Local Variables & Function Arguments

Local Variables

• They only exist inside a scope.
  • scope: Defined within a pair of braces.

  {            // start of scope
        int i; // allocate memory for i
        ...    // use i
  }            // end of scope, release memory used by i

Pass By Value

• By default, a variable which is passed into a function is copied, as is it’s return value.
• This is effectively taking double the memory, as memory is consumed by the original variable and the one copied from it.

  int func(int y) {   // y is a copy of what is passed into func
      return y;       // copies y into the function's return value
  }

Pass By Address

• Involves declaring the function parameter as a pointer, and then supplying the function with a reference to the original variable.
• No double allocation of memory is involved.

  void func(int* y) { // y will be a pointer to the caller's variable x
      *y = 1;
  }
  
  int x = 0;
  func(&x);           // x will now have the value 1

Pass By Reference

• Acts like a pointer which is automatically dereferenced when used.

  void func(int &y) { // y will be a reference to the caller's variable x
      y = 1;
  }
  
  int x = 0;
  func(x)             // x will now have the value 1

Pass By Const Reference

• For read-only access to class objects, usually more efficient than pass by value.

  class MyClass {...};
  
  void func(const MyClass &mc) { // mc will be a reference to the caller's object
    // do stuff
  }
  
  MyClass my_class;
  func(my_class) // pass object to func