Declaring a variable as constant in C and C++ means that its value cannot be changed once it is set.

In C, you can declare a constant using the keyword "const". For example:

const int MAX_AGE = 100;

There are many different ways to make the variable as constant

1. Using const keyword: The const keyword specifies that a variable or object value is constant and can’t be modified at the compilation time.

// C program to demonstrate const specifier
#include 
int main()
{
	const int num = 1;

	num = 5; // Modifying the value
	return 0;
}

1. 2. Using enum keyword: Enumeration (or enum) is a user defined data type in C and C++. It is mainly used to assign names to integral constants, that make a program easy to read and maintain.

// In C and C++ internally the default
// type of 'var' is int
enum VARS { var = 42 };

// In C++ 11 (can have any integral type):
enum : type { var = 42; }

// where mytype = int, char, long etc.
// but it can't be float, double or
// user defined data type.

Note: The data types of enum are of course limited as we can see in above example.

1. 3. Using constexpr keyword: Using constexpr in C++(not in C) can be used to declare variable as a guaranteed constant. But it would fail to compile if its initializer isn’t a constant expression.

#include 

int main()
{
	int var = 5;
	constexpr int k = var;
	std::cout << k;
	return 0;
}

Above program will throw an error i.e.,

error: the value of ‘var’ is not usable in a constant expression

because the variable ‘var’ in not constant expression. Hence in order to make it as constant, we have to declare the variable ‘var’ with const keyword.

1. 4. Using Macros: We can also use Macros to define constant, but there is a catch,

#define var 5

Since Macros are handled by the pre-processor(the pre-processor does text replacement in our source file, replacing all occurrences of ‘var’ with the literal 5) not by the compiler.
Hence it wouldn’t be recommended because Macros doesn’t carry type checking information and also prone to error. In fact not quite constant as ‘var’ can be redefined like this,

C++

// C++ program to demonstrate the problems
// in 'Macros'
#include 
using namespace std;

#define var 5
int main() {
	printf("%d ", var);

	#ifdef var
	#undef var

	// redefine var as 10
	#define var 10
	#endif

	printf("%d", var);
	return 0;
}

C

// C program to demonstrate the problems
// in 'Macros'
#include 

#define var 5
int main()
{
	printf("%d ", var);

#ifdef var
#undef var

// redefine var as 10
#define var 10
#endif

	printf("%d", var);
	return 0;
}

Output:

5 10

Note: preprocessor and enum only works as a literal constant and integers constant respectively. Hence they only define the symbolic name of constant. Therefore if you need a constant variable with a specific memory address use either ‘const’ or ‘constexpr’ according to the requirement.