1. const

A variable or an object declared by keyword const is not motifiable.

2. Roles

(1) Declare a constant variable

const int LENGTH = 20;

(2) Type checking
const variable is commonly confused with #define micro, however, they are technically different. The constant variable declared by const is typed, so it can be type-checked by compiler through static type checking. But the micro defined by #define is a typeless placeholder. It does nothing more than a string replacement while the program is compiled, so the static type checking has nothing to do with it.

(3) Foolproof
Prevent your stupid co-workers from changing the value of a predefined constant.

bad coding style:

// This is fxxking important!!!
// DO NOT change its value, or I will KILL you!!!
int TOTAL_NUM = 100;

good coding style:

const int TOTAL_NUM = 100;

(4) Memory saving
In the assembly language, a const variable is referred by a memory address, rather than a concrete value, like #define does. So, a const variable has only one copy in the memory while #define micro may have multiple copies.

3. Scope

(1) A nonconst variable can be referred in another file.

// example1.cpp
int example;
// example2.cpp
#include<iostream>
using namespace std;

extern int example;

int main(){
    cout << example << endl;
}
// g++ -o executable example2.cpp example1.cpp
// ./executable
// 0

(2) A variable declared by keyword const is local to file by default, which means if you want it to be referred in another file, you have to explicitly declare that using extern.

// example1.cpp
extern const int example = 100;
// example2.cpp
#include<iostream>
using namespace std;

extern int example;

int main(){
    cout << example << endl;
}
// g++ -o executable example2.cpp example1.cpp
// ./executable
// 100

Notice that in order to be used externally, a const variable has to not only be declared by extern but also to be initialized because its value is unmotifiable after declarition.

4. Utility and common mistakes

(1) Unmotifiable after declarition

const int a = 123;
a = 666; // error!

(2) Must be initialized

const int b; // error!