C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  template metaprogramming          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Mark Guest Posted: Wed Nov 16, 2005 1:44 am    Post subject: template metaprogramming is there a way to make so i dont have to call the following with the extra ::val portion? template<int r, int g, int b> struct RGB { static const int val = 65536 * r + 256 * g + b; }; ie RGB<50,100,150> instead of RGB<50,100,150>::val Back to top Victor Bazarov Guest Posted: Wed Nov 16, 2005 1:56 am    Post subject: Re: template metaprogramming Mark wrote: Quote: is there a way to make so i dont have to call the following with the extra ::val portion? template<int r, int g, int b struct RGB { static const int val = 65536 * r + 256 * g + b; }; ie RGB<50,100,150> instead of RGB<50,100,150>::val You don't "call" it. You probably use it in an expression. If it is an expression that expects an int, you can do template<int r, int g, int b> struct RGB { operator int() const { return 65536 * r + 256 * g + b; } }; and use it like int a = RGB<50,100,150>(); V Back to top lancediduck@nyc.rr.com Guest Posted: Wed Nov 16, 2005 2:18 am    Post subject: Re: template metaprogramming templates do not have to evaluate expressions until they are actually instantiated. So just decarling typedef RGB<100,1,1> my_fav_color; does not assign a value to val. When val is actually referenced, then it is evaluated. In any case, how would you use the calculated value without accessing "val"??? Back to top Mark Guest Posted: Wed Nov 16, 2005 8:27 am    Post subject: Re: template metaprogramming Victor Bazarov, well. i meant use in an expression... it's similar to a function in some sense, even if it doesnt operate the same way (ie, it's precomputed). anyways RGB<50,100,150>() is no better than RGB<50,100,150>::val ... i'm just trying to be lazy, and it looks ugly : lancedid... (AT) nyc (DOT) rr.com, how would you use the calculated value without accessing "val"??? well.. i suppose that's what im asking. RGB<50,100,150> is only supposed to "return" a single value. so there is no ambiguity between it returning ::val or ::coconut. i'm wondering if there is another way to write the template so that it automatically produces this value... without having to assign it to some dummy variable first such as val. and what exactly would typedef RGB<100,1,1> my_fav_color do??? would you have to use it like my_fav_color::val or what? Back to top Erik Wikström Guest Posted: Wed Nov 16, 2005 9:24 am    Post subject: Re: template metaprogramming On 2005-11-16 02:44, Mark wrote: Quote: is there a way to make so i dont have to call the following with the extra ::val portion? template<int r, int g, int b struct RGB { static const int val = 65536 * r + 256 * g + b; }; ie RGB<50,100,150> instead of RGB<50,100,150>::val Wouldn't it be better as a function? inline int RGB(int r, int g, int b) { return 65536 * r + 256 * g + b; } And then when using it you replace the < and > with ( and ) and it's all you wanted, ie. RGB(50, 100, 150). Erik Wikström -- "I have always wished for my computer to be as easy to use as my telephone; my wish has come true because I can no longer figure out how to use my telephone" -- Bjarne Stroustrup Back to top John Harrison Guest Posted: Wed Nov 16, 2005 12:30 pm    Post subject: Re: template metaprogramming Erik Wikström wrote: Quote: On 2005-11-16 02:44, Mark wrote: is there a way to make so i dont have to call the following with the extra ::val portion? template<int r, int g, int b struct RGB { static const int val = 65536 * r + 256 * g + b; }; ie RGB<50,100,150> instead of RGB<50,100,150>::val Wouldn't it be better as a function? inline int RGB(int r, int g, int b) { return 65536 * r + 256 * g + b; } And then when using it you replace the < and > with ( and ) and it's all you wanted, ie. RGB(50, 100, 150). Erik Wikström As a function it's not a compile time constant. Why the OP would want an RGB value as a compile time constant I'm not sure but maybe there's a reason. Of course the other option is a macro #define RGB(r, g, b) (65536*(r) + 256*(g) + (b)) Now it's a compile time constant and you have the convenient syntax. john Back to top Mark Guest Posted: Wed Nov 16, 2005 6:02 pm    Post subject: Re: template metaprogramming Erik, as John said, it's not compile time constant. John, that's not compile time either. that just replaces any occurance of RGB(r, g, b) with (65536*(r) + 256*(g) + (b)), it still has to do the calculations in run time. Why do I want it during compile time? Because I'm never going to use any variables, only constants... Oh well. Just wondering if there was a better way. Thanks guys. Back to top Erik Wikström Guest Posted: Wed Nov 16, 2005 6:10 pm    Post subject: Re: template metaprogramming On 2005-11-16 19:02, Mark wrote: Quote: Erik, as John said, it's not compile time constant. John, that's not compile time either. that just replaces any occurance of RGB(r, g, b) with (65536*(r) + 256*(g) + (b)), it still has to do the calculations in run time. Why do I want it during compile time? Because I'm never going to use any variables, only constants... Oh well. Just wondering