C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  efficiency and uninitialized variables Goto page 1, 2, 3  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message nick Guest Posted: Mon Jan 24, 2005 11:23 am    Post subject: efficiency and uninitialized variables I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Karthik Kumar Guest Posted: Mon Jan 24, 2005 8:27 pm    Post subject: Re: efficiency and uninitialized variables nick wrote: Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. This essentially depends on if you want to use the value of 'a' between int a = 0; // do you want to use 'a' somewhere here. // then probably a better idea to assign // it a value in the beginning. // Else a better option would be // to declare 'a' at the point of invocation. // something like, // int a = dosomething(); // // a = dosomething(); Quote: [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Joe Guest Posted: Mon Jan 24, 2005 8:28 pm    Post subject: Re: efficiency and uninitialized variables Actually, it's one of the reasons that C++ allows you to put variable declarations anywhere you wish. What I would do is instead of: nick wrote: Quote: int a=0; a = dosomething(); or Quote: int a; a = dosomething(); // somewhere further down I would just use int a = dosomething(); // somewhere right before I was going to use a That way, you don't have to pay for an initialization that will never be used and the compiler will warn you about uses of 'a' before you intended to use it. Let the compiler help you as much as possible, and don't do more work than you need to. joe [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Peter Koch Larsen Guest Posted: Mon Jan 24, 2005 8:28 pm    Post subject: Re: efficiency and uninitialized variables "nick" <maskofzero (AT) hotmail (DOT) com> skrev i en meddelelse news:1106542546.765114.54470 (AT) f14g2000cwb (DOT) googlegroups.com... Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. Your second version will likely be faster. I doubt this is something that you could measure, though. But why not the obvious int a = dosomething()? If it is possible, this is the solution to go for. /Peter [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Patrick Leslie Polzer Guest Posted: Mon Jan 24, 2005 8:32 pm    Post subject: Re: efficiency and uninitialized variables nick wrote: Quote: I usually init my variables [...] but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down Assuming that the O/S does not blank the .data area automatically (and I don't think the major operating systems do that) it is of course more efficient to leave superfluous assignments out (thus saving the operations "Load into register" -> "Modify value" -> "Save to memory"). However, compiler should not have any problems finding out that this is superfluous (as in the example above the variable is not volatile). Leslie [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Pete Becker Guest Posted: Mon Jan 24, 2005 8:35 pm    Post subject: Re: efficiency and uninitialized variables nick wrote: Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. It's more efficient to initialize a variable once instead of initializing it once and assigning to it once. The way to do that is: int a = dosomething(); // somewhere further down It's also clearer, because it doesn't initialize the variable with a value that won't be used. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Falk Tannhäuser Guest Posted: Mon Jan 24, 2005 8:37 pm    Post subject: Re: efficiency and uninitialized variables The classical way to define and initialise a variable is int a = dosomething(); Any C++ tutorial worth its name should explain this at the very beginning... Quote: I usually init my variables eg. int a=0; a = dosomething(); This is only useful if the assignment is done conditionally, as for example in int a = 0; if(some_condition) a = dosomething(); use(a); or int a = 0; try { a = dosomething(); } catch(...) { ... // Error handling } use(a); Quote: but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. Defining uninitialised variables that are assigned somewhere later should be avoided whenever possible, except in particular cases as in std::cout << "Enter a number: "; int a; std::cin >> a; if(!std::cin) ... // Error handling else use(a); Falk [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (ne Spange Guest Posted: Mon Jan 24, 2005 8:42 pm    Post subject: Re: efficiency and uninitialized variables Hello Nick, nick schrieb: Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. From the point of micro-optimization this situation should not be worth worring about and usually the compiler will produce the same code. The **real** question arises, why you don't use the C++ way and simple write int a = dosomething(); or, if you don't modify a in the following code: const int a = dosomething(); which reduces the risk of variable abuses and is clean, tight, and self-documenting code ;-) Greetings from Bremen, Daniel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top edA-qa mort-ora-y Guest Posted: Mon Jan 24, 2005 8:44 pm    Post subject: Re: efficiency and uninitialized variables nick wrote: Quote: int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. With a non-optimizing compiler it may be marginally more efficient (saves 1 short CPU instruciton on most architectures). With an optimizing compiler this is most likely done automatically (it won't bother assinging unused values, and indeed will likely give you a warning about it being