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One application of a pointer to pointer is the creation of memory arrays. The

following program demonstrates the creation, manipulation and freeing of a memory

array.

OptionExplicit

'Declareapointertoanintpointer

DimmyMemArrayAsIntegerPtrPtr

DimAsIntegeri,j

'Create10rowsofintegerpointers

myMemArray=Callocate(10,Sizeof(IntegerPtr))

'Add10columnsofintegerstoeachrow

Fori=0To9

myMemArray[i]=Callocate(10,Sizeof(Integer))

'Addsomedatatothememorysegment

Fori=0To9

Forj=0To9

myMemArray[i][j]=Int(Rnd*10)

Next

'Printoutdata

Fori=0To9

Forj=0To9

Print"i,j=";i;",";j;"MemArray=";myMemArray[i][j]

Next

'Freememorysegment

Fori=0To9

DeallocatemyMemArray[i]

'Freethepointertopointer

DeallocatemyMemArray

Sleep

End

Listing 8.6: memarray.bas

Analysis:

Line 4 dimensions a pointer to an integer pointer, myMemArray, which

will simulate a two dimensional array, with myMemArray pointing to a list of integer

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pointers. This list of pointers will comprise the “rows” of the array. Line 5 just declares

some working variables that are used in the For-Next loops later in the program.

Line 8 creates the rows of the array by allocating a memory segment that will

contain 4 integer pointers, which are initialized in lines 11 through 13. Remember that

the index method of accessing a pointer automatically does the pointer arithmetic for

you. The code in lines 11 through 13 iterates through the memory segment created in

line 8 and initializes the memory segment with pointers to memory segments that will

contain integers. In other words, you have a pointer that is pointing to a list of pointers.

These newly created memory segments are the columns for each row. Each column index

will be a pointer to a memory location containing an integer.

Lines 16 through 20 add some random numbers to the memory array. Notice that

by using the indexing method you can access the memory array just like a normal array. i

points to the row (the list of pointers) and j points to the individual columns within that

row which contain the integer data. The same method is used in lines 23 through 27 to

print out the array values.

Lines 30 through 32 free the individual rows of memory that were created in line 8. It is

important that each row be freed before freeing myMemArray. If you were to just free

myMemArray, the rows would still be in memory, but unaccessible, causing a memory

leak. Once all the rows have been freed, myMemArray can be freed in line 34. Since the

program is terminating, Deallocating the memory is not strictly required in this instance,

but if you needed to reuse the memory, then you must Deallocate in the method

described, otherwise you will get a memory leak while the program is running. Lines 36

and 37 close the program in the usual way.

Running the program should produce a result similar to the following.

i,j=0,1MemArray=5

i,j=0,2MemArray=1

i,j=0,3MemArray=8

i,j=0,4MemArray=5

i,j=1,0MemArray=4

i,j=1,1MemArray=3

i,j=1,2MemArray=8

i,j=1,3MemArray=8

i,j=1,4MemArray=7

i,j=2,0MemArray=1

i,j=2,1MemArray=8

i,j=2,2MemArray=7

i,j=2,3MemArray=5

i,j=2,4MemArray=3

i,j=3,0MemArray=0

i,j=3,1MemArray=0

i,j=3,2MemArray=3

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i,j=3,3MemArray=1

i,j=3,4MemArray=1

i,j=4,0MemArray=9

i,j=4,1MemArray=4

i,j=4,2MemArray=1

i,j=4,3MemArray=0

i,j=4,4MemArray=0

Output 8.4: Output of memarray.bas

As you can see from the output, the memory array behaves exactly like a

predefined array. This structure is useful for adding dynamic arrays to type definitions,

which normally cannot hold a dynamic array. You will see this in more detail in the

chapter on composite types.

One last note on this program. If you run the program more than once, you will

notice that the values in the array are always the same, even though the program is

generating random numbers. This is because the program did not seed the random

number generator using the Randomize statement. To get different numbers for each run,

add RandomizeTimerbefore calling the Rnd function.

A Look Ahead

Pointers are an efficient and powerful data type, and you will be seeing more of

them in this book, including the next chapter where you will explore the string data types

available in FreeBasic.

