24.2 Assignment Type Conversions

Certain type conversions occur automatically in assignments and certain other contexts. These are the conversions assignments can do:

• Converting any numeric type to any other numeric type.
• Converting void * to any other pointer type (except pointer-to-function types).
• Converting any other pointer type to void *. (except pointer-to-function types).
• Converting 0 (a null pointer constant) to any pointer type.
• Converting any pointer type to bool. (The result is 1 if the pointer is not null.)
• Converting between pointer types when the left-hand target type is upward compatible with the right-hand target type. See Compatible Types.

These type conversions occur automatically in certain contexts, which are:

• An assignment converts the type of the right-hand expression to the type wanted by the left-hand expression. For example,

  double i;
  i = 5;
  

  converts 5 to double.

• A function call, when the function specifies the type for that argument, converts the argument value to that type. For example,

  void foo (double);
  foo (5);
  

  converts 5 to double.

• A return statement converts the specified value to the type that the function is declared to return. For example,

  double
  foo ()
  {
    return 5;
  }
  

  also converts 5 to double.

In all three contexts, if the conversion is impossible, that constitutes an error.