What is the << operator doing in C++?

In the example below, what exactally is the << operator doing? I'm guessing it is not a bitwise operator.

std::cout << "Mouse down @ " << event.getPos() << std::endl;

I understand what the code will do here: Use standard out, send this text, send an end of line. Just I've never come accross the use of this << apart from on raw binary.

I'm starting out with C++. And, as an operator of sorts, it's hard to search for a description of this and what it means. Can someone enlighten me and/or give me a pointer as to what to google for?

Thanks Ross

From stackoverflow

• It is a "bitwise left shift" operator.

  n << p

  Shifts the bits of n left p positions. Zero bits are shifted into the low-order positions. 3 << 2 is 12.

  In the context of the question it pushes something into 'cout' which is the current output stream.

  Nick Meyer : -1 Not in this context...

  Carl Smotricz : @Nick Brooks: I've edited your last line to make it a bit clearer, but it's probably too late to save your answer from the hate it's getting.

• The operator<< is being overloaded. Check out Operator Overloading.

• This is sometimes called the 'stream insertion operator', and this is the most common use: to insert data into a stream. Sometimes, however, I've seen it overloaded to insert data into other objects when doing things like serialization, for example.

• The answer is: The << operator does left shifts by default for integral types, but it can be overloaded to do whatever you want it to!

  This syntax for piping strings into a stream was first (I think) demonstrated in C++ inventor Bjarne Stroustroup's eponymous book The C++ Programming Language. Personally, I feel that redefining an operator to do IO is gimmicky; it makes for cool-looking demo code but doesn't contribute to making code understandable. Operator overloading as a technique has been widely criticized in the programming language community.

  ---

  EDIT: Since nobody else has mentioned this yet:

  operator<< is defined in the ostream class, of which cout is an instance. The class definition sits in the iostream library, which is #include'd as <iostream>.

  rubenvb : I think it's a great tool to say exactly what you want in your code, and very useful when using stringstreams and fstreams for piping data into something

  Billy ONeal : Actually, if you want to be pedantic, `operator<<` is a free function with an `ostream` as it's first argument.

  Carl Smotricz : @Billy ONeal: Surely you're more of an expert than I am, but isn't it an instance method of the `ostream` class, with (whatever's to the right) as its first argument?

  Exception : I don't think operator overloading should be used at all because class methods can achieve the same functionality. In the case of `ostream`, the `operator<<` functionality can be achieved by having a method such as `write_formatted` that is chainable.

  Billy ONeal : @Exception: Thank you for your opinion. Unfortunately what you're looking for breaks down when someone tries to do something like `myOstream << myClass`. Someone can add `operator<<` s just by adding an additional method; however one cannot just stick methods onto standard library classes.

  Billy ONeal : @Carl: Actually, we're both right -- several of the operator<

  avakar : @Exception: I also don't think that the stream insertion operator should have an overloaded meaning for integral types, when the same functionality can be achieved by adding a library function `lshift`.

  Steve Jessop : "Operator overloading as a technique has been widely criticized in the programming language community". Although the biggest direct criticism I've seen of operator overloading in C++ came with the design of Java, which itself (hypocritically, C++ fans could argue) uses operator overloading, for example in String addition. It just doesn't allow mere programmers to overload operators - that right is reserved to Language Designers ;-) (In reality, the biggest direct criticism of C++ operator overloading might be the average programmer's use of it...)

• Like any operators in c++, << is doing operations. Using overloading, with an ostream left operand (std::cout is of ostream type), it's used as a stream operator to print data of various types. For example, you can do

  int x = 10;
  std::string y = " something";
  std::cout << x << y << std::endl;
  

  This will output "10 something".

  @ is not replaced by anything in this context. operator<< just dump the result.

  std::endl is not only the end of line, it also flushes the result to the output device.

  Ross : thanks for flagging the '@' & std::endl too.

• Try writing a program where you create an object and call the overloaded << operator,

  class x {
      //declare some pvt variables
      //overload << operator
  };
  
  int main() {
      x obj;
      cout << obj;
  }
  

  By doing so you will understand the rationale behind using the following statement

  cout << string_var << int_var;
  

  You can assume 'string' and 'int' as classes that have overloaded << operator even though not true.