C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Perform output if stream is good          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message William Payne Guest Posted: Sat Sep 25, 2004 10:42 pm    Post subject: Perform output if stream is good Hi, I have a function declared as: void foo(const std::string& s, std::ostream& verbose_output); I want foo() to write a lot of data to the ostream if it's a valid stream. If it's valid or not should depend on user input (command line arguments actually). If the user decides he/she wants verbose output, I will pass std::cout as the last argument when calling foo(). But what should I pass if the user doesn't want any output? And how should foo() check if the stream is good? verbose_output.good()?. Thanks for any replies / WP Back to top Victor Bazarov Guest Posted: Sun Sep 26, 2004 1:16 am    Post subject: Re: Perform output if stream is good "William Payne" <mikas493_no_spam (AT) student (DOT) liu.se> wrote... Quote: void foo(const std::string& s, std::ostream& verbose_output); I want foo() to write a lot of data to the ostream if it's a valid stream. If it's valid or not should depend on user input (command line arguments actually). If the user decides he/she wants verbose output, I will pass std::cout as the last argument when calling foo(). But what should I pass if the user doesn't want any output? And how should foo() check if the stream is good? verbose_output.good()?. Yes... Invent your own stream with '/dev/null' buffer which will always be valid and pass when you don't need verbosity. The problem is that if you have a reference to a stream, it must be some real stream, otherwise where did you get a reference to it. Of course, you could use 'ostringstream' instead, but I am not sure what you make it "bad"... Victor Back to top William Payne Guest Posted: Sun Sep 26, 2004 8:19 am    Post subject: Re: Perform output if stream is good "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote Quote: "William Payne" <mikas493_no_spam (AT) student (DOT) liu.se> wrote... void foo(const std::string& s, std::ostream& verbose_output); I want foo() to write a lot of data to the ostream if it's a valid stream. If it's valid or not should depend on user input (command line arguments actually). If the user decides he/she wants verbose output, I will pass std::cout as the last argument when calling foo(). But what should I pass if the user doesn't want any output? And how should foo() check if the stream is good? verbose_output.good()?. Yes... Invent your own stream with '/dev/null' buffer which will always be valid and pass when you don't need verbosity. The problem is that if you have a reference to a stream, it must be some real stream, otherwise where did you get a reference to it. Of course, you could use 'ostringstream' instead, but I am not sure what you make it "bad"... Victor Thanks for your reply, Victor. I think I will just add a third parameter to foo(), a boolean telling it if to use the stream or not. / WP Back to top Pete Becker Guest Posted: Sun Sep 26, 2004 2:45 pm    Post subject: Re: Perform output if stream is good William Payne wrote: Quote: Hi, I have a function declared as: void foo(const std::string& s, std::ostream& verbose_output); I want foo() to write a lot of data to the ostream if it's a valid stream. If the stream isn't valid writes to it do nothing. So all you need to do is set badbit. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top Julián Albo Guest Posted: Sun Sep 26, 2004 5:54 pm    Post subject: Re: Perform output if stream is good Victor Bazarov wrote: Quote: I want foo() to write a lot of data to the ostream if it's a valid stream. If it's valid or not should depend on user input (command line arguments actually). If the user decides he/she wants verbose output, I will pass std::cout as the last argument when calling foo(). But what should I pass if the user doesn't want any output? And how should foo() check if the stream is good? verbose_output.good()?. Invent your own stream with '/dev/null' buffer which will always be valid and pass when you don't need verbosity. I have this tiny class that can be useful: //********************************************************* // Null ostream class. //********************************************************* class Nullbuff : public std::streambuf { public: Nullbuff () { setbuf (0, 0); } protected: int overflow (int) { setp (pbase (), epptr () ); return 0; } int sync () { return 0; } }; class Nullostream : public std::ostream { public: Nullostream () : std::ostream (& buff) { } private: Nullbuff buff; }; -- Salu2 Back to top David Hilsee Guest Posted: Sun Sep 26, 2004 8:45 pm    Post subject: Re: Perform output if stream is good "William Payne" <mikas493_no_spam (AT) student (DOT) liu.se> wrote Quote: "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message news:DLo5d.262179$Fg5.185548 (AT) attbi_s53 (DOT) .. "William Payne" <mikas493_no_spam (AT) student (DOT) liu.se> wrote... void foo(const std::string& s, std::ostream& verbose_output); I want foo() to write a lot of data to the ostream if it's a valid stream. If it's valid or not should depend on user input (command line arguments actually). If the user decides he/she wants verbose output, I will pass std::cout as the last argument when calling foo(). But what should I pass if the user doesn't want any output? And how should foo() check if the stream is good? verbose_output.good()?. Yes... Invent your own stream with '/dev/null' buffer which will always be valid and pass when you don't need verbosity. The problem is that if you have a reference to a stream, it must be some real stream, otherwise where did you get a reference to it. Of course, you could use 'ostringstream' instead, but I am not sure what you make it "bad"... Victor Thanks for your reply, Victor. I think I will just add a third parameter to foo(), a boolean telling it if to use the stream or not. Pointers are typically used for "optional" parameters. If you use a pointer instead of a reference, you can pass a null pointer when you don't want the output, and check to see if the pointer is null instead of using a separate boolean value. I think most programmers would find that approach more appealing. I'm assuming that foo() will completely ignore the stream when the boolean flag indicates that it should, of course. -- David Hilsee Back to top Ron Natalie Guest Posted: Mon Sep 27, 2004 5:46 pm    Post subject: Re: Perform output if stream is good "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote Quote: Invent your own stream with '/dev/null' buffer which will always be valid and pass when you don't need verbosity. You can just write your own streambuf class that just has no-op put functions. 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