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• Variables in C++


A variable is a value that may change during the execution of a program. C++ language has specified some rules which should be followed in order to give a variable name in a C++ program. These rules are as follows :


• A variable name must start with an alphabet.
• In general, the total length of a valid variable name could be between 1 to 8, though some compiler may allow larger length i.e. length until 255 characters.
• A variable name must not contain a space or a comma.
• A variable name may contain an underscore.


Let's see an example of how we can declare variable in a program :



#include<iostream>

using namespace std;

int main()
{
	int size = 1000;
	float ink_prc = 9.50;
	short dist2 = 20;
	char alpha_1 = 'a';
	
	cout<< "The value in variable size is : " << size << "\n";
	cout<< "The value in variable inc_prc is : " << ink_prc  << "\n";
	cout<< "The value in variable dist2 is : " << dist2 << "\n";
	cout<< "The value in alpha_1 is : " << alpha_1 << "\n";
	
	return 1;
}





Output




The value in variable size is : 1000
The value in variable inc_prc is : 9.500000
The value in variable dist2 is : 20
The value in alpha_1 is : a





• Keywords in C++


Keywords are the special words whose meaning is already defined in C++ programming language. Keywords in C++ are also known as reserved words because name of any keyword cannot be used for a variable name in a C++ program. There are about 64 keywords defined in C++ language(an extension to 32 keywords in C). Let's see a table of these keywords defined in C++ language.



keywords	keywords	keywords	keywords
if	else	enum	extern
float	do	for	goto
double	int	long	near
register	return	short	signed
static	struct	switch	typedef
union	unsigned	void	while
auto	break	char	case
far	const	continue	default
namespace	explicit	operator	friend
public	private	protected	virtual
new	inline	mutable	typename
dynamic_cast	reinterpret_cast	static_cast	const_cast
this	throw	using	class
true	bool	template	class
catch	try	asm	wchar_t
delete	false






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• Constants


Constant is a value that remains the same and does not change or it cannot be modified. C++ language allows us to define constants in a program, using the keyword const and the value of such constant does not change throughout the course of execution of the program, even when one intentionally tries to modify it. Let's look at an example to define a constant.



// declaring a constant in C++

#include<iostream>

using namespace std;

int main()
{
	const int distance = 10;
	cout<< "The value of constant distance is : " <<  distance;
	return 0;
}





Output




The value of constant distance is : 10







Trying to modify the a constant value in C++


Let's try to modify a constant value declared with const keyword in a C++ program named Cons.cpp and see what happens.

Cons.cpp

//Trying to modify a constant in C++

#include<iostream>

using namespace std;

int main()
{
	const int kg = 1000;
	kg = 2000;
	cout<< "The value of kg is : " <<  kg;
	return 0;
}





Output




Cons.cpp: In function 'int main()':
Cons.cpp:10:7: error: assignment of read-only variable 'kg'
  kg = 2000;
       ^~~~



As you can see in the output of the last program, the compiler has thrown an error stating that because kg is a declared at a constant using const keyword, that's why its value is just read-only and cannot be modified.





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