CONST
Reference: CONST in c++
If you decalre something with const, you're saying that you are not going to modify that thing.
Combning with pointers
const int* a = new int;
// or
int const* a = new int;
// they mean that you can not modify the content of the memory address to which a pointed.
int* const a = new int;
// it means that you can not modify what memory address a pointed to.
const int* const a = new int;
// both above can not
Read it backward, like the compiler does. For instance :
-
const int * A; ==> "A is a pointer to an int that is constant."
(or, depending on how you prefer to write it) -
int const* A; ==> "A is a pointer to a const int"
-
int * const A; ==> "A is a const pointer to an int."
-
const int* const A; ==> "A is a const pointer to an int that is constant".
Const int * A does not mean that A actually points to a const variable. It just means that the compiler will not allow you to modify the pointed value through A.
for instance :
int val = 10;
int const * a = &val;
*a = 30; //this will NOT compile, you're not allowed to modify "val" through "a".
val = 30; //this will compile.
Same with :
int val = 10;
int val2 = 30;
const int * const A = &val;
A = &val2; //will NOT compile : you can't modify what A points to.
*A = 30; //will NOT compile : you can't modify val through A
val = 30; //this will compile, val is not constant
Compiling things:
// the code
const int MAX_AGE = 90;
int* a = new int;
a = (int*)&MAX_AGE;
*a = 85;
std::cout << *a << " " << MAX_AGE << std::endl;
// The output: 85 90
//I used the memory inspector in debug mode and saw the value pointed by &MAX_AGE actually being changed from 90 to 85 . So what the hell? Why is 90 the output for _MAX_AGE_, if the memory was actually modified!??
//I took one step further and inspected the generated .asm file (as taught by Cherno) to realize that the compiler optimized the code by replacing all read operations of MAX_AGE with the literal 90 . That cleared everything!! It's reasonable for a compiler to eliminate the need of accessing a variable if its never gonna change: just replace its occurrence with the said value from the get-go. One less operation each time it's used.
With classes and methods
class Entity
{
private:
int m_X, m_Y;
mutable int var; // even in a const decalred method, you can modify this variable
public:
int GetX() const // const after the method name declares that it would not modify the class member.
{
var = 5; // you can modify it;
return m_X;
}
int GetX()
{
m_X = 5;
return m_X;
}
};
void PrintEntity(const Entity& e)
{
// const declare means that you can not modify the completely things of e, so if you call one method without const declared, you'll get an error
std::cout << e.GetX() << std::endl;
}
int main()
{
Entity e;
}