gcc – Page 2 – Coding Friends

Vector

An vector is very similar to a array, in that it is able to store data within one variable name. The main difference is that a vector is more of a class structure that allows you do to different tasks to it and also it acts similar to a stack FIFO (First In First Out) theory.

Here is SGI vector functions (SGI Vectors)

The vector allows for a dynamic approach of creating and inserting data onto a seemless endless array of data. When a new element is placed onto the vector (push_back), this creates a new block of memory associated with the vector and the value is attached (you can only attached the value the same as the initial setup of the vector, an vector is able to use classes, variables etc to push/pop of the vector/stack).

This is the example code

#include <iostream>
#include <vector>
 
using namespace std;
 
int main()
{
       // create a vector of intergers
       vector<int> vectorInt;
       // push_back, inserts onto the top of the list values
       vectorInt.push_back(2);              // insert 2
       vectorInt.push_back(3);              // insert 3
 
       // a iterator is basically a type of object that can reference part of the vector
       vector<int>::iterator iteratorInt;
 
       // display all of the emplements inside the vector
       for (iteratorInt = vectorInt.begin(); iteratorInt != vectorInt.end(); iteratorInt++)
       {
              cout << *iteratorInt << "\n";              // display what the iterator pointers to 
       }
       // display the size of a vector 
       cout << "size = " << vectorInt.size() << "\n";
 
       // the capacity = how big the vector is, since there has been no pull/remove/erase from the vector
       // then it is the same size.
       cout << "capacity = " << vectorInt.capacity() << "\n";
 
       cout << "Last element " << (int)vectorInt.back() << "\n";              // the last element
       vectorInt.pop_back();                            // delete the last element
       cout << "Last element " << (int)vectorInt.back() << "\n";              // the last element now
 
       cout << "capacity = " << vectorInt.capacity() << "\n";       // capacity is still 2
       cout << "size = " << vectorInt.size() << "\n";              // but the size is 1
 
       cout << "max size = " << vectorInt.max_size() << "\n";       
       return 0;
}

if you save the code as vectorcpp.cpp, then compile and run, the output will be

2
3
size = 2
capacity = 2
Last element 3
Last element 2
capacity = 2
size = 1
max size = 1073741823

and as you can see the with removing the last element from the list (number 3), the size of the vector is reduced, but the capacity is the same since that is how much had been assigned to the vector.

Note : the (int) in front of the vectorInt.back(), will decast the return value into a int(eger) value for the standard console output (std::cout).

Array

Arrays are ways of having a block of memory allocated to a single variable type, this allows for holding the national lottery numbers within an array of 6 instead of actually having 6 different variables.

e.g.

int value1 = 1;
int value2 = 2;
int value3 = 3;
int value4 = 4;
int value5 = 5;
int value6 = 6;

and instead you could just have

int values[6] = {1,2,3,4,5,6};

this is a example of how to manipulate the array structure.

#include <iostream>
 
int main()
{
       // array structure
       int values[5];       // set asside 5 blocks of int(eger) size of continuous memory blocks
                     // the actual points into the array are 0-4 (e.g. 5 points but start at 0)
 
       int values2[6] = {0,1,2,3,4,5}; // exmaple of settings the values on define time.
 
       // direct assigning the values.
       values[0] = 5;
       values[1] = 4;
       values[2] = 3;
       values[3] = 2;
       values[4] = 1;
 
       //could use a position pointer
       for (int i = 0; i < 5; i++)
       {
              // display the values
              std::cout << "Value " << i << " : " << values[i] << "\n";
       }
 
       int *int1p = &values[0];
       std::cout << "Pointer value = " << *int1p << "\n";
       int1p++;              // increament the pointer to the next part of the array
       std::cout << "Pointer value = " << *int1p << "\n";
       return 0;
}

save as arrays.cpp and then compile the program, the result once executed will be

Value 0 : 5
Value 1 : 4
Value 2 : 3
Value 3 : 2
Value 4 : 1
Pointer value = 5
Pointer value = 4

and include three different ways to access the values within the array.

