN="LEFT">-2,147,483,648 to 2,147,483,647</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Int64</TD>
		<TD ALIGN="LEFT">8</TD>
		<TD ALIGN="LEFT">-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Integer</TD>
		<TD ALIGN="LEFT">4</TD>
		<TD ALIGN="LEFT">Same as LongInt</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Cardinal</TD>
		<TD ALIGN="LEFT">4</TD>
		<TD ALIGN="LEFT">0 to 2,147,483,647</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Single</TD>
		<TD ALIGN="LEFT">4</TD>
		<TD ALIGN="LEFT">1.5 &#165; 10<SUP>-45</SUP> to 3.4 &#165; 10<SUP>38</SUP></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Double</TD>
		<TD ALIGN="LEFT">8</TD>
		<TD ALIGN="LEFT">5.0 &#165; 10<SUP>-324</SUP> to 1.7 &#165; 10<SUP>308</SUP></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Real</TD>
		<TD ALIGN="LEFT">8</TD>
		<TD ALIGN="LEFT">5.0 &#165; 10<SUP>-324</SUP> to 1.7 &#165; 10<SUP>308</SUP> (same as Double)</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Extended</TD>
		<TD ALIGN="LEFT">10</TD>
		<TD ALIGN="LEFT">3.4 &#165; 10<SUP>-4932</SUP> to 1.1 &#165; 10<SUP>4932</SUP></TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Comp</TD>
		<TD ALIGN="LEFT">8</TD>
		<TD ALIGN="LEFT">-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Currency</TD>
		<TD ALIGN="LEFT">8</TD>
		<TD ALIGN="LEFT">-922,337,203,685,477.5808 to 922,337,203,685,477.5807</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Boolean</TD>
		<TD ALIGN="LEFT">1</TD>
		<TD ALIGN="LEFT">True or False</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">Variant</TD>
		<TD ALIGN="LEFT">16</TD>
		<TD ALIGN="LEFT">Varies</TD>
	</TR>
</TABLE>
</P>
<P>Examining Table 1.1, you might notice that an Integer is the same as a LongInt.
So why does Object Pascal have two different data types that are exactly the same?
Essentially, it's a holdover from days gone by. In a 16-bit programming environment,
an Integer requires 2 bytes of storage and a LongInt requires 4 bytes of storage.</P>
<P>In a 32-bit programming environment, however, both require 4 bytes of storage
and have the same range of values. Delphi 4 produces only 32-bit programs, so an
Integer and a LongInt are identical. Most programmers use Integer rather than LongInt.</P>
<P>You might also notice that the Int64 and Comp (computational) types have an identical
range of values. The difference between these two types is in the way they are treated
internally by the compiler. The Int64 type is an integer type, whereas the Comp type
is a real type. Probably you will have very little reason to use the Comp type in
your programs.</P>
<P>Notice also that the Real and Double data types are identical. In previous versions
of Delphi, the Real type was a 6-byte variable. Now it is an 8-byte variable. This
change was made to make the Real data type compatible with today's processors. The
Real type is considered obsolete and you should use Double rather than Real in your
Delphi applications.</P>


<BLOCKQUOTE>
	<P>
<HR>
<strong>NOTE:</strong> The Int64 data type is new to Delphi 4. There are many reasons for
	an integer type of this size. One of the most compelling is the need for an integer
	value that can hold the huge values required by today's larger hard drives. For example,
	Windows contains a function called GetDiskFreeSpaceEx, which can return values much
	larger than 2,147,483,647 (the maximum value of an Integer). A 64-bit integer data
	type was needed for reasons like this. 
<HR>
<BR>
	
<HR>
<strong>NOTE:</strong> The Single, Double, Extended, and Currency data types use floating-point
	numbers (numbers with decimal places). The other data types deal only with integer
	values. You cannot assign a value containing a decimal fraction to an integer data
	type. For example, the following code will generate a compiler error:</P>
	<PRE>var
  X : Integer;
{ Later... }
X := 3.75;</PRE>

</BLOCKQUOTE>

<PRE></PRE>


<BLOCKQUOTE>
	<P>You don't really have to worry about this too much, because the compiler is very
	good at telling you what you can and cannot do. By the way, you'd be surprised how
	few times you need floating-point numbers in most Windows programs. 
<HR>


</BLOCKQUOTE>

<H3></H3>
<H4>Converting Between Data Types</H4>
<P>Object Pascal performs conversion between different data types when possible.
Take the following code snippet for an example:</P>
<P>
<PRE>var
  Res : SmallInt;
  Num1 : Integer;
  Num2 : Integer;
{ Later... }
  Num1 := 200;
  Num2 := 200;
  Res  := Num1 * Num2;
</PRE>
<P>In this case I am trying to assign the result of multiplying two Integers to a
SmallInt. Even though this formula mixes two data types, Object Pascal is able to
perform a conversion. Would you like to take a guess at the result of this calculation?
You might be surprised to find out that the result is -25,536. What!? If you look
at Table 1.1, you'll see that a SmallInt can have a maximum value of 32,767. What
happens if you take a SmallInt with a value of 32,767 and add 1 to it? You will get
a value of -32,768. This is essentially the same as the odometer on a car turning
over from 99,999 to 00,000 when you drive that last mile. To illustrate, perform
the following steps:</P>

