class - mutable method X.this is not callable using a immutable object error in D -

i have d code copied page: http://ddili.org/ders/d.en/class.html

import std.stdio;  struct s {     (int x) {         this.x = x;     }     int x; }  class foo {     s      o;       char[] s;     int    i;  // ...      this(s o, const char[] s, int i)     {         this.o = o;         this.s = s.dup;         this.i = i;     }      foo dup() const     {         return new foo(o, s, i);     }      immutable(foo) idup() const     {         return new immutable(foo)(this.o, this.s, this.i);     } }  void main() {     auto var1 = new foo(s(5), "hello", 42);     auto var2 = var1.dup();     immutable(foo) imm = var1.idup();      writeln(var1);     writeln(var2);     writeln(imm); } 

the issue have "mutable method a.foo.this not callable using immutable object" error when compile it.

you're receiving error because called new immutable(foo)(this.o, this.s, this.i); looks immutable constructor, , have constructor mutable object defined, default. can solve writing constructor , marking immutable, must how mark methods const, there better , easier solution.

try marking constructors , methods pure if can. if mark constructor pure, constructor can used both mutable , immutable objects. alternatively, can mark idup method pure, construct new foo, mutable object, , return immutable. because pure functions can safely create immutable data if data returned isn't referenced elsewhere. in other words, uniquely owned memory can moved out immutable types.