Re: Why can't I init a new var from const var?
On 02/12/2017 12:15 AM, Random D user wrote:
I can init a variable from mutable source without defining any
constructor or assignment operators, but not if the source is const. I
would imagine the behavior to be the same with mutable and const source,
since it's just reading the source and copying it.
It works as long as your type has no mutable indirections (pointers,
array, class objects). You can't just copy an indirection from const to
mutable, because the referenced (possibly immutable) data would be
incorrectly typed as mutable.
Is there a reason for this? Or is this a bug?
I can workaround this by making copies or casting, that just creates
ugly code everywhere.
Here's an example (with dmd 2.073):
struct Foo
{
[...]
this( const Foo source )
{
buf = source.buf.dup;
}
[...]
char[] buf;
}
Foo fun(const ref Foo foo, Foo foo2)
{
Foo bar = foo; // Error: cannot implicitly
convert expression (foo) of type const(Foo) to Foo
Initializations like this don't call the constructor. Instead, they do
"copy construction" and call a "postblit function" if it's defined.
See https://dlang.org/spec/struct.html#struct-postblit
So, if you want to call a function whenever your struct gets copied, you
define a postblit function. Like so: this(this) { buf = buf.dup; }
Or if you want to do the dup-ing in specific places only, call the
constructor explicitly: Foo bar = Foo(foo);
Foo baz = foo2;// Ok, No need for constructors
or opAssign
Note that this does not call the constructor, and does not do the dup.
If you want this to do the dup, postblit is the way.
[...]
//Foo bar = Foo(foo); // This works provided there is non-const
opAssign defined.
//Foo bar = cast(Foo)foo; // This seems to work as well
I wouldn't have expected the cast to call the constructor, but
apparently it does. It's in the spec here:
https://dlang.org/spec/expression.html#CastExpression
"Casting a value v to a struct S, when value is not a struct of the same
type, is equivalent to: S(v)"
So a cast like that is really just a fancy way of calling the constructor.
This seems a bit inconsistent, though. On postblit the spec says that
copy construction happens with "the same type". The compiler apparently
sees Foo and const(Foo) as the "same type" in that context. But with the
cast, the qualifier mismatch is enough to trigger the constructor call
which is supposed to happen when the types are not the same.
return bar;
}
Foo foo;
foo = fun(foo, foo);
Re: Why can't I init a new var from const var?
On 02/11/2017 03:15 PM, Random D user wrote:
> I can init a variable from mutable source without defining any
> constructor or assignment operators, but not if the source is const. I
> would imagine the behavior to be the same with mutable and const source,
> since it's just reading the source and copying it.
>
> Is there a reason for this? Or is this a bug?
> I can workaround this by making copies or casting, that just creates
> ugly code everywhere.
Just to make sure, you don't need to define all those overloads. This is
sufficient but you have to use a different (and idiomatic construction
syntax.):
struct Foo
{
this( const(Foo) source )
{
buf = source.buf.dup;
}
void opAssign( const(Foo) source )
{
buf = source.buf.dup;
}
char[] buf;
}
Foo fun(const ref Foo foo, Foo foo2)
{
auto bar = Foo(foo);
// Foo bar = foo; // Error: cannot implicitly
convert expression (foo) of type const(Foo) to Foo
Foo baz = foo2;// Ok, No need for constructors
or opAssign
auto baz2 = const(Foo)(foo2);
Foo bar2;
bar2 = foo; // uses opAssing( const Foo ) /
opAssign( const ref Foo )
Foo bar3;
bar3 = foo2;// uses opAssign( const Foo ) /
opAssign( Foo )
Foo bar4;
bar4 = cast(const Foo)foo2; // uses opAssing( const Foo )
//Foo bar = Foo(foo); // This works provided there is non-const
opAssign defined.
//Foo bar = cast(Foo)foo; // This seems to work as well
return bar;
}
void main() {
}
When you do this:
Foo bar = foo;
There is an implicit conversion in place, which D does not allow during
construction. The idiomatic syntax is the following:
auto bar = Foo(foo);
The construction on the right-hand side is now explicit.
Likewise, prefer the following syntax:
auto baz2 = const(Foo)(foo2);
Notes:
When you want to treat lvalue sources differently from rvalue sources,
you can define the following overloads as well (note const(Foo) syntax,
which is preferred):
this( ref const(Foo) source )
{
buf = source.buf.dup;
}
void opAssign( ref const(Foo) source )
{
buf = source.buf.dup;
}
Alternatively, you can define an 'auto ref' function that automatically
takes both lvalues and rvalues (lvalue by-ref, rvalues by-move). Note
the empty template parentheses:
struct Foo
{
this()( auto ref const(Foo) source )
{
buf = source.buf.dup;
}
void opAssign()( auto ref const(Foo) source )
{
buf = source.buf.dup;
}
char[] buf;
}
Ali
Why can't I init a new var from const var?
I can init a variable from mutable source without defining any
constructor or assignment operators, but not if the source is
const. I would imagine the behavior to be the same with mutable
and const source, since it's just reading the source and copying
it.
Is there a reason for this? Or is this a bug?
I can workaround this by making copies or casting, that just
creates ugly code everywhere.
Here's an example (with dmd 2.073):
struct Foo
{
this( Foo source )
{
buf = source.buf.dup;
}
this( const Foo source )
{
buf = source.buf.dup;
}
this( const ref Foo source )
{
buf = source.buf.dup;
}
void opAssign( Foo source )
{
buf = source.buf.dup;
}
void opAssign( const Foo source )