C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Strange C++ construct          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Trevor Morgan Guest Posted: Wed Feb 08, 2006 3:00 pm    Post subject: Strange C++ construct Can anyone help we with this construct? I thought I knew the language pretty well, but I'm really not sure what this is doing. This code: int i = (1,2,3); compiles fine, and sets i to 3. What is the exact meaning of this syntax? Where is it described in the standard or in Stroustrup? Thanks in advance. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top ThosRTanner Guest Posted: Wed Feb 08, 2006 5:00 pm    Post subject: Re: Strange C++ construct Trevor Morgan wrote: Quote: Can anyone help we with this construct? I thought I knew the language pretty well, but I'm really not sure what this is doing. This code: int i = (1,2,3); compiles fine, and sets i to 3. What is the exact meaning of this syntax? Where is it described in the standard or in Stroustrup? It's the comma operator (not to be confused with the , function parameter separator, or the , list separator) - who's effect is to: a) evaluate the thing on the left hand side & throw the result away b) evaluate the thing on the right hand side and return that as the result of the operation. The brackets ensure that the , is taken as a comma operator, rather than a list separator. So int i = (1, 2, 3) says evaluate 1 & discard evaluate 2 & discard set i to 3. In c++, that's a singularly ropy way of doing things, you could just do 1; 2; int i = 3; it looks as though that code might have been ported from c, where it makes a lot more sense. The , operator is useful for avoiding the dreaded assignment in if statement syntax: instead of if (a = complexfunction(b)) you can write if (a = complexfunction(b), a != 0) which is clearer (well, I think it is), and it doesn't precipitate compiler warnings [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel T. Guest Posted: Wed Feb 08, 2006 5:00 pm    Post subject: Re: Strange C++ construct In article <1139401358.826611.264500 (AT) g44g2000cwa (DOT) googlegroups.com>, "Trevor Morgan" <trevor_morgan (AT) yahoo (DOT) com> wrote: Quote: Can anyone help we with this construct? I thought I knew the language pretty well, but I'm really not sure what this is doing. This code: int i = (1,2,3); compiles fine, and sets i to 3. What is the exact meaning of this syntax? Where is it described in the standard or in Stroustrup? Thanks in advance. Stroustrup, "The C++ Programming Language" section 6.2.2. The comma operator evaluates the left hand argument then evaluates the right hand argument and returns it. So for example: int a, b; b = ( a = 2, a + 1 ); assert( b == 3 ); -- Magic depends on tradition and belief. It does not welcome observation, nor does it profit by experiment. On the other hand, science is based on experience; it is open to correction by observation and experiment. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alec Ross Guest Posted: Wed Feb 08, 2006 5:00 pm    Post subject: Re: Strange C++ construct In message <1139401358.826611.264500 (AT) g44g2000cwa (DOT) googlegroups.com>, Trevor Morgan <trevor_morgan (AT) yahoo (DOT) com> writes .... Quote: This code: int i = (1,2,3); compiles fine, and sets i to 3. What is the exact meaning of this syntax? Where is it described in the standard or in Stroustrup? Its the comma operator. On the RHS it evaluates the expression L to R, and the whole expression takes the value of the rightmost term. Sorry, no refs to hand, but I'm sure you can look up the operator. -- Alec Ross [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ivan Vecerina Guest Posted: Wed Feb 08, 2006 5:00 pm    Post subject: Re: Strange C++ construct "Trevor Morgan" <trevor_morgan (AT) yahoo (DOT) com> wrote in message news:1139401358.826611.264500 (AT) g44g2000cwa (DOT) googlegroups.com... : Can anyone help we with this construct? I thought I knew the language : pretty well, but I'm really not sure what this is doing. : : This code: : : int i = (1,2,3); : : compiles fine, and sets i to 3. What is the exact meaning of this : syntax? Where is it described in the standard or in Stroustrup? Look for the "comma operator". When used outside of contexts where it is given a special meaning as an argument or value separator, the comma is treated as an operator that evaluates its operands in left-to-right order, and returns the rightmost value. Here's the relevant extract from the C++ standard: <<<< 5.18 Comma operator [expr.comma] <1> The comma operator groups