Bug 18284 – Can call struct method through a null pointer

The following example causes a runtime error as expected import std.stdio; class A { void print() { writeln("hello"); } } void main() { A a; assert(a is null); a.print(); // Runtime error as expected } However, by making `A` a struct, and accessing it through a pointer, there is no runtime error. import std.stdio; struct A { void print() { writeln("hello"); } } void main() { A* a; assert(a is null); a.print(); // No runtime error. Weird! } Adding a field to the struct, and accessing that field in the method will cause a runtime error, as expected. import std.stdio; struct A { int i; void print() { writeln(i); } } void main() { A* a; assert(a is null); a.print(); // Runtime error as expected }

Try it again with print() being final on the class. The reason your code causes a runtime error with the class is the method is virtual, and so needs to be looked up in the vtable. No such lookup is necessary for the struct, and so no null-deref happens, and so no runtime error.