how to access struct member using [] operator?

Dear all For example, I have a struct struct point{int x;int y} point a; Is there an easy way to access x and y by using a["x"] and a["y"] I guess I need to overload [], but can't figure out how. Someone can help? Thank you very much

On Sunday, 25 September 2016 at 04:54:31 UTC, grampus wrote: > Dear all > > For example, I have a struct > struct point{int x;int y} > point a; > > Is there an easy way to access x and y by using a["x"] and a["y"] > > I guess I need to overload [], but can't figure out how. > > Someone can help? Thank you very much You can't do that. There's a problem because the string passed is not known at compile-time, so the return type of "opIndex(string member)" cannot be inferred. It works only for one type: °°°°°°°°°°°°°°°°°°° import std.stdio; struct Something { int x,y; float z; int opIndex(string member) { alias T = typeof(this); foreach(m;__traits(allMembers, T)) if (m == member) { static if (is(typeof(__traits(getMember, T, m)) == int)) return __traits(getMember, T, m); } assert(0); } } void main(string[] args) { Something s; writeln(s["x"]); } °°°°°°°°°°°°°°°°°°° If you add a template parameter to opIndex then you can't call it with the array syntax so it becomes pointless to use an operator overload.

On Sunday, 25 September 2016 at 09:01:44 UTC, Namespace wrote: > ---- > import std.stdio; > > struct Something > { > int x, y; > float z; > > auto opIndex()(string member) { > switch (member) { > case "x": return this.x; > case "y": return this.y; > case "z": return this.z; > default: assert(0); > } > } > } > > void main(string[] args) > { > Something s; > writeln(s["x"]); > writeln(s["z"]); > } > ---- Doesn't work. s["x"] is returned as float in this example. The reason is, opIndex cannot magically change return type based on the passed-in string.

On Sunday, 25 September 2016 at 10:44:38 UTC, pineapple wrote: > On Sunday, 25 September 2016 at 04:54:31 UTC, grampus wrote: >> Dear all >> >> For example, I have a struct >> struct point{int x;int y} >> point a; >> >> Is there an easy way to access x and y by using a["x"] and a["y"] >> >> I guess I need to overload [], but can't figure out how. >> >> Someone can help? Thank you very much > > If they all share the same type, you can use a switch like @Namespace suggested. > > If the "x" and "y" strings are available at compile-time, you can use a mixin. > > auto getattr(string attr)(point a){ > mixin(`return a.` ~ attr ~ `;); > } > > auto x = a.attr!"x"; > > Otherwise, no. D types aren't dynamic in the same way that Python's types are. Thank you all for the clear reply. Now I know how far I can go on this.

On Sunday, 25 September 2016 at 09:01:44 UTC, Namespace wrote: > On Sunday, 25 September 2016 at 04:54:31 UTC, grampus wrote: >> Dear all >> >> For example, I have a struct >> struct point{int x;int y} >> point a; >> >> Is there an easy way to access x and y by using a["x"] and a["y"] >> >> I guess I need to overload [], but can't figure out how. >> >> Someone can help? Thank you very much > > ---- > import std.stdio; > > struct Something > { > int x, y; > float z; > > auto opIndex()(string member) { > switch (member) { > case "x": return this.x; > case "y": return this.y; > case "z": return this.z; > default: assert(0); > } > } > } > > void main(string[] args) > { > Something s; > writeln(s["x"]); > writeln(s["z"]); > } > ---- I think this approach should work for me. Thank you Namespace:)

On Sunday, 25 September 2016 at 09:01:44 UTC, Namespace wrote: > On Sunday, 25 September 2016 at 04:54:31 UTC, grampus wrote: >> Dear all >> >> For example, I have a struct >> struct point{int x;int y} >> point a; >> >> Is there an easy way to access x and y by using a["x"] and a["y"] >> >> I guess I need to overload [], but can't figure out how. >> >> Someone can help? Thank you very much > > ---- > import std.stdio; > > struct Something > { > int x, y; > float z; > > auto opIndex()(string member) { > switch (member) { > case "x": return this.x; > case "y": return this.y; > case "z": return this.z; > default: assert(0); > } > } > } > > void main(string[] args) > { > Something s; > writeln(s["x"]); > writeln(s["z"]); > } > ---- WooW I have to say that I'm mesmerized ! How can this works ? "member" is run time variable so the return type shouldn't be inferable.