left-to-right. expression: assignment-expression expression , assignment-expression A pair of expressions separated by a comma is evaluated left-to-right and the value of the left expression is discarded. The lvalue-to-rvalue (4.1), array-to-pointer (4.2), and function-to-pointer (4.3) standard conversions are not applied to the left expression. All side effects (1.9) of the left expression, except for the destruction of temporaries (12.2), are performed before the evaluation of the right expression. The type and value of the result are the type and value of the right operand; the result is an lvalue if its right operand is. <2> In contexts where comma is given a special meaning, [Example: in lists of arguments to functions (5.2.2) and lists of initializers (8.5) ] the comma operator as described in clause 5 can appear only in parentheses. [Example: f(a, (t=3, t+2), c); has three arguments, the second of which has the value 5. ] Quote: hth, Ivan -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form Brainbench MVP for C++ <> http://www.brainbench.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Wed Feb 08, 2006 7:10 pm    Post subject: Re: Strange C++ construct Trevor Morgan wrote: Quote: Can anyone help we with this construct? I thought I knew the language pretty well, but I'm really not sure what this is doing. This code: int i = (1,2,3); compiles fine, and sets i to 3. What is the exact meaning of this syntax? Where is it described in the standard or in Stroustrup? Read about "the comma operator". It's been around for ages. "TC++PL", Special Edition, has it on page 123. The Standard has it in the subclause 5.18. V -- Please remove capital As from my address when replying by mail [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Trevor Morgan Guest Posted: Thu Feb 09, 2006 11:06 am    Post subject: Re: Strange C++ construct Thanks all for your help. Now I remember that I have seen this before - we had a long debate once in a code-review meeting as to whether this was legal, and had to go back to Stroustrup to convince ourselves. I'd forgotten all about it - I was assuming that it was something to do with brackets, not commas. Of course, in C++ you can even overload the comma operator, but I can't imagine a circumstance when you'd want to do that, apart from possibly creating yourself a bit of job-security.... [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Carl Barron Guest Posted: Fri Feb 10, 2006 11:06 am    Post subject: Re: Strange C++ construct In article <1139476878.601967.58460 (AT) g44g2000cwa (DOT) googlegroups.com>, Trevor Morgan <trevor_morgan (AT) yahoo (DOT) com> wrote: Quote: Of course, in C++ you can even overload the comma operator, but I can't imagine a circumstance when you'd want to do that, apart from possibly creating yourself a bit of job-security.... Or writing a mini language within C++ code such as phoenix. the comma operator could combine the two expression templates its passed into one that executes both expressions in order when it is executed at runtime. such as print to count a bar graph of '.' of a vector<int>. using namespace phoenix; using namespace std; //... int iii; for_each(data.begin(),data.end(), if_(arg1 > 0) [ for_(var(iii) = 0;var(iii)!=arg1;++var(iii)) [ cout << val('.') // no semicolons in the []'s here ], // note comma here... cout << val('\n') ] ); this equivalent to [produces the same results as] using namespace std; struct foo { void operator () (int x) { if(x > 0) { for(int i=0;i!=x;++i) cout << '.'; cout << '\n'; } }; // ... for_each(data.begin(),data.end(),foo()); as an example. Its not job security but usage of an available library to put the loop code in the loop statement. For this example its 'overkill' but in the context of a real application it can produce 100's of implementation created functors for simple things, enhancing STL algorithms by place small code segments directly in the algorithm call. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thiruvalluvan Guest Posted: Sat Feb 11, 2006 12:06 pm    Post subject: Re: Strange C++ construct Trevor Morgan wrote: Quote: Of course, in C++ you can even overload the comma operator, but I can't imagine a circumstance when you'd want to do that, apart from possibly creating yourself a bit of job-security.... Did you see Boost parameter library which overloads comma operator and produces some interesting usage? http://www.boost.org/libs/parameter/doc/html/index.html [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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