Jeremy Pack said: (by the date of Thu, 7 Jun 2007 14:14:49 -0700)
1 - Call functions when you know the exact identifier (whatever type you decided the identifier should be ...), parameters, and return value. 2 - Call functions when you just put a whole bunch of possible parameters into a map, and then the call succeeds if it can find the values it needs, for instance:
I'm thinking about the most obvious usage scenario (for me) - if you ever used AutoCAD you'll understand instantly, if not, then I'll try to explain with an example: Command: ci // user invokes a command "ci" because he wants to draw a circle on the screen. // A search in the reflection database is performed. // "ci" is an alias to "circle". // (alias matching can be done outside Reflection library) // // Method circle has four(!) signatures: // // [1] accept a 2D point and a radius // Shape* Circle(point2d p, double radius); // // [2] accept three 2D points - circle is inscribed on them // Shape* Circle(point2d p1, point2d p2, point2d p3); // // [3] accept two 2D points, which also define the circle's diameter // Shape* Circle(point2d p1, point2d p2); // // [4] accept two pointers to Shape* to which circle must be tangent, and a radius // Shape* Circle(Shape* s1, Shape2* s2, double radius); // // User can give arguments to create a circle using any of them. He has // just written a command "ci" - he has not decided yet which function he is // going to use. // // Therefore the default (first) function has a lengthy description like this one: // description_1: // std::string("CIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]:") // // It means that user can do two things: // // 1. give a 2D point argument (by clicking) on screen which means that he agreed // to use the first function signature, and has just given the first // argument for it: a 2D point. // 2. or write "3P"/"3" or "2P"/"2" or "Ttr"/"T"/"t" which means that he // has decided to use another function signature. // // for now, let's just lick on the screen, which will produce the coordinates: CIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: 10,20 // the description for the second argument is a bit lenghty too, the reflection // library needs to know a radius: // description_2: // std::string("Specify radius of circle:") // again, clicking on a screen produces a number in the command line: Specify radius of circle or: 15 Command: // And the circle is drawn. The full command sequence looks like this: Command: ci CIRCLE Specify center point for circle or [3P/2P/Ttr (tan tan radius)]: 10,20 Specify radius of circle or [Diameter]: 15 // (**) Command: ///////////// Note that the function description_1 describes: 1. the default method to draw a circle, 2. its first argument type, 3. a way to call different than default method - the Reflaction library must recognize the signatures for that to work, and identify them. This can be done by giving them longer names, like: [1] is "circle" [2] is "circle 3p" [3] is "circle 2p" [4] is "circle ttr" The 'extra' name after space allows to distinguish them, and to call the correct one, when user types eg.: "t". Perhaps the code for handling that "t" is not inside the Reflection library itself, but the Reflection library certainly must provide a way to implement that by me, when I'm using it. The second description_2 describes just the second argument. Important thing to note: AutoCAD is not aware what types of arguments those functions expect. It's the user's job to provide the right ones. A very simple mouse-click-feedback mechanism simply sends the mouse-click results to the command line. They can be coordinates (if an empty space is clicked), or Shape* (if another circle or line is clicked). If given wrong arguments the method called by reflection library will simply bail out with an error. (**) The "[Diameter]" allows user to specify circle diameter instead of radius. But diving into that detail will unnecessarily complicate the (rather simple) example. I hope that my explanation was clear? I can give you much more examples like that. I've been using AutoCAD almost as long as I'm coding in C++ ;-) And I wouldn't mind if I wrote a replacement for AutoCAD in my free time. Heh, what was a free time, again? apparently something I seem to have now, as I'm posting like crazy to boost mailing list today ;) -- Janek Kozicki |