Templates

Templates, Templates, Templates. A template is a way to declassify the type of variable used, e.g. if there was a function to add up two integer values, but you had two double values, then the int function would not function correctly, so there would be a need to function overload the function to allow for the double increment as well. But just envisage that the same function would be required for characters, floats, personally created ones etc, then there would be more functions per type of variable than the actual work done.

A template negates this by allowing any class type passed to the function, as long as the type has the standard increment process defined, it will work with any type.

The syntax for a template is

template <class (template variable name) > (return type) function name (parameters)

Hopefully the code will help in the explanation.

#include <iostream>
 
// the definition is similar to a function, (return type) (function name) (parameters...) instead
// it has a template <class (name of template)>
template <class T> T TemplateAdd(T a, T b)
{
              return (a+b);              // use the internal adding for the variable of type T
              // e.g if defined as a int, then use int add internal, or if a double, use a double etc.
}
 
// template class, same as a function apart from a class 
template<class T> class TemplateClass
{
private:
       T value;       // the internal value
public : 
       TemplateClass(T va)       // set the default internal value in the constructor
       {
              value = va;
       }
 
       T ReturnValue()              // return the default value, all using the template class
       {
              return value;
       }
};
 
int main()
{
       // standard template for a function, pass in two integer values, and also two doubles, it will both work the same
       std::cout << "Adding " << TemplateAdd(3,1) << " " <<  TemplateAdd(3.4, 5.4) << "\n";
 
       // create a int class with the default value of 3
       TemplateClass<int> intTemplate(3);
       std::cout << intTemplate.ReturnValue() << "\n";
 
       // using the same constructor, create a char class with the default value 'b'
       TemplateClass<char> charTemplate('b');
       std::cout << charTemplate.ReturnValue() << "\n";
 
       return 0;
}

Save as templates.cpp, then compile the program. The result displayed will be
Adding 4 8.8
3
b

So using the templates will cut down on the standard function/class types but you do need to be careful to not use any of the non internal class calls within the template, e.g. if you create a new type and do not proved a function to increment the values this will not work.

Just a note that you could add in more different types of templates by
template (return value) (function name) (parameters , e.g. S input1, T input2)

Friends

A friend of a class allow access to the private members of that class, because there are some cases where this is required when the whole of the created class is required to be viewable, e.g. when a private member is not being able to set with the class methods.

To declare a friend within a class, for example to declare a function / class as a friend.

friend int FuncToFriendClass(int a);              // friend to the function.
friend class ClassToClassFriend;              // friend to a class

This will allow the function FuncToFriendClass to gain access to all objects within the class, and the class ClassToClassFriend as well.

The source code

int FuncToFriendClass(int a);              // declare the function, for the class to make a friend.
 
class FriendlyClass 
{
private:
       int private_data;
 
       friend int FuncToFriendClass(int a);              // friend to the function.
       friend class ClassToClassFriend;              // friend to a class
public:
       FriendlyClass()
       {
              private_data = 5;
       }
};
 
int FuncToFriendClass(int a)
{
       FriendlyClass friendClass;                            // the friend to this funciton.
       return friendClass.private_data + a;
}
 
class ClassToClassFriend
{
public:
       int FriendClassPrivateDataMinus(int b)
       {
              FriendlyClass classFriend;
              return classFriend.private_data - b;
       }
};
 
int main()
{
       ClassToClassFriend classFriend;
       std::cout << "Friendly function to class =" << FuncToFriendClass(3) << "\n";
       std::cout << "Friendly class to class = " << classFriend.FriendClassPrivateDataMinus(2) << "\n";
       return 0;
}

If you save that as friend.cpp and then compile, the output will be
Friendly function to class = 8
Friendly class to class = 3

Pointers

Pointers are pointers to memory, the variable that is defined as a pointer still holds the same area in memory as the type of the pointer e.g. char and int are of different size in memory, just do a

cout << sizeof(char) << sizeof(int);

to find out.

The actually pointer holds the address of the variable it is pointing to, e.g.