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String Data Types

Strings are an essential part of programming, and proper manipulation of strings

is an essential skill for any programmer. You will need to use strings in virtually every

program you will write, from games to data entry programs. Strings are important

because humans use strings to communicate concepts and ideas. A menu in a game

communicates the various game options to the user. An address stored in a database

communicates the location of the person who lives at that address. Programs are

solutions to problems, and in most cases, must communicate that solution to the user

through the use of strings.

Computers however, only understand numbers. The string “¡FreeBasic está

fresco!“ has no meaning to the computer. For the very first computers this did not matter

much, since those early computers were just glorified calculators, albeit calculators that

took up a whole room. However it soon became evident to computer engineers that in

order for the computer to be a useful tool, it needed to be able to somehow recognize

string data, and be able to manipulate that string data.

Since computers only understand numbers, the solution was to convert alpha-

numeric characters to numbers via translation tables. The familiar ASCII code table is one

such translation scheme that takes a set of alpha-numeric characters and converts them

to numbers. The “A” character is encoded as decimal 65, the exclamation point as

decimal 33 and the number 1 is encoded as decimal 49. When you press any key on your

keyboard, a scancode for that key is generated and stored in the computer as a number.

Humans group letters together to form words, and words to form sentences. A

certain arrangement of letters and words mean something according to the semantic

rules the writer's language. When you read the string “FreeBasic is cool!” you understand

the meaning of the words if you can read the English language. The computer however

doesn't know what FreeBasic is and doesn't know what it means to be cool. When you

read that something is “cool” you know that this is slang for great or excellent. All the

computer can do is store the string in memory in a way that preserves the format of the

string so that a human reading the string will understand its meaning.

While computers have grown in both speed and capacity, the basic computing

processes haven't changed since the 1950's. The next revolution in computing won't be

quantum processors or holographic memory; it will be when computers can understand

language.

The solution to the storage problem was to simply store string data in memory as a

sequence of bytes, and terminate that sequence of bytes with a character 0. To put it

another way, a string in computer memory is an array of characters, terminated with a

character 0 to signal the end of the string. Virtually all early programming languages, and

many modern ones, have no native String data type. A string in C is a Null (character 0)

terminated array of Char, where Char can be interpreted as both a character and number.

While this scheme accurately reflects the internal structure of a string in memory, it is

hard to work with and error prone, and doesn't reflect the way humans work with string

data. A better solution to the problem was the creation of the native String data type. The

internal structure of a string has not changed, it is still stored in memory as a sequence

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of bytes terminated by a Null character, but the programmer can interact with string data

in a more natural, humanistic way.

FreeBasic has four intrinsic string data types, listed in the following table.

String Type

Declaration

Purpose

Dynamic String

Dim myString as String

8-bit string that is

dynamically allocated and

automatically resized.

Fixed Length String

Dim myString as String *

length

8-bit, fixed-length string.

Zstring

Dim myString as Zstring *

length

Dim myString as Zstring Ptr

8-bit, fixed-length, Null

terminated character string.

A Zstring is a C-style string.

Wstring

Dim myString as Wstring *

length

Dim myString as Wstring Ptr

16-bit, fixed-length, Null

terminated string used with

Unicode functions.

Table 9.1: Intrinsic FreeBasic String Data Types

All FreeBasic strings can hold up to 2 GB of data, or up to the amount of memory

on your computer.

Dynamic Strings

Dynamic strings are variable-length strings that the compiler automatically

allocates and resizes as needed. Dynamic strings are actually structures stored in

memory, called a string descriptor, that contain a pointer to the string data, the length of

the string data and the size of the string allocation. Dynamic strings are allocated in 36

byte blocks, which reduces the amount of resizing that the compiler must do if the string

size changes. The following program displays the internal string structure data for a

typical dynamic string

OptionExplicit

'Stringdescriptor

TypeStringDesc

strdataAsZstringPtr

lengthAsInteger

sizeAsInteger

EndType

'Createastring

DimAsStringmyString="ThisisadynamicstringinFreeBasic!"

'Dimensionapointertothestringdescriptor

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DimAsStringDescPtrStringData=Cptr(stringdescPtr,@myString)

'Printstringdescriptordata

Print"Data:";*(StringData>strdata)

Print"Length:";StringData>length

Print"Size:";StringData>size



Sleep

End

Listing 9.1: dynastring.bas

Analysis:

Lines 4 through 8 defines the string descriptor data type. The strdata

field is a pointer to a NULL terminated string in memory. The length fields contains the

number of characters in the string. The size field indicates the length of the current

memory-block allocation. Line 11 creates a dynamic string and initializes the value of the

string. Line 13 creates a pointer, StringData, that points to the string descriptor of

myString. Notice that the address of operator is used to return the address of the string

descriptor, which is then cast as a StringDesc Ptr using the Cptr function.