unused). -- edA-qa mort-ora-y Idea Architect http://disemia.com/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Harris Guest Posted: Mon Jan 24, 2005 8:45 pm    Post subject: Re: efficiency and uninitialized variables [email]maskofzero (AT) hotmail (DOT) com[/email] (nick) wrote (abridged): Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down Conceptually, yes. In practice, usually no. It depends on the compiler, but most compilers should optimise away the inefficiency, at least for ints in simple situations. If the situation isn't simple, eg "if" statements are involved, then the extra safety of the zero initialisation might be worth while anyway. For more complex types, eg a smart pointer or std::string, it's harder to be sure about what the compiler will be able to optimise. Generally it is better to defer declaring the variable until its initial value is known: int a = dosomething(); -- Dave Harris, Nottingham, UK [ 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: Mon Jan 24, 2005 10:37 pm    Post subject: Re: efficiency and uninitialized variables nick wrote: Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. Actually int a = dosomemething(); is the most efficient of all. V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Nicola.Musatti@ObjectWay. Guest Posted: Mon Jan 24, 2005 10:41 pm    Post subject: Re: efficiency and uninitialized variables What makes 0 better than any random value? In general I don't like my code to contain statements unrelated to its purpose; in the case of native types it is not much a question of performance as those are likely to get initialized at program startup in a very efficient way; rather, it is a matter of making the meaning of my code as easy to understand as possible. Note also that in C++ variables do not have to be declared at the beginning of a block, so it is usually possible to initialize them in a useful way, e.g. in your example one may often move the declaration to the point of first use. By the way, if you really prefer to initialize your variables, why not use a more recognisable pattern? Consider also that 0 is a rather common value to find in unitialized memory. Cheers, Nicola Musatti [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top jcoffin@taeus.com Guest Posted: Mon Jan 24, 2005 10:41 pm    Post subject: Re: efficiency and uninitialized variables nick wrote: Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. It may be more efficient (but then again it may not -- the compiler may skip assigning the initial value if it notices that it's never used). IMO, an improvement over either one is (if possible) to delay defining the variable until the point at which it can be initialized: int a(dosomething()); The obvious time this can't be done is if you have to initialize it inside of some inner block: int a; if (x) a = dosomething(); else a = dosomething_else(); This can sometimes be worked around with something like: int a = x ? dosomething() : dosomething_else(); -- Later, Jerry. The universe is a figment of its own imagination. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Skandinavisches Seminar Guest Posted: Mon Jan 24, 2005 11:53 pm    Post subject: Re: efficiency and uninitialized variables "nick" <maskofzero (AT) hotmail (DOT) com> schrieb im Newsbeitrag news:1106542546.765114.54470 (AT) f14g2000cwb (DOT) googlegroups.com... Quote: I usually init my variables eg. int a=0; I apologize if the following is obvious to you. I usually don't bother to initialize variables of plain data types, if I'm declaring them before I use them for some reason. However, I always do so for pointers, e.g., int a; // I don't care, not initialized. int* p = 0; If you forget yourself and dereference an unitialized pointer, you are likely to get a segmentation fault. According to my understanding of the matter, if the contents of `p' are by chance a valid address, then you won't get one, but this would be even worse, especially if you're assigning to `*p', whereas the compiler will catch it if you try to dereference a null pointer. (I can't test this on this machine---no compiler.) Laurence Finston http://www.gnu.org/software/3dldf/LDF.html Reply To: [email]lfinsto1 (AT) gwdg (DOT) de[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan W Guest Posted: Mon Jan 24, 2005 11:54 pm    Post subject: Re: efficiency and uninitialized variables nick wrote: Quote: I usually init my variables eg. int a=0; a = dosomething(); but i am wondering if it may be more efficient to just int a; a = dosomething(); // somewhere further down even if it is only marginally more efficient it would be good to know. On Intel platform, using both MSVC 6.0 and 7.1, here's what I got: // In debug mode int a=0; // 7 bytes to initialize a = dosomething(); // 8 bytes to call and store result int b; // NO code generated b = dosomething(); // 8 bytes to call and store result int c = dosomething(); // 8 bytes int d(dosomething()); // 8 bytes // In release mode int a=0; // NO bytes a = dosomething(); // 7 bytes to call and store result int b; // NO bytes b = dosomething(); // 7 bytes to call and store result int c = dosomething(); // 7 bytes int d(dosomething()); // 7 bytes No difference at all, except in debug mode. (MSVC 7.1 wanted to inline dosomething()... I moved it into a different compilation unit to defeat this.) Other compilers might be slightly different... I wouldn't worry about it. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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