C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  runtime performance impact of template usage          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Aaron Anodide Guest Posted: Tue Jul 29, 2003 5:39 am    Post subject: runtime performance impact of template usage Hello, I am using the following template class as a shorthand for zero-ing memory: template<class T> class ZeroMem : public T { public: ZeroMem(void) { ZeroMemory( this, sizeof(T) ); } }; Then, I do things like: ZeroMem<MYSTRUCT> mystruct; My question is: Does using this have any runtime performance impact? My hope is that the inline constructor causes this usage of this template to be eqiv to a macro. Thanks, Aaron Anodide Back to top John Harrison Guest Posted: Tue Jul 29, 2003 6:19 am    Post subject: Re: runtime performance impact of template usage "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote Quote: Hello, I am using the following template class as a shorthand for zero-ing memory: template<class T class ZeroMem : public T { public: ZeroMem(void) { ZeroMemory( this, sizeof(T) ); } }; Then, I do things like: ZeroMem My question is: Does using this have any runtime performance impact? My hope is that the inline constructor causes this usage of this template to be eqiv to a macro. Thanks, Aaron Anodide The *only* way to find out would be to look at the machine code generated by your compiler. C++ does not require inline funcitons to be actually inlined, its only a hint. In any case the overhead for a function call is nano-seconds, are you claiming that this would make a noticeable difference to your program? Have you timed anything? Unless the answer to both those questions is yes, you should be concentrating on writing clear code, not tricks with templates. I think your method is somewhat dubious because you are creating different types from what you really want. Isn't a template function better template <class T> void ZeroMem(T& obj) { ZeroMemory(&obj, sizeof(T)); } MYSTRUCT mystruct; ZeroMem(mystruct); john Back to top Aaron Anodide Guest Posted: Tue Jul 29, 2003 6:55 am    Post subject: Re: runtime performance impact of template usage "John Harrison" <john_andronicus (AT) hotmail (DOT) com> wrote Quote: "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote in message news:6MnVa.96702$R92.60016 (AT) news2 (DOT) central.cox.net... Hello, I am using the following template class as a shorthand for zero-ing memory: template<class T class ZeroMem : public T { public: ZeroMem(void) { ZeroMemory( this, sizeof(T) ); } }; Then, I do things like: ZeroMem My question is: Does using this have any runtime performance impact? My hope is that the inline constructor causes this usage of this template to be eqiv to a macro. Thanks, Aaron Anodide The *only* way to find out would be to look at the machine code generated by your compiler. C++ does not require inline funcitons to be actually inlined, its only a hint. In any case the overhead for a function call is nano-seconds, are you claiming that this would make a noticeable difference to your program? Have you timed anything? Unless the answer to both those questions is yes, you should be concentrating on writing clear code, not tricks with templates. I think your method is somewhat dubious because you are creating different types from what you really want. Isn't a template function better template void ZeroMem(T& obj) { ZeroMemory(&obj, sizeof(T)); } MYSTRUCT mystruct; ZeroMem(mystruct); Thanks for the tip. This is a good idea. Sincerely, Aaron Anodide Quote: john Back to top John Harrison Guest Posted: Tue Jul 29, 2003 6:57 am    Post subject: Re: runtime performance impact of template usage Quote: I think your method is somewhat dubious because you are creating different types from what you really want. Isn't a template function better template <class T void ZeroMem(T& obj) { ZeroMemory(&obj, sizeof(T)); } MYSTRUCT mystruct; ZeroMem(mystruct); Thanks for the tip. This is a good idea. Sincerely, Aaron Anodide Don't forget to add inline. template inline void ZeroMem(T& obj) john Back to top John Carson Guest Posted: Tue Jul 29, 2003 12:24 pm    Post subject: Re: runtime performance impact of template usage "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote Quote: Hello, I am using the following template class as a shorthand for zero-ing memory: template<class T class ZeroMem : public T { public: ZeroMem(void) { ZeroMemory( this, sizeof(T) ); } }; Then, I do things like: ZeroMem My question is: Does using this have any runtime performance impact? My hope is that the inline constructor causes this usage of this template to be eqiv to a macro. Thanks, Aaron Anodide I think that it is rather neat the way you derive from a template parameter but, on a more practical level, what's wrong with MYSTRUCT mystruct = {0}; ? -- John Carson 1. To reply to email address, remove donald 2. Don't reply to email address (post here instead) Back to top Aaron Anodide Guest Posted: Tue Jul 29, 2003 3:22 pm    Post subject: Re: runtime performance impact of template usage "John Carson" <donaldquixote (AT) datafast (DOT) net.au> wrote Quote: "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote in message news:6MnVa.96702$R92.60016 (AT) news2 (DOT) central.cox.net Hello, I am using the following template class as a shorthand for zero-ing memory: template<class T class ZeroMem : public T { public: ZeroMem(void) { ZeroMemory( this, sizeof(T) ); } }; Then, I do things like: ZeroMem My question is: Does using this have any runtime performance impact? My hope is that the inline constructor causes this usage of this template to be eqiv to a macro. Thanks, Aaron Anodide I think that it is rather neat the way you derive from a template parameter slightly OT, but ATL does this all over the place. I didn't think it up. Quote: but, on a more practical level, what's wrong with MYSTRUCT mystruct = {0}; Honestly, I wasn't aware of that. I've seen reams of example code using ZeroMemory on windows or memset(). I've never seen ={0}; Is this standard c++? Thanks, Aaron Quote: ? -- John Carson 1. To reply to email address, remove