C/C++ - const with Pointer or Reference

const with Normal Variables

Two ways to add const for normal variables:

const TYPE NAME = VALUE; // more common
TYPE const NAME = VAULE;

Both means this variable cannot be assigned to another value.

For example,

#include <iostream>

using namespace std;

int main(void)
{
	const int i = 1;
	int const j = 1;
	i = 2; // error
	j = 2; // error
	cout << "i = " << i << endl;
	cout << "j = " << j << endl;

	return 0;
}

The same error cames out for i and j:

const.cpp:9:4: error: cannot assign to variable 'i' with const-qualified type 'const int'
        i = 2;
        ~ ^
const.cpp:7:12: note: variable 'i' declared const here
        const int i = 1;
        ~~~~~~~~~~^~~~~
const.cpp:10:4: error: cannot assign to variable 'j' with const-qualified type 'const int'
        j = 2;
        ~ ^
const.cpp:8:12: note: variable 'j' declared const here
        int const j = 1;
        ~~~~~~~~~~^~~~~
2 errors generated.

const and Reference

Also two ways to add const to a reference:

const TYPE &NAME = VALUE; // more common
TYPE const &NAME = VAULE;

Both have the same meaning.
There are two limits for them:

1. This reference cannot be assigned to another variable
2. The variable being referenced cannot change its value with this reference,
   but it can change its value without using that reference.

For example,

#include <iostream>

using namespace std;

int main(void)
{
	int i = 1, j = 2;
	int const &r1 = i;
	const int &r2 = i;

	// change value with reference
	r1 = 3;  // error
	r2 = 3;  // error

	// change value
	i = 4;

	// change reference object
	r1 = j;  // error
	r2 = j;  // error

	return 0;
}

constant reference can only be read.
if the value of the variable it referenced to has be changed,
it can only change the value without that reference.

const and Pointer

This one is complicated.

But we can use the position of const to remember which one const is decorating:

TYPE* const pNAME;  // 1
TYPE const *pNAME;  // 2
const TYPE *pNAME;  // 3
const TYPE* const pNAME;  // 4

For 1,
const decorates pNAME,
that means pNAME cannot be changed (No pNAME = ...).

For 2,
const decorates *pNAME,
so *pNAME cannot be changed (No *pNAME = ...).

For 3,
const decorates TYPE *pNAME.
it is the same as 2,
saying that *pNAME cannot be changed (No *pNAME = ...).

For 4,
const decorates pNAME and TYPE*,
so both pNAME and TYPE* cannot be changed (No pNAME = ... or *pNAME = ...).

#include <iostream>

using namespace std;

int main(void)
{
	int i = 1, j = 2;
	int* const p1 = &i;
	int const *p2 = &i;
	const int *p3 = &i;
	const int* const p4 = &i;

	// change value with pointer
	*p1 = 2;
	*p2 = 2;  // error
	*p3 = 2;  // error
	*p4 = 2;  // error

	// change value
	i = 3;

	// change pointer position
	p1 = &j;  // error
	p2 = &j;
	p3 = &j;
	p4 = &j;  // error

	return 0;
}