How can stream manipulators in C++ be functions?

When calling a function in C++, function's name is written followed by () to distinguish it as a function call. Why can't I call stream manipulator functions in the same way?

Why this isn't allowed?:

cout << "Hello!" << endl();

Isn't endl a variable holding \n?

Thanks!

Solution

Isn't endl a variable holding \n?

No, it is not. std::endl is a function defined in global namespace

  template<typename _CharT, typename _Traits>
    inline basic_ostream<_CharT, _Traits>& 
    endl(basic_ostream<_CharT, _Traits>& __os)
    { return flush(__os.put(__os.widen('\n'))); }

In expression std::cout << endl_or_something right hand side of << is an argument of a call to operator<< (first argument is std::ostream implicitly). So endl_or_something should be an int, double or other type that can be converted to one of the possible arguments of operator<<. There is an overloaded version of this operator that takes pointers to functions ( functions which take reference to std::ostream and return reference to std::ostream):

  // [27.6.2.5] formatted output
  // [27.6.2.5.3]  basic_ostream::operator<<
  //@{
  /**
   *  @brief  Interface for manipulators.
   *
   *  Manipulators such as @c std::endl and @c std::hex use these
   *  functions in constructs like "std::cout << std::endl".  For more
   *  information, see the iomanip header.
  */
  __ostream_type&
  operator<<(__ostream_type& (*__pf)(__ostream_type&))
  {
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// DR 60. What is a formatted input function?
// The inserters for manipulators are *not* formatted output functions.
return __pf(*this);
  }

Since std::endl signature matches, it can be used in expression

std::cout << "Hello!" << std::endl;

or equivalently

std::cout << "Hello!";
std::endl( std::cout);

Note however that this manipulator is often mistakenly used when a simple newline is desired, leading to poor buffering performance. In such cases use just "\n".

Why this isn't allowed?:
cout << "Hello!" << endl();

std::endl takes one argument, std::ostream. You can see that it can be called with:

return __pf(*this);

means

return std::endl( *this); // std::endl( std::cout);

There is no version of std::endl that takes no parameters so it could be called with

std::endl()

In expression

std::cout << std::endl;

it denotes an argument to operator<<, it is passed as a pointer to function and then called in body of operator<<.