operator<< cannot output std::endl -- Fix?

Question

The following code gives an error when it's supposed to output just std::endl:

#include <iostream>
#include <sstream>

struct MyStream {
  std::ostream* out_;
  MyStream(std::ostream* out) : out_(out) {}
  std::ostream& operator<<(const std::string& s) {
    (*out_) << s;
    return *out_;
  }
};

template<class OutputStream>
struct Foo {
  OutputStream* out_;
  Foo(OutputStream* out) : out_(out) {}
  void test() {
    (*out_) << "OK" << std::endl;
    (*out_) << std::endl; // ERROR     
  }
};

int main(int argc, char** argv){
  MyStream out(&std::cout);
  Foo<MyStream> foo(&out);
  foo.test();
  return EXIT_SUCCESS;
}

The error is:

stream1.cpp:19: error: no match for 'operator<<' in '*((Foo<MyStream>*)this)->Foo<MyStream>::out_ << std::endl'
stream1.cpp:7: note: candidates are: std::ostream& MyStream::operator<<(const std::string&)

So it can output a string (see line above the error), but not just the std::endl, presumably because std::endl is not a string, but the operator<< definition asks for a string.

Templating the operator<< didn't help:

  template<class T>
  std::ostream& operator<<(const T& s) { ... }

How can I make the code work? Thanks!

Answer 1

You need to add this to your struct MyStream:

  std::ostream& operator<<( std::ostream& (*f)(std::ostream&) )
  {
      return f(*out_);
  }

std::endl is a function that appends a newline and flushes the underlying stream; this function signature accepts that function and applies it to the ostream member.

Then, as a test, defining foo::test as

  void test() {
    (*out_) << "start";
    (*out_) << std::endl;
    (*out_) << "done";
  }

will correctly output

start
done