The value variable

Value = 10, address within the memory for value = 4

The pointer

ValuePointer = 4 (the address of the value), address within the memory = 200

The valuepointer actually equals 4, the memory of the value variable, if you use the * (pointer) then you will get the value pointed to, 10, and if you use the & (address) you will get 200 which is the place in memory where the valuepointer is placed in memory.

For example

int main()
{
int value1 = 10;
int *valp = &value1;       // equals the memory address of value1
 
std::cout << "Value 1 = " << value1 << " address = " << &value1 << "\n";
std::cout << "Value p = " << *valp << " address = " << &valp << " pointed to memory " << valp << "\n";
std::cout << "The value p pointerd to memory is the same as the memory address as value 1\n";
std::cout << "The value p = the pointerd memory value that is assoicated with the value that is in valp (address of memory)\n";
 
(*valp)++;              // correct increament the value that is pointed to by the address held by vapl
std::cout << "Value 1 = " << value1 << " address = " << &value1 << "\n";
std::cout << "Value p = " << *valp << " address = " << &valp << " pointed to memory " << valp << "\n";
 
*valp++;              // incorrect will increament the address that is held valp by sizeof(int) (usually 4)
std::cout << "Value 1 = " << value1 << " address = " << &value1 << "\n";
std::cout << "Value p = " << *valp << " address = " << &valp << " pointed to memory " << valp << "\n";
 
return 0;
}

Local variables

This is a tutorial about variable scope, a variable is a means to store data within the context of the program. For example, if you wish to store a numerical value then you would not store that number in a string, but within a integer or floating point number.

There are different types of variables, integer / string etc, but the scope of a variable is the area in which the variable is defined within block of code that it is situated.

This is some c++ code to demonstrate the difference between local variables and global.

#include <iostream>
 
// global variable
int value1 = 0;
 
void globalVariable()
{
       // output the global variable
       std::cout << "Global " << value1 << "\n";
}
 
int main(void)
{
       int value1 = 1;                     // local function variable
       globalVariable();              // output the global
       std::cout << "Local " << value1 << "\n";       // output the local
 
       // the value1 here is local to the for loop
       for (int value1 = 10; value1 < 15; value1++)
       {
              std::cout << "For loop : " << value1 << "\n";
       }
 
       // output the global and local and notice they have not changed with the very local for loop 
       // with the same variable name.
       globalVariable();
       std::cout << "Local " << value1 << "\n";
       return 0;       
}

the output would be

Global 0
Local 1
For loop : 10
For loop : 11
For loop : 12
For loop : 13
For loop : 14
Global 0
Local 1

To demonstrate that the global /local and looping locals do only work within there area.

Add two numbers

This tutorial will read in two numbers from the console line and output the answer of the two integer values.

The cin refers to the console input, the >> allows the console input value placed into the variable (in this case the val1, val2), NOTE. << would mean output. With using the exception, and the try catch block of code allows for any errors in the conversion of the input into a integer value. Within the printf, there are a few conversion types, the %d means using the corresponding parameter within printf and output a integer value, %s means a string. When I say the corresponding parameter, if there are two %d within the output string, it will display in order the values attached to the printf(...) call. The code

#include <iostream>
#include <exception>
 
//using the namespace std (standard) for the cin -> console input
using namespace std;       
 
int main()
{
       // setup the defaul values.
       int val1 =0, val2 =0;
       try 
       {
              // output to the console
              printf("Please enter number 1 : ");
              // read in the input, if not a integer value, this will
              // cause a error 
              cin >> val1;       
              printf("Please enter number 2 : ");
              cin >> val2;
       }
       catch (exception& e)
       {
              // write out any error exception
              printf("Not a valid input %s\n", e.what());
       }
       // output the answer of the two inputted values.
       printf("Answer : %d\n", (val1 + val2));
       return 0;
}

save this as cppaddtwonumbers.cpp. Once compiled (g++ cppaddtwonumbers.cpp -o cppaddtwonumbers (the -o means to output to the filename else a a.out file name would be created))
Once executed the program, cppaddtwonumbers within Windows, or ./cppaddtwonumbers within Linux/Unix.

The output would be

Please enter number 1 : 23
Please enter number 2 : 41
Answer : 64