The DOs and DON’Ts of C++ Pointers

This article outlines some of the common mistakes that happen with C++ Pointers.

BASIC OVERVIEW

  1. Pointers are usually indicated by using the '*' symbol before the
    variable name. To declare a pointer the syntax is,
    [data type] * [variable name];
    int *p;
    as an example.

  2. Pointers need to point to a variable of the same type. So if you declare
    int pointer, it will either need to reference another int pointer or
    the address of an int variable. Here is an example of an int pointer referencing the address of an int
    variable. The '&' symbol means address.
    int v;
    int *p;
    p = &v;

  3. Here is an example of an int pointer
    referencing another int pointer.
    int v = 6;
    int *p1 = &v;
    int *p2;
    p2 = p1;

  4. Dereferencing also uses the '*' symbol. What dereferencing does is if
    your pointer points at a variable, when you dereference the pointer
    you can retrieve the value of the variable or alter the variable.
    Here is an example of dereferencing a pointer so that you can
    retrieve the value of the variable.
    int v = 5;
    int *p = &v;
    if(*p == 5)
       v = 6;

  5. Here is an example of dereferencing a pointer so that you can
    alter the value.
    int v;
    int *p = &v;
    *p = 6; // Actually changes v.

  6. The main importance of pointers is the ability to dynamically
    create new instances of objects. The 'new' keyword is used
    to create a variable in memory. It's really important to make
    pointers point to dynamic copies. If a reference is lost to
    a new variable, it turns into a memory leak in your application.
    Here is an example of the 'new' and 'delete' keywords. 'delete'
    just helps us clean up the new variable so we don't get memory
    leaks. 'NULL' is the same as zero.
    int *p = new int(); // Makes new int in memory.
    delete p; // Removes the new int from memory.
    p = NULL; // Make sure we don't access the old int address.

  7. An advantage to using pointers is the ability to dynamically create
    arrays. With a standard array you are stuck with creating a fixed
    size. With a pointer array, you can dynamically create almost any
    size you want. In the previous example we dynamically created a
    single int with 'new int()'. We can create a pointer array by
    inserting a number between the parenthesis like 'new int(100)'.
    The cleanup is a little different because we use array syntax
    '[100]'. The 100 is optional, so you have the option to just
    use '[]'.

    int *p = new int(100); // Dynamically creates an 100 element int array.
    delete [100]p; // Explicitly removes all 100 elements from the array.
    p = NULL; // Make sure we don't access the old int address.

DON'T

Dereferencing is common with pointers. A mistake that is often
made is setting a value to a dereferenced pointer before that
pointer has been initialized.

int i = 2;
int *p;
*p = i;

DON'T

The compiler can catch a lot of your mistakes, but it won't
catch one's like these. This problem is similar to the one
above. This time the pointer has been initialized, but not
with an address of a variable that can hold its data type.

int i = 2;
int *p = 0; // Initialized, but not pointing to an address
*p = i;

DON'T

The same problem happens if we initialize the pointer to a
non address and try to set a value to that dereferenced pointer.

int i;
int *p = 0; // Initialized, but not pointing to an address
*p = 2;

DON'T

The same problem happens if we initialize the pointer to a
non address and try to set a value to that dereferenced pointer.
Here's another look of the same error.

int i;
int *p = NULL; // Initialized, but not pointing to an address
*p = 2;

DO

If you are going to initialize your pointers, you need to use
a real address. This is how you do it.

int i;
int *p = &i;

DO

Here is an example of how to properly set a value to your
dereferenced pointer, step by step.

int i;
int *p;
p = &i;
*p = 2; // This actually sets i to 2.

DO

Here is an example of how to properly set a value to your
dereferenced pointer, in fewer steps.

int i;
int *p = &i;
*p = 2; // This actually sets i to 2.