Lines 16 through 18 then print out the string descriptor data. The member

operator, ->, is used with type definition pointers and is comparable to using *PtrName.

The member operator will be discussed in the chapter on composite types. The program

is closed in the usual way.

Running the program in FBIde will produce the following result.

Data:ThisisadynamicstringinFreeBasic!

Length:38

Size:72

Output 9.1: Output of dynastring.bas

Dynamic strings use Zstrings strings internally so that you can pass a dynamic

string to a function that expects a Null terminated string and the function will work

correctly. The Windows api, the C runtime library and most third-party libraries written in

C expect a Null terminated string as a string parameter.

Caution In other versions of Basic, strings have been used to load and manipulate

binary data. If the binary data contains a Null character, this could cause problems with

FreeBasic strings, since the Null character is used to terminate a string. While it is

possible to have an embedded Null in a dynamic string (since dynamic strings have a

string descriptor), if you pass this data to a third party library function that is expecting a

C-style string, the data will read only up to the Null character, resulting in data loss.

Instead of using strings to read binary data, you should byte arrays instead.

Since a dynamic string is actually a pointer to a string descriptor, you cannot use

dynamic strings in type definitions that you are going to save to a file. You should use a

fixed length string instead.

Fixed Length Strings

Fixed length strings are defined using a length parameter, and can only hold

strings that are less than or equal to the defined size. Trying to initialize a fixed length

string with data that is longer than the defined size will result in the string being

truncated to fit.

The following short program illustrates creating and using a fixed length string.

OptionExplicit

'Defineafixedlengthstring

DimmyFStringAsString*20

'Savesomedatinthestring

myFString="Thisshouldfit."

PrintmyFString

'Thisdatawillbetruncated.

myFString="Thisstringwillbetruncated."

PrintmyFString

Sleep

End

Listing 9.2: fixedstr.bas

Analysis:

Lines 4 defines a fixed length string that can hold up to 20 characters.

Line 7 loads the variable with a string and line 8 prints the contents of the variable. Since

the string "This should fit." is smaller than 20 characters, so the whole string will be

printed out. Line 11 tries to initialize the string with data that is longer than 20 characters

so the string will be truncated when it is printed out in line 12.

When you run the program you should see the following output.

Thisshouldfit.

Thisstringwillbe

Output 9.2: Output of fixedstr.bas

As you can see from the output, if a string is too long to fit in the variable, it will be

truncated to fit. Fixed length strings are very useful when creating random access records

using type definitions. This technique is used in the chapter on file handling.

Zstrings

Zstrings are Null terminated, C-style strings. The main purpose of a Zstring is to

interface with third-party libraries that expect a C-style strings, however they are useful

even if you do not plan to pass them to third-party libraries. Zstrings can be defined just

like a fixed-length string, DimmyZstringasZstring*10, and FreeBasic will handle

them just like fixed strings, automatically truncating data to fit the defined size.

Unlike fixed strings however, Zstrings can be dynamically managed by declaring a

Zstring pointer and using the associated memory functions. When using a dynamically

allocated Zstring, you must be careful not to overwrite the the size of the string, as this

will overwrite parts of memory not contained in the string and may cause the program or

even the operating system to crash.

When using either type of Zstring, FreeBasic will manage the terminating Null

character for you, but the storage size of a Zstring will be 1 less than the defined size

since the the Null character occupies the last character position. When calculating the

size of a Zstring be sure to add 1 to the value to account for the Null terminator. You must

also not include the character 0 in any of your data, or the data will be truncated since

FreeBasic will see the Null character as the end of the string.

The following program shows both uses of the Zstring data type.

OptionExplicit

'DeclareafixedlengthZstring

DimmyFixedZstringAsZstring*20+1

'DeclareadynamicZstring

DimmyDynaZstringAsZstringPtr

'Loadsomedataintothefixedzstring

Print"123456789012345678901"

myFixedZstring="Thisshouldfit."

PrintmyFixedZstring

myFixedZstring="Thiswillbetruncated."

PrintmyFixedZstring

'Createadynamiczstring

Print"123456789012345678901"

myDynaZstring=Callocate(20+1)

*myDynaZstring="Let'saddsomedata."

Print*myDynaZstring

'Resizethestring:20forcurrentdata+5fornew

'data+1forNull

myDynaZstring=Reallocate(myDynaZstring,20+5+1)

Print"12345678901234567890123456"

*myDynaZstring="Let'saddsomemoredata."

PrintmyDynaZstring[0]

'Deallocatethememorysegment

DeallocatemyDynaZstring

Sleep

End

Listing 9.3: zstring.bas

Analysis:

Line 4 declares a fixed length Zstring. Notice that the desired data length

is 20 so 1 is added to to allocate space for the terminating Null character. Line 6 declares

a pointer to a Zstring that will be managed dynamically. Line 9 prints out a ruler to

compare the print statements in line 11 and 13. Line 10 initializes the Zstring with some

data that is small enough to fit in the declared size. Line 12 adds some data that is too

long to fit within the Zstring, which will be truncated to fit.

Line 16 prints another ruler for comparison with the dynamic Zstring data. Line 17

Callocates enough space for a 20-byte string along with the terminating Null character.

Since myDynaZstring is defined as a Zstring Ptr, the indirection operator is used to

initialize the variable data in line 18. Line 22 resizes the Zstring to hold 5 additional

characters plus the terminating Null using the Reallocate function. Another ruler is

printed in line 23 to show the new size of the data loaded into the variable in line 24. Line

25 uses the pointer index method to print out the contents of the variable. Line 28

deallocates the allocated memory.

When you run this program you should get the following output.

123456789012345678901

Thisshouldfit.

Thiswillbetruncat

123456789012345678901

Let'saddsomedata.

12345678901234567890123456

Let'saddsomemoredata.

Output 9.3: Output of zstring.bas

As you can see, the fixed length version of the Zstring data type behaves exactly

like the fixed string data type and the dynamic version behaves exactly like any other

pointer-based memory segment. Since there is no intrinsic method for determining the

size of a Zstring allocation, you must be careful not to over-run the buffer. The best way

to do this is to keep a current allocation size in a separate variable that can be referenced

as needed.

You will notice in Line 25 of the listing that the pointer index method was used to

print the contents of the resized Zstring. What would the output be if you changed the

index from 0 to 6? Remember that a pointer index behaves much like an array index, and

a Zstring could be viewed as an array of characters. If you were to use 6 as the index, the

output would be addsomemoredata. The Print statement would start at character

position 6 and print everything up to the terminating Null character.

You can directly assign a fixed length Zstring to a dynamic string. For a dynamic

Zstring, the pointer must be dereferenced by using * or the pointer index method. Keep

in mind that the behavior of the indexing method is the same for assignments as it is for

the Print statement.

Wstrings

Dynamic, fixed and Zstrings all use 1 byte per character. Wstrings, also called wide

strings, use 2 bytes per character and is generally used in conjunction with Unicode

strings functions. Unicode is, strictly speaking, a character coding scheme designed to

associate a number for every character of every language used in the world today, as

well as some ancient languages that are of historical interest. In the context of

developing an application, Unicode is used to internationalize a program so that the end-

user can view the program in their native language.

Wstrings can be both fixed length and dynamic and are similar to Zstrings.

Wstrings will be discussed in more detail in the chapter on Unicode.

String Functions

Creating and using strings in your application will often consist of manipulating

those strings and FreeBasic has a rich set of string functions, which are listed in the

following table.

Function

Syntax

Comment

Asc

B = Asc(string)

B = Asc(string, position)

Returns the character code

of the first character in a

string as a uninteger, or the

character code of character

at position.

Bin

B = Bin(number)

B = Bin(number, digits)

Returns the binary form of

number as a string. Can

optionally return only

number of digits.

Chr

B = Chr(code)

B = Chr(code, code, ...)

Returns the character

represented by Ascii Code. If

multiple codes are passed to

Chr, the function will return

a string of characters.

Hex

B = Hex(number)

B = Hex(number, digits)

Returns the hexadecimal

form of number as a string.

Can optionally return only

number of digits.

Instr

B = Instr(string, substring)

B = Instr(start, string,

Returns the position of

substring within string as an

100

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