cpp key workds

  • decltype
    decltype is a keyword that is used to obtain the type of an expression or an entity

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    #include <iostream>

    int main() {
        int x = 42;
        double y = 3.14;

        // Using decltype to declare a variable with the same type as another variable
        decltype(x) result1 = x;  // result1 has the type int
        decltype(y) result2 = y;  // result2 has the type double

        // Using decltype with an expression
        int a = 10;
        int b = 20;
        decltype(a + b) sum = a + b;  // sum has the type int

        return 0;
    }

  • friend
    In C++, the friend keyword is used to grant non-member functions or other classes access to the private and protected members of a class. When a function or class is declared as a friend of another class, it is allowed to access private and protected members of that class as if it were a member of that class.

  • const

    1
    2
    3
    4
    const int var = 1;
    int* a = new int;
    *a = 2; // change the pointed int
    a = (int*)&var; // change pointer to pointing another int
1
2
3
4
const int var = 1;
const int* a = new int; // cannot change the value pointed by a; same as written `int const* a = new int`
*a = 2; // illegal
a = (int*)&var; //  valid
1
2
3
4
5
6
7
8
9
10
const int var = 1;
int* const a = new int; // cannot change the pointer address, but can change the pointed value;
*a = 2; // valid
a = (int*)&var; //  invalid

```cpp
const int var = 1;
const int* const a = new int; 
*a = 2; // invalid
a = (int*)&var; //  invalid
1
2
3
4
5
6
7
8
9
10
11
class Entity
{
private: 
    int m_x, m_y;
public:
    int GetX() const // const here means the method will not change state of the object
    {
        return m_x;
    }

}
  • const is a promise, you can bypass it actually.