DON'T

When you use a static pointer, you can run into memory issues.
This problem results in how static works. The first time the
code is executed is when the static pointer is set. The
second time the code is reached, the local variables will have
different memory addresses causing a memory error.

int i = 2;
static int *p = &i;
*p = 1234; // crashes on second lap because i has a different
           // memory address each time this code executes.
  /*
	There are 3 easy solutions to this problem.
	(1) Don't make p static
	(2) Make i static
	(3) Correct the code by splitting it into two statements:
		int i = 2;
		static int *p;
                p = &i;
		*p = 1234;
  */

DO

When working with pointers, type casting is always good so you
know what kind of objects you are working with.

int i;
int *p = (int*)&i;
*p = 1;

DO

The memset function is commonly used with pointers. It helps us
be sure that memory was allocated when we made our variables.
Here is an example of how to use it.

#include  // let's us use the memset function
//...
int i;
if( memset(&i, 0, sizeof(int)) )
   return -1; // Memory was not allocated

DO

The memcpy function is commonly used with pointers. It helps us
be sure that memory was copied properly when we do an assignment.
Here is an example of how to use it.

#include  // let's us use the memset function
//...
int i = 6;
int j;
if( i != *(int*)(memcpy(&j, &i, sizeof(int))) )
   return -1; // Memory was not allocated

DO

In more advanced concepts of C++, ADTs (Abstract Data Types) are
used. Most implementations require lists of some kind. In order
to construct your own list without using arrays, you will need
to use pointers. To make your own objects you can use struct
or class. Both have data members and methods. For a list
implementation, your object will need a pointer to the same type
of object to link them together. Here is a brief example of that
kind of situation. This is a basic structure object with a Value
data member and a pointer to let us create a list. Elements in
abstract lists are commonly referred to as nodes.

You'll see that Node is a pointer with the same type as the
object that it is part of. This is intentional. We want our
list elements to be able to point to other elements in the
list. It makes sense that all the elements in the list
would be the same type of elements.

 struct aMyArray
 {
 public:
    int Value;
    aMyArray *Node;
 }

HowTo MySQL

The following are example commands common to doing database operations in MySQL. AnhD V Nguyen contributed this helpful guide.

-- Same as sysobjects in MS SQL
show tables;

-- Show all Database in MySQL
show databases;

-- Show table detail same as sp_help tblOrders
show columns from Orders;

-- Create Database
CREATE DATABASE Northwind;

-- Drop Database
Drop DATABASE mycs416;

-- Create table Categories
create table Categories
(
	CategoryID INT AUTO_INCREMENT PRIMARY KEY,
	CategoryName varchar (15),
	Description TEXT,
	Picture TEXT
);

-- Create table Customers
create table Customers
(
	CustomerID varchar (40) PRIMARY KEY,
	CompanyName varchar (40),
	ContactName varchar (30),
	ContactTitle varchar (30),
	Address varchar (60),
	City varchar (15),
	Region varchar (15),
	PostalCode varchar (10),
	Country varchar (15),
	Phone varchar (24),
	Fax varchar (24)
);


-- Create table Employees
create table Employees
(
	EmpoyeeID INT AUTO_INCREMENT PRIMARY KEY,
	LastName varchar (20),
	FirstName varchar (10),
	Title varchar (30),
	TitleOfCourtesy varchar (25),
	BirthDate date,
	HireDate date,
	Address varchar (60),
	City varchar (15),
	Region varchar (15),
	PostalCode varchar (10),
	Country varchar (15),
	HomePhone varchar (24),
	Extension varchar (4),
	Photo TEXT,
	Notes TEXT,
	ReportsTo Int
);

-- Create table Order Details
create table Order Details
(
	OrderID INT PRIMARY KEY,
	ProductID INT,
	UnitPrice Decimal(9,2),
	Quantity Integer,
	Discount INT
);


-- Create table Orders
create table Orders
(
	OrderID INT AUTO_INCREMENT PRIMARY KEY,
	CustomerID Text,	
	EmployeeID Int,
	OrderDate DateTime,
	ReqireDate DateTime,
	ShippedDate DateTime,
	ShipVia Int,
	Freight Decimal (9,2),
	ShipName Text,
	ShipAddress varchar (60),
	ShipCity varchar (15),
	ShipRegion varchar (15),
	ShippPostalCode varchar (10),
	ShipCountry varchar (15)
);


-- Create table Products
create table Products
(
	ProductID INT AUTO_INCREMENT PRIMARY KEY,
	ProductName Text,	
	SupplierID Int,
	CategoryID Int,
	QuantityPerUnit Text,
	UnitPrice Decimal (9,2),
	UnitsInStock Int,
	UnitsOnOrder Int,
	ReorderLevel Int,
	Discontinued varchar (3)
);

