C++ Constants

Introduction

When we want to have a read-only object or promise not to change the value of some object in the current scope, we make it a constant.

C++ uses the const type qualifier to mark the object as a read-only.

We say that our object is now immutable.

To define an integer constant with a value of 5, for example, we would write: 

int main() 
{ 
    const int n = 5; 
}

We can now use that constant in places such as an array size: 

int main() 
{ 
    const int n = 5; 
    int arr[n] = { 10, 20, 30, 40, 50 }; 
}

Constants are not modifiable, attempt to do so results in a compile-time error: 

int main() 
{ 
    const int n = 5; 
    n++; // error, can't modify a read-only object 
}

An object declared const cannot be assigned to; it needs to be initialized.

So, we can't have: 

int main() 
{ 
    const int n;       // error, no initializer 
    const int m = 123; // OK 
}

const modifies an entire type, not just the object.

So, const int and int are two different types.

The first one is said to be const-qualified.




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