donald 2. Don't reply to email address (post here instead) Back to top John Carson Guest Posted: Wed Jul 30, 2003 1:53 am    Post subject: Re: runtime performance impact of template usage "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote Quote: "John Carson" <donaldquixote (AT) datafast (DOT) net.au> wrote in message news:3f266784$1 (AT) usenet (DOT) per.paradox.net.au... I think that it is rather neat the way you derive from a template parameter slightly OT, but ATL does this all over the place. I didn't think it up. but, on a more practical level, what's wrong with MYSTRUCT mystruct = {0}; Honestly, I wasn't aware of that. I've seen reams of example code using ZeroMemory on windows or memset(). I've never seen ={0}; Is this standard c++? Thanks, Aaron Yes, it is standard. It can't be used for dynamically allocated memory, but it works for all other memory allocation. It is just a special case of the rules for struct initialisation. Given a plain struct (no constructor etc.) like struct S { int x, char *str, int y; }; You can initialise it with, say, S s = {5, "Name", 9}; The rules say that if you only partially initialise the struct, e.g., S s = {5}; then everything in the remainder of the struct is initialised to zero, i.e., the preceding line is equivalent to S s = {5, 0, 0}; Thus if you enter S s = {0}; then this is equivalent to S s = {0, 0, 0}; -- John Carson 1. To reply to email address, remove donald 2. Don't reply to email address (post here instead) Back to top Aaron Anodide Guest Posted: Wed Jul 30, 2003 3:01 am    Post subject: Re: runtime performance impact of template usage "John Carson" <donaldquixote (AT) datafast (DOT) net.au> wrote Quote: "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote in message news:xiwVa.100493$R92.59493 (AT) news2 (DOT) central.cox.net "John Carson" <donaldquixote (AT) datafast (DOT) net.au> wrote in message news:3f266784$1 (AT) usenet (DOT) per.paradox.net.au... I think that it is rather neat the way you derive from a template parameter slightly OT, but ATL does this all over the place. I didn't think it up. but, on a more practical level, what's wrong with MYSTRUCT mystruct = {0}; Honestly, I wasn't aware of that. I've seen reams of example code using ZeroMemory on windows or memset(). I've never seen ={0}; Is this standard c++? Thanks, Aaron Yes, it is standard. It can't be used for dynamically allocated memory, but it works for all other memory allocation. It is just a special case of the rules for struct initialisation. Given a plain struct (no constructor etc.) like struct S { int x, char *str, int y; }; You can initialise it with, say, S s = {5, "Name", 9}; The rules say that if you only partially initialise the struct, e.g., S s = {5}; then everything in the remainder of the struct is initialised to zero, i.e., the preceding line is equivalent to S s = {5, 0, 0}; Thus if you enter S s = {0}; then this is equivalent to S s = {0, 0, 0}; Thanks for the info. I wonder why so many people use memset or ZeroMemory on stack variable structs then? What about nested structs? struct S { int x; }; struct T { int y; S s; }; will your initialization scheme work here too? Aaron Quote: -- John Carson 1. To reply to email address, remove donald 2. Don't reply to email address (post here instead) Back to top John Carson Guest Posted: Wed Jul 30, 2003 3:44 am    Post subject: Re: runtime performance impact of template usage "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote Quote: Thanks for the info. I wonder why so many people use memset or ZeroMemory on stack variable structs then? The ={0} approach is a technique inherited from C. People brought up on constructors may not be familiar with it or may not have bothered to remember it. Stroustrup discusses it in TC++PL. Quote: What about nested structs? struct S { int x; }; struct T { int y; S s; }; will your initialization scheme work here too? Aaron Yes. Just to make it slightly more interesting, suppose we have: struct S { int w, x; }; struct T { int y; S s; }; You could explicitly initialise this with: T t = {3, {5, 7}}; The nested brackets, however, only make the code visually clearer. The following is equivalent: T t = {3, 5, 7}; The same rules apply. If you only partially initialise, then everything not explicitly initialised is set to zero. Thus T t = {3}; sets the nested S structure to zero, while T t = {0}; sets everything to zero. -- John Carson 1. To reply to email address, remove donald 2. Don't reply to email address (post here instead) Back to top Aaron Anodide Guest Posted: Wed Jul 30, 2003 6:45 am    Post subject: Re: runtime performance impact of template usage "John Carson" <donaldquixote (AT) datafast (DOT) net.au> wrote Quote: "Aaron Anodide" <anodide (AT) hotmail (DOT) com> wrote in message news:cyGVa.105163$R92.76256 (AT) news2 (DOT) central.cox.net Thanks for the info. I wonder why so many people use memset or ZeroMemory on stack variable structs then? The ={0} approach is a technique inherited from C. People brought up on constructors may not be familiar with it or may not have bothered to remember it. Stroustrup discusses it in TC++PL. Thanks again for the background info. It's very interesting to learn something I did not know previously. As a footnote, I wonder if the guys writing the Unit test cases for my C++ compiler forgot about the ={0} as well..... Aaron Quote: What about nested structs? struct S { int x; }; struct T { int y; S s; }; will your initialization scheme work here too? Aaron Yes. Just to make it slightly more interesting, suppose we have: struct S { int w, x; }; struct T { int y; S s; }; You could explicitly initialise this with: T t = {3, {5, 7}}; The nested brackets, however, only make the code visually clearer. The following is equivalent: T t = {3, 5, 7}; The same rules apply. If you only partially initialise, then everything not explicitly initialised is set to zero. Thus T t = {3}; sets the nested S structure to zero, while T t = {0}; sets everything to zero. -- John Carson 1. To reply to email address, remove donald 2. 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