• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 432
  • Last Modified:

ostream

To C++ experts,

    If I want my print() function to be able to print things both on files and screen, is that better to declare the funtion as "void print(ostream &os)"
or "void print(ostream os)" ? or any other format ? and why ?

    thanks.

meow.
0
meow00
Asked:
meow00
  • 2
2 Solutions
 
efnCommented:
Passing the stream by reference as in

void print(ostream & os)

is preferable.  Stream objects are not designed to be passed around by value, so you should only have one stream object and pass it around by reference.

A stream object needs to maintain state, and the state changes when you output to the stream.  It may be buffering the output or maintaining a file position.  If you pass the stream by value, the print function will change the state of a temporary copy of the stream object, so the original stream object will be confused by operations going on behind its back.

--efn
0
 
AxterCommented:
According to the C++ standard, you're not allowed to pass a stream object by value.

A few compilers, like VC++ 6.0 will let you compile this type of code, but will then crash at run time.

So never pass a stream object by value.  It will either fail to compiler, or crash.
0
 
AxterCommented:
FYI:

Section 27.4.4 of C++ Standard:
private:
basic_ios(const basic_ios& ); // not defined
basic_ios& operator=(const basic_ios&); // not defined

basic_istringstream derives from basic_ios via the following inheritance.

basic_istringstream->basic_istream->basic_ios

The above section of the C++ standard shows the copy constructor and assignment operator as private and not defined.

That means any derived class like ostream can not be pass by value.
In a compliant implementation, you'll get a compile error if you try to pass by value.
0

Featured Post

Concerto Cloud for Software Providers & ISVs

Can Concerto Cloud Services help you focus on evolving your application offerings, while delivering the best cloud experience to your customers? From DevOps to revenue models and customer support, the answer is yes!

Learn how Concerto can help you.

  • 2
Tackle projects and never again get stuck behind a technical roadblock.
Join Now