-- Create table Shippers
create table Shippers
(
	ShipperID INT AUTO_INCREMENT PRIMARY KEY,
	CompanyName varchar (40),	
	Phone varchar (24)
);


-- Create table Suppliers
create table Suppliers
(
	SupplierID INT AUTO_INCREMENT PRIMARY KEY,
	CompanyName varchar (40),	
	ContactName varchar (30),
	ContactTitile varchar (30),
	Address varchar (60),
	City varchar (15),
	Region varchar (15),
	PostalCode varchar (10),
	Country varchar (15),
	Phone varchar (24),
	Fax varchar (24),
	HomePage varchar (50)
);


-- Create table tblCust
create table tblCust
(
	CustomerID varchar (30) PRIMARY KEY,
	UserName varchar (30),	
	Pass varchar (30)
);

-------------------------------------------------------

/* Use the Alter table statement to add the 
   following constraints to (PK)Categories ---> (FK)Products:         	*/
        alter table Products
        add foreign key(CategoryID) 
        references Categories (CategoryID);

/* Use the Alter table statement to add the 
   following constraints to (PK)Supplies ---> (FK)Products:         	*/
        alter table Products
        add foreign key(SupplierID) 
        references Suppliers (SupplierID);

/* Use the Alter table statement to add the 
   following constraints to (PK)Products ---> (FK)OrderDetails:         */
        alter table OrderDetails
        add foreign key(ProductID) 
        references Products (ProductID);


/* Use the Alter table statement to add the 
   following constraints to (PK)Orders ---> (FK)OrderDetails:         	*/
        alter table OrderDetails
        add foreign key(OrderID) 
        references Orders (OrderID);

/* Use the Alter table statement to add the 
   following constraints to (PK)Orders ---> (FK)Employees:    	 	*/
        alter table Employees
        add foreign key(EmployeeID) 
        references Orders (EmployeeID);

/* Use the Alter table statement to add the 
   following constraints to (PK)Shippers ---> (FK)Orders:    	 	*/
        alter table Orders
        add foreign key(ShipperID) 
        references Shippers (ShipVia);


/* Use the Alter table statement to add the 
   following constraints to (PK)Customers ---> (FK)Orders:    	 	*/
        alter table Orders
        add foreign key(CustomerID) 
        references Customers (CustomerID);


/* Use the Alter table statement to add the 
   following constraints to (PK)tblCust ---> (FK)Customers:    	 	*/
        alter table Customers
        add foreign key(CustomerID) 
        references tblCust (CustomerID);

--=======================Load Data from Text File ========================

load data infile 'c:categories.txt' into table Categories(CategoryID,
CategoryName, Description, Picture);


load data infile 'c:suppliers.txt' into table Suppliers(
	CompanyName,	
	ContactName,
	ContactTitile,
	Address,
	City,
	Region,
	PostalCode,
	Country,
	Phone,
	Fax, HomePage);

load data infile 'c:Products.txt' into table Products(
	ProductName,	
	SupplierID,
	CategoryID,
	QuantityPerUnit,
	UnitPrice,
	UnitsInStock,
	UnitsOnOrder,
	ReorderLevel,
	Discontinued);


load data infile 'c:Employees.txt' into table Employees(
	LastName,
	FirstName,
	Title,
	TitleOfCourtesy,
	BirthDate,
	HireDate,
	Address,
	City,
	Region,
	PostalCode,
	Country,
	HomePhone,
	Extension,
	Photo,
	Notes,
	ReportsTo);

load data infile 'c:Shippers.txt' into table Shippers(
	CompanyName,	
	Phone);


load data infile 'c:Customers.txt' into table Customers(
	CustomerID,
	CompanyName,
	ContactName,
	ContactTitle,
	Address,
	City,
	Region,
	PostalCode,
	Country,
	Phone,
	Fax);


load data infile 'c:tblCust.txt' into table tblCust(
	CustomerID,
	UserName,	
	Pass);



load data infile 'c:Orders.txt' into table Orders(
	CustomerID,
	EmployeeID,
	OrderDate,
	ReqireDate,
	ShippedDate,
	ShipVia,
	Freight,
	ShipName,
	ShipAddress,
	ShipCity,
	ShipRegion,
	ShippPostalCode,
	ShipCountry);

