• Sean Eskapp (18/18) Jan 21 2011 The following code yields results as commented.
• =?UTF-8?B?QWxpIMOHZWhyZWxp?= (47/65) Jan 21 2011 Help anybody?

Sean Eskapp <eatingstaples gmail.com> writes:

The following code yields results as commented.

import std.stdio;

class A
{
	int b;
}

void main()
{
	const A a = new A;
	writeln(typeof(a).stringof); // const(A)
	writeln(typeof(a.b).stringof); // const(int)

	writeln((const A).stringof); // const(A)
	writeln(typeof((const A).b).stringof); // int
}

Given this information, how do I work around this in templates? I'd like a
template which can type its params based on members of T, for instance:

void foo(T)(T x, typeof(T.member) y)

but this will yield non-const second parameters even if T is const. Help
anybody?

=?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:

Sean Eskapp wrote:
 The following code yields results as commented.

 import std.stdio;

 class A
 {
 	int b;
 }

 void main()
 {
 	const A a = new A;
 	writeln(typeof(a).stringof); // const(A)
 	writeln(typeof(a.b).stringof); // const(int)

 	writeln((const A).stringof); // const(A)
 	writeln(typeof((const A).b).stringof); // int
 }

 Given this information, how do I work around this in templates? I'd 

like a
 template which can type its params based on members of T, for instance:

 void foo(T)(T x, typeof(T.member) y)

 but this will yield non-const second parameters even if T is const. 

Help anybody?

Ironically, parameter list is not a useful place to constraint the 
parameter types. At the least, you probably want some conversions. If 
so, would this work:

import std.traits;

void foo(T, M)(T x, M m)
     if (isImplicitlyConvertible!(M, typeof(T.member)))
{}

class C
{
     int member;
}

void main()
{
     auto c = new C;
     foo(c, 42);
}

If you want to ensure that the member is assignable, you can try this:

import std.traits;

void foo(T, M)(T x, M m)
     if (__traits(compiles, { x.member = m; }))
{}

class C
{
     int member;
}

void main()
{
     const auto c = new C;  // <-- compilation error
     foo(c, 42);
}

But the error message is only somewhat helpful:

deneme.d(26369): Error: template deneme.foo(T,M) if 
(__traits(compiles,delegate ()
{
x.member = m;
}
)) does not match any function template declaration
deneme.d(26369): Error: template deneme.foo(T,M) if 
(__traits(compiles,delegate ()
{
x.member = m;
}
)) cannot deduce template function from argument types !()(const(C),int)

Ali