union & struct & aggregate & constexpr

using `aggregate initialization` on `classes` and `structs`

aggregate initialization syntax

1	Type objectName = {argument1,...,argumentN}

alternatively

1	Type objecname {argument1,...,argumentN}

aggregate initialization applied to classes and structs

#include <iostream>
#include <string>
using namespace std;

class Aggregate1
{
public:
    int num;
    double pi;
};

class Aggregate2
{
public:
    char hello[6];
    int impYear[3];
    string world;
};

int main()
{
    Aggregate1 a1{2023, 3.14};
    cout << "approximately pi is " << a1.pi << endl;

    Aggregate2 a2{{'h', 'e', 'l', 'l', 'o', '\0'}, {2020, 2021, 2023}, {"test"}};
    cout << "the string is " << a2.world << endl;

    return 0;
}

`constexpr` with `classes` and `objects`

The basic effect is improving the performance of application.

e.g. using constexpr with class

#include <iostream>
using namespace std;

class Human
{
    int Age;

public:
    constexpr Human(int HumanAge) : Age(HumanAge) {}
    constexpr int GetAge() const { return Age; }
};
int main()
{
    constexpr Human somePerson(15);
    const int hisAge = somePerson.GetAge();
    //cout << hisAge << endl;
    Human anotherPerson(18);

    return 0;
}

uncomment line 16 output is 15. Comparing line 17 and 14-15, the later is optimized for performance.

differences in using dot operator`.` and pointer operator`->`

If you have a pointer to an object, the pointer operator would be best suited. If you have instantiated an object as a local variable on the stack, then the dot operator is best suited.

All my class members are private, and my class does not contain any declared friend class or function. Who can access these members?

None except member methods of the same class ref 1

members of a class are private by default as opposed to those in a struct

e.g.

class Human
{
    int Age;//Age is private
};

contrarily

struct Human
{
    int Age;// Age is public
};

A programme for calculating area and circumference of a circle

#include <iostream>
#include <string>
using namespace std;

class CalCircle
{
    double Radius;
    const double Pi;

public:
    CalCircle(double urRadius)
        : Radius(urRadius), Pi(3.14) {}
    void GetArea()
    {
        double circleArea = 0;
        circleArea = Pi * Radius * Radius;
        cout << "the area is " << circleArea << endl;
    }

    void GetCircumference()
    {
        double circleCircumference=0;
        circleCircumference=2*Pi*Radius;

        cout <<" the circumference is " << circleCircumference<< endl;
    }
};

int main()
{
    double yourRadius=0;
    cout << "plz input ur Radius ";
    cin >>yourRadius;

    CalCircle MyCircle(yourRadius);
    MyCircle.GetArea();

    return 0;
}