• Shriramana Sharma via Digitalmars-d-learn (25/25) Oct 22 2014 Hello. Please see the following code:
• Steven Schveighoffer (19/42) Oct 23 2014 This is not the same. In the above, the x outside the parens is ALWAYS a...

Shriramana Sharma via Digitalmars-d-learn writes:

Hello. Please see the following code:

import std.stdio ;
struct Pair {
	int x, y ;
	this (int x, int y) { x = x ; y = y ; }
}
void main() {
	auto P = Pair(1, 2) ;
	writeln(P.x, ' ', P.y) ;
}

This outputs 0 0, whereas the equivalent C++ code outputs 1 2 correctly:


struct Pair {
	int x, y ;
	Pair(int x, int y) : x(x), y(y) {}
} ;
int main() {
	auto P = Pair(1, 2) ;
	std::cout << P.x << ' ' << P.y << std::endl ;
}

It seems to me that D should either not permit argument names to
shadow the member names, since it has no initializer lists and all
members are automatically initialized. Comments?

-- 
Shriramana Sharma ஶ்ரீரமணஶர்மா
श्रीरमणशर्मा

Steven Schveighoffer <schveiguy yahoo.com> writes:

On 10/23/14 1:03 AM, Shriramana Sharma via Digitalmars-d-learn wrote:
 Hello. Please see the following code:

 import std.stdio ;
 struct Pair {
 	int x, y ;
 	this (int x, int y) { x = x ; y = y ; }
 }
 void main() {
 	auto P = Pair(1, 2) ;
 	writeln(P.x, ' ', P.y) ;
 }

 This outputs 0 0, whereas the equivalent C++ code outputs 1 2 correctly:


 struct Pair {
 	int x, y ;
 	Pair(int x, int y) : x(x), y(y) {}
 } ;

This is not the same. In the above, the x outside the parens is ALWAYS a 
member. D does not have this syntax. Change it to the same as your D 
implementation, and you get the same result (actually worse, because C++ 
will not initialize x and y for you).

 int main() {
 	auto P = Pair(1, 2) ;
 	std::cout << P.x << ' ' << P.y << std::endl ;
 }

 It seems to me that D should either not permit argument names to
 shadow the member names, since it has no initializer lists and all
 members are automatically initialized. Comments?

You're missing the "or" part of that statement :)

But 2 things:

x = x;

This should produce an error, or at least a warning I think, as it does 
nothing. However, even with dmd -w, it does not. I know I have seen the 
compiler complain about noop statements before, I just don't know under 
what circumstances

and 2, you would use the same mechanism as you use with C++ to 
initialize the items inside the ctor:

this.x = x;

Note, the rules for shadowing are the same as for any function 
parameters to any function vs. members or module variables. Nothing is 
inconsistent here.

-Steve