digitalmars.D.learn - opUnary with ++ and -- on a struct that has a dynamic array
Hi, Is there a way to get post increment and pre increment working properly in this scenario? import std.stdio; struct A { int[] a; this(int a) { this.a = [a]; } auto opUnary(string op)(){ return A(mixin(op ~ "this.a[0]")); } } void main() { auto a = A(0); int b = 0; writeln(a++, ":", b++); // 1, 0 <-- wrong post increment result writeln(++a, ":", ++b); // 2, 2 } If I switch the order of the mixin expression (i.e. "this.a[0]" ~ op) then the pre increment does not work. Any tips? Cheers - Ali
Feb 11 2018
On Monday, 12 February 2018 at 03:13:43 UTC, aliak wrote:Hi, Is there a way to get post increment and pre increment working properly in this scenario? import std.stdio; struct A { int[] a; this(int a) { this.a = [a]; } auto opUnary(string op)(){ return A(mixin(op ~ "this.a[0]")); } } void main() { auto a = A(0); int b = 0; writeln(a++, ":", b++); // 1, 0 <-- wrong post increment result writeln(++a, ":", ++b); // 2, 2 }writeln(a++) translates to: A copy = a; a.opUnary!"++"; writeln(copy); copy.a[] and a.a[] are the same reference, you increment a.a[0]/copy.a[0] in opUnary to make this work you will need a postblit constructor: struct A { .... this(this) { a = a.dup; //make a copy a; } } In fact, you'll find exactly the same scenario in docs: https://dlang.org/spec/struct.html#struct-postblitIf I switch the order of the mixin expression (i.e. "this.a[0]" ~ op) then the pre increment does not work. Any tips? Cheers - Ali
Feb 11 2018
On Monday, 12 February 2018 at 06:16:21 UTC, rumbu wrote:writeln(a++) translates to: A copy = a; a.opUnary!"++"; writeln(copy); copy.a[] and a.a[] are the same reference, you increment a.a[0]/copy.a[0] in opUnary to make this work you will need a postblit constructor: struct A { .... this(this) { a = a.dup; //make a copy a; } } In fact, you'll find exactly the same scenario in docs: https://dlang.org/spec/struct.html#struct-postblitAh! Awesome. Yes that works, thank you! Cheers
Feb 12 2018