if there was a better way. Thanks guys. If you are not going to use _any_ variables then you should be able to hardcode the value directly. Erik Wikström -- "I have always wished for my computer to be as easy to use as my telephone; my wish has come true because I can no longer figure out how to use my telephone" -- Bjarne Stroustrup Back to top John Harrison Guest Posted: Wed Nov 16, 2005 6:17 pm    Post subject: Re: template metaprogramming Mark wrote: Quote: Erik, as John said, it's not compile time constant. John, that's not compile time either. that just replaces any occurance of RGB(r, g, b) with (65536*(r) + 256*(g) + (b)), it still has to do the calculations in run time. Not if you are using constants for the values of r, g and b. RGB(0, 0, 0) is a compile time constant. Quote: Why do I want it during compile time? Because I'm never going to use any variables, only constants... So RGB will give a compile time constant. Quote: Oh well. Just wondering if there was a better way. Thanks guys. Perhaps you need to quote the code you say doesn't compile. You must be doing something wrong. john Back to top Mark Guest Posted: Wed Nov 16, 2005 11:46 pm    Post subject: Re: template metaprogramming Erik Wikström wrote: Quote: If you are not going to use _any_ variables then you should be able to hardcode the value directly. Well. I could. But I'd have to do it for many values... not a huge deal, was just curious. John Harrison wrote: Quote: Not if you are using constants for the values of r, g and b. RGB(0, 0, 0) is a compile time constant. Hm.. so it doesn't actually perform any calculations during run time...? It won't attempt to do (65536*(0) + 256*(0) + (0)) when the program is run, but actually pre-determine the answer is 0? That's interesting. Wish there was a way to test that out... But yes, I shall use this then. So, in theory, I could do #define RGB(r, g, b) (65536*(r) + 256*(g) + (b)) and then do #define WHITE RGB(255,255,255) and it will replace all occurences of "WHITE" with 16777215? Quote: Perhaps you need to quote the code you say doesn't compile. You must be doing something wrong. Hm? Did I say something doesn't compile? Back to top John Harrison Guest Posted: Thu Nov 17, 2005 8:02 am    Post subject: Re: template metaprogramming Mark wrote: Quote: Erik Wikström wrote: If you are not going to use _any_ variables then you should be able to hardcode the value directly. Well. I could. But I'd have to do it for many values... not a huge deal, was just curious. John Harrison wrote: Not if you are using constants for the values of r, g and b. RGB(0, 0, 0) is a compile time constant. Hm.. so it doesn't actually perform any calculations during run time...? It won't attempt to do (65536*(0) + 256*(0) + (0)) when the program is run, but actually pre-determine the answer is 0? That's interesting. Wish there was a way to test that out... Easy, array bounds in C++ must be compile time constants (at least in a standard conforming compiler). So try this char x[RGB(0, 0, 1)]; When that compiles (which it will) it proves that RGB with constant values produces compile time constants. Quote: But yes, I shall use this then. So, in theory, I could do #define RGB(r, g, b) (65536*(r) + 256*(g) + (b)) and then do #define WHITE RGB(255,255,255) and it will replace all occurences of "WHITE" with 16777215? Correct. Quote: Perhaps you need to quote the code you say doesn't compile. You must be doing something wrong. Hm? Did I say something doesn't compile? You were so confident that you were right, I thought you must have tried to compile something. john Back to top peter koch Guest Posted: Thu Nov 17, 2005 11:00 pm    Post subject: Re: template metaprogramming John Harrison skrev: Quote: Mark wrote: Erik, as John said, it's not compile time constant. John, that's not compile time either. that just replaces any occurance of RGB(r, g, b) with (65536*(r) + 256*(g) + (b)), it still has to do the calculations in run time. Not if you are using constants for the values of r, g and b. RGB(0, 0, 0) is a compile time constant. Just wanted to add that it does not matter if you use an inline function or a macro here - unless you need a compile time constant. On all real-world compilers the inline function will be replaced by a constant value (no run-time overhead). /Peter Quote: Why do I want it during compile time? Because I'm never going to use any variables, only constants... So RGB will give a compile time constant. Oh well. Just wondering if there was a better way. Thanks guys. Perhaps you need to quote the code you say doesn't compile. You must be doing something wrong. john Back to top Mark Guest Posted: Fri Dec 02, 2005 2:45 am    Post subject: Re: template metaprogramming oh. well then. i guess i learned something about my compiler today very nifty. thanks guys. so there was a better solution than RGB<1,2,3>::val after all. Quote: You were so confident that you were right, I thought you must have tried to compile something. oh. i see what you were getting at now. didn't realize i just shot your point down without looking into it. sorry John you were right. 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