How should I correctly assign cout to a static ostream reference variable?

Question

I'm defining a class like this:

class StaticRuntimeContext {
 public:
  enum Verbosity {
    kHIGH,
    kMEDIUM,
    kLOW,
    kSILENT
  };
  static void Construct();
  static std::ostream& stdout1() {return stdout1_;}
  static std::ostream& stdout2() {return stdout2_;}
  static std::ostream& stdout3() {return stdout3_;}
  static std::ostream& stderr() {return stderr_;}
 protected:
 private:
  static std::ostream& stdout1_;
  static std::ostream& stdout2_;
  static std::ostream& stdout3_;
  static std::ostream& stderr_;
};

I'm defining the construct function as:

void StaticRuntimeContext::Construct() {
  std::ostream& test = cout;
  stdout1_ = cout;
  stdout2_ = cout;
  //stdout3_ = NULL;
  stderr_ = cerr;
}

I cannot understand why assigning cout to test (std::ostream&) is OK to compile but the compiler produces error messages for the rest like "stdout1_=cout". The error message is:

/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/bits/ios_base.h:791:5: error: ‘std::ios_base& std::ios_base::operator=(const std::ios_base&)’ is private

I'm wondering what I should do to correctly assign cout to these ostream reference variables. Thanks!

Answer 1

It's because references have value semantics, and the operator = is copying the object instead of assigning a new reference.

Instead of references you should define static pointers, assign them in Construct, and return references in your accessors

  static std::ostream& stdout1() {return *stdout1_;}
  static std::ostream& stdout2() {return *stdout2_;}
  static std::ostream& stdout3() {return *stdout3_;}
  static std::ostream& stderr()  {return *stderr_;}
 protected:
 private:
  static std::ostream* stdout1_;
  static std::ostream* stdout2_;
  static std::ostream* stdout3_;
  static std::ostream* stderr_;

void StaticRuntimeContext::Construct() {
  stdout1_ = &cout;
  stdout2_ = &cout;
  stdout3_ = &cout;
  stderr_ = &cerr;
}

EDIT: And you must add this in your .cpp file

std::ostream* StaticRuntimeContext::stdout1_ = NULL;
std::ostream* StaticRuntimeContext::stdout2_ = NULL;
std::ostream* StaticRuntimeContext::stdout3_ = NULL;
std::ostream* StaticRuntimeContext::stderr_ = NULL;