static and const

const can be use 1) functions parameter, 2) function return value and 3) member function.

When you use pointer or reference as function parameter, You should always put const in front of it. Becuause 90% you don’t need to modify it.

When function return build-in value, don’t use const at all.

const int foo() {
   return 3;
}
int x = foo();  // copies happily
foo() = 4;  // not valid anyway for built-in types

When function return user-defined type value, don’t use const at all. It will not allow you to use rvalue and move.

time operator+(const time &t){
time temp;
return temp.bla = bla+t.bla;
}

When function return reference or pointer, you can use const to restrict modify it.

class String{
const char& operator[](int position);
}

When const used in member function. There are two usages: detail can be seen effective c++ item 21

In const member function, you can’t change member variable,(If you change it, it will compile error).
Only const obj can call const function. But non-const obj can call ALL funcitons( const or non-const).

time operator+(const time & t1) const{
   this->m_a = 100// will compile error
}

char& operator[int index]; //for non-const  obj
const char& operator[int index] const;  for const obj

In C++, global and static variables initialized to default values.

You can’t initialize a static member variable inside the class declaration, you need to put it in a .cpp file. But you can if the static data member is const of integer or enumeration.

class{  //in .h file
  static int obj_num;  // you can’t initialize
  const static int months = 12; // you can  initialize
};

//In .cpp file
Int class::obj_num = 0; // no static keyword anymore.
// obj_num will be default value(0) even you don’t init it.

Static member function has two usages: 1)It can be invoked just by class name, not object instance, so you can define math class and define a lot static math function inside it. Just like name space.
2) It can’t access class data member, only can access class static member data.

static can be used restrain the scope

Int global = 0; //All files
static int one_file = 50; //just in this file
main(){
Static int inside_main = 100; //just in this block
}

for const, you need to tell the difference below:

const char *p; //const char
char* const p; //const p
const char * const p; //both const
// use * as separation symbol

1.1.6 static and const