--=============================================================
--Export to text file

select * into outfile 'c:order.txt' from orders;

-- Or

Select * from orders into outfile 'c:orders';

-- Or
select * INTO OUTFILE "c:/result.text" FIELDS TERMINATED BY ',' OPTIONALLY
ENCLOSED BY '"' LINES TERMINATED BY "n" FROM test_table;

-- MS SQL run from Dos-Prompt
bcp pubs..publishers out 'c:publishers.txt' -c -Sglamour -Usa -Ppassword

-- limit row_count OFFSET offset
select * from ordres limit 5,10; -- # Retrieve rows 6-15

select * from orders limit 95,-1; # Retrieve rows 96-last.

select * from orders LIMIT 5;     # Retrieve first 5 rows

--================================================================

HowTo Connect HTML with MS SQL

This example script shows you how to get started in web/database development by connecting an HTML page to an MS SQL database via VbScript.

<SCRIPT LANGUAGE="VBScript">

   Dim objConn
   Dim objRec
   Dim sIn
   Dim Query

   ' Put an SQL query into a string
   Query = "SELECT * FROM MyTable WHERE name LIKE 'target%'"

   ' This is our connection to the MS SQL Database
   strConnect = "Server=TESTING1;Database=MSSQLDB;UID=user;PWD=password"

   ' We access the database thru an ADO connection
   Set objConn = CreateObject ("ADODB.connection")

   ' This tells which provider / driver to use
   objConn.Provider="SQLOLEDB"

   ' Opens the database connection
   objConn.Open strConnect

   ' Creates a recordset to hold the data coming back from the query
   Set objRec = CreateObject ("ADODB.Recordset")

   ' Causes the query to execute
   objRec.Open Query, objConn

   ' Output field names
   document.write (objRec.Fields(0).Name)
   document.write (" ")
   document.write (objRec.Fields(1).Name)
   document.write (" ")
   document.write (objRec.Fields.Count)
   document.write ("<br />")

   ' Output each record
   do while not objRec.EOF
      document.write (objRec.Fields(0).Value)
      document.write (" ")
      document.write (objRec.Fields(1).Value)
      document.write ("<br />")
      objRec.MoveNext
   loop

   ' Close the recordset
   objRec.close
   set objRec = nothing

   ' Close the database connection
   objConn.close
   set objConn = nothing

</SCRIPT>

HowTo Connect HTML with MS Access

This example script shows you how to get started in web/database development by connecting an HTML page to an MS Access database via VbScript.

<SCRIPT LANGUAGE="VBScript">

   Dim objConn
   Dim objRec
   Dim strConnect
   Dim Query

   ' Put an SQL query into a string
   Query = "select * from orders;"

   ' Access file must reside locally. A URL won't work
   strConnect = "C:/My Documents/NorthWind.mdb"

   ' We access the database thru an ADO connection
   Set objConn = CreateObject ("ADODB.connection")

   ' This tells which provider / driver to use
   objConn.Provider="Microsoft.Jet.OLEDB.4.0"

   ' Opens the database connection
   objConn.Open strConnect

   ' Creates a recordset to hold the data coming back from the query
   Set objRec = CreateObject ("ADODB.Recordset")

   ' Causes the query to execute
   objRec.Open Query, objConn

   ' Output each record
   do while not objRec.EOF
      document.write (objRec.Fields(0).Value)
      document.write (" --->> ")
      document.write (objRec.Fields(1).Value)
      document.write ("<br />")
      objRec.MoveNext
   loop

   ' Close the recordset
   objRec.close
   set objRec = nothing

   ' Close the database connection
   objConn.close
   set objConn = nothing

</script>

Sketches

I used this week to take a break from coding and worked on a few sketches instead. It allowed me to add a second page to the sketches section.


I’ve been using manual ftp commands to update this site. In case I forget, this is how to set permissions with chmod as an ftp command.
quote site chmod 755 filename.pl