What is the difference between Initializing and Assignment inside a constructor?

Foo(int num): bar(num)

bar

num

What is the difference between Initializing and Assignment inside a constructor?

Foo(int num): bar(num) {};

Foo(int num)
{
bar = num;
}

Cost of Member Initialization = Object Construction
Cost of Member Assignment = Object Construction + Assignment

Foo(int num) : bar() {bar = num;}

Foo(int num): bar(num){}

When do you HAVE TO use Member Initializer list?

A code example:

class MyClass
{
public:
//Reference member, has to be Initialized in Member Initializer List
int &i;
int b;
//Non static const member, must be Initialized in Member Initializer List
const int k;

//Constructor’s parameter name b is same as class data member
//Other way is to use this->b to refer to data member
MyClass(int a, int b, int c):i(a),b(b),k(c)
{
//Without Member Initializer
//this->b = b;
}
};

class MyClass2:public MyClass
{
public:
int p;
int q;
MyClass2(int x,int y,int z,int l,int m):MyClass(x,y,z),p(l),q(m)
{
}

};

int main()
{
int x = 10;
int y = 20;
int z = 30;
MyClass obj(x,y,z);

int l = 40;
int m = 50;
MyClass2 obj2(x,y,z,l,m);

return 0;
}

MyClass2

MyClass

Important points to Note while using Member Initializer Lists: