Hello Sorry, I have several questions on C++ instead of any boost library. But I believe the boost guys could easily give me help. :-) 1) I found code fragment like this, esp. in macro definitions: do { \ \ // ... \ } while(0) Why is the do/while clause needed here? 2) I found large amount of code like this in a class implementation: AClass::func() { this->func1(); } Why is 'this->' needed? Thanks for any help. B/Rgds Max

hello Nat, Thanks you very much for your explanation! for 1), I never heard of that idiom. In fact, after playing with C++ for more thant 10 years, I am still keeping finding interesting corners of the language. :) What I get from your information is that, the do/while clause is a MUST-HAVE and is obviously intentional. for 2), it seems that 'this->' is a question of coding style and generally we could omit it. I really appreciate your help! and I believe the moderator of the list will not yell to us for such topic. :-) B/Rgds Max

1) I found code fragment like this, esp. in macro definitions:

do { \ \ // ... \ } while(0)

Why is the do/while clause needed here?

That's a macro idiom that dates back to classic C. It packages a compound statement { stmt1; stmt2; } as if it were a single statement. If you write #define YOURMACRO(arg) { stmt1; stmt2; } and a subsequent coder writes this: if (some_condition) YOURMACRO(1); else std::cout << "Whoops, error.\n"; he or she will get a nasty surprise. It works to drop the semicolon after YOURMACRO(1), but that imposes a new requirement: some macros must be followed by semicolons, others must NOT be followed by semicolons, and the coder must know something about the macro's implementation to use it correctly. If you write #define YOURMACRO(arg) do { stmt1; stmt2; } while (0) then it's appropriate (required) to write a semicolon after the macro invocation, and subsequent coders can do that consistently without needing to know which kind of macro this is.

2) I found large amount of code like this in a class implementation:

AClass::func() { this->func1(); }

Why is 'this->' needed?

There are a few different reasons. 1. Some people write that as a stylistic preference, since it clarifies that you're referencing a data member rather than a local variable. 2. In some IDEs, when you write "this->", the IDE pops up a list of members from which to choose. 3. In some arcane scenarios such as a template subclass derived from a template base class, the language sometimes requires this->baseClassMember (though I believe MSVC still permits a plain baseClassMember reference as an extension). But if you ask any more such questions on this list, the moderator might yell at both of us. ;-) _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org

Hello Marshall Thanks. With your additional point, the things are getting more clearer. B/Rgds Max ----- Original Message ---- From: Marshall Clow To: boost-users@lists.boost.org Sent: 2008-07-17 10:26:23 Subject: Re: [Boost-users] Several questions on C++

At 9:47 AM +0800 7/17/08, boost_www wrote:

hello Nat,

Thanks you very much for your explanation!

for 1), I never heard of that idiom. In fact, after playing with C++ for more thant 10 years, I am still keeping finding interesting corners of the language. :)

What I get from your information is that, the do/while clause is a MUST-HAVE and is obviously intentional.

Absolutely.

for 2), it seems that 'this->' is a question of coding style and generally we could omit it.

Generally, but not always. If you want to call a member function (_especially_ in templated/library code), then you should qualify it with "this->", because you don't know if there will be a function named "func1" available when the code is compiled. -- -- Marshall Marshall Clow Idio Software <mailto:marshall@idio.com> It is by caffeine alone I set my mind in motion. It is by the beans of Java that thoughts acquire speed, the hands acquire shaking, the shaking becomes a warning. It is by caffeine alone I set my mind in motion. _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org

Hello John Also many thanks to you for your explanation. B/Rgds Max ----- Original Message ---- From: John Femiani To: boost-users@lists.boost.org Sent: 2008-07-17 01:36:03 Subject: Re: [Boost-users] Several questions on C++

1) I found code fragment like this, esp. in macro definitions:

do { \ \ // ... \ } while(0)

Why is the do/while clause needed here?

Probably so the statements inside the do...while are in their own block, with its own scope. If simple curly braces were used, then sometimes it would break 'if' statements in a weird way, so using do {} while(0) is more robust. Example: #define FOO { ... } if (cond) FOO; //<--- breaks else FOO;

2) I found large amount of code like this in a class implementation:

AClass::func() { this->func1(); }

Why is 'this->' needed?

Probably because the class is a template with a template base class, so explicitly using this-> is one way to make sure that g++ recognizes func1 as an inheritted member function. Another possiblity is that typeing '->' triggers the autocomplete function in some editors, so typing 'this->' provides them with a convenient way to recall & type the correct function name within the scope of 'this'. _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org