const in C++

It's mostly found when declaring a variable something like const int a = 3; However, it is a sort of "promise" for human being since it will usually not make a difference in the compiled code.

Just as we can break a "promise" sometimes, there is also a workaround to modify const variable.

const variable

It's self-explanatory. We know what it means when we see it.

const int a = 3;

// illegal
// a = 5;

const with pointer

This is when the most confusing part because there are following variations

const int* a;
int* const a;
int const* a;
const int* const a;

what I would like to think is that const decorate something that is in its left. For example,

• int const * means int is constant because int is in the left.

If there is nothing in its left, const looks for its right side.

• const int * means the same as above. int is constant not int*

Then it's fairly easy.

// int is constant
// content cannot be changed
const int* a;

// ok (can be assigned to another pointer)
a = nullptr;

// `const` decorate `*`
// pointer cannot be re assigned
int* const a = new int;

// But, this is ok
*a = 33;

// not ok
// a = nullptr;

// `const` decorate `int`
// it's same as `const int* a`
int const* a = new int;