<DL>
	<DT></DT>
	<DD><B>1. </B>Start with a new application and place a label and button on the form.
	<P>
	<DT></DT>
	<DD><B>2. </B>Double-click the button to create an event handler for the button's
	OnClick event.
	<P>
	<DT></DT>
	<DD><B>3. </B>Modify the event handler so that it looks like this:
	<P>
</DL>



<BLOCKQUOTE>
	<PRE>procedure TForm1.Button1Click(Sender: TObject);
var
  X : SmallInt;
begin
  X := 32767;
  X := X + 1;
  Label1.Caption := IntToStr(X);
end;	</PRE>

</BLOCKQUOTE>

<PRE></PRE>

<DL>
	<DT></DT>
	<DD><B>4. </B>Run the program and click the button.
	<P>
</DL>

<P>You should see the caption of the label change to -32768 when you click the button
(in case you wondering, the IntToStr function translates an integer value to a string).
This exercise illustrates that 32767 plus 1 equals -32768! Okay, maybe not quite.</P>
<P>This example really illustrates what is known as <I>overflow</I> or <I>wrapping</I>.
You should be aware of the maximum possible values your variables can contain and
choose the data type that is large enough to guarantee that the variable will contain
the value without overflowing. For the most part, you won't go too far wrong if you
use the Integer data type as your data type of choice. You are unlikely to run into
the problem of wrapping because the Integer data type gives you an approximate range
of -2 billion to +2 billion.</P>
<P>Okay, where was I? Oh, yes, I was talking about automatic type conversion. In
some cases, Object Pascal cannot perform a conversion. If that is the case, you will
get a compiler error that says something along the lines of Incompatible types: `Integer'
and `Real'. This compiler error is telling you that you are trying to assign a value
that cannot be stored by this particular data type. Another compiler error you might
see has to do with what is called <I>range checking</I>. Take this code, for instance:</P>
<P>
<PRE>var
  X : Byte;
begin
  X := 1000;
end; 
</PRE>
<P>This code will generate a compiler error that states Constant expression violates
subrange bounds. The compiler is telling you that you can't assign a value of 1000
to the variable X because X is declared as a Byte and a Byte can only hold values
from 0 to 255.</P>


<BLOCKQUOTE>
	<P>
<HR>
<strong>TIP:</strong> Learn to treat compiler hints and warnings as errors. The compiler
	is trying to tell you that something is not quite right in your code, and you need
	to respect that warning. Ultimately, you should strive for warning-free compiles.
	In rare cases, a warning cannot be avoided, but be sure to examine all warnings closely.
	Do your best to understand the reason for the warning and correct it if possible.
	
<HR>


</BLOCKQUOTE>

<H3></H3>
<H3><A NAME="Heading18"></A>Object Pascal Operators</H3>
<P><I>Operators</I> are used to manipulate data. Operators perform calculations,
check for equality, make assignments, manipulate variables, and perform other, more
esoteric duties that most programmers never do. There are a lot of operators in Object
Pascal. Rather than present them all here, I will list only the most commonly used
ones. Table 1.2 contains a list of those operators.</P>
<P>
<H4>TABLE 1.2. COMMONLY USED OBJECT PASCAL OPERATORS.</H4>
<P>
<TABLE BORDER="1">
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT"><I>Operator</I></TD>
		<TD ALIGN="LEFT"><I>Description</I></TD>
		<TD ALIGN="LEFT"><I>Example</I></TD>
	</TR>
	<TR ALIGN="CENTER" VALIGN="TOP">
		<TD ALIGN="CENTER" COLSPAN="3">
			<P ALIGN="CENTER"><B>Mathematical Operators</B>
		</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">+</TD>
		<TD ALIGN="LEFT">Addition</TD>
		<TD ALIGN="LEFT">x := y + z;</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">-</TD>
		<TD ALIGN="LEFT">Subtraction</TD>
		<TD ALIGN="LEFT">x := y - z;</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">*</TD>
		<TD ALIGN="LEFT">Multiplication</TD>
		<TD ALIGN="LEFT">x := y * z;</TD>
	</TR>
	<TR ALIGN="LEFT" VALIGN="TOP">
		<TD ALIGN="LEFT">/</TD>
		<TD ALIGN="LEFT">Real number division</TD>
		<TD ALIGN="LEFT">x := y / 3.14;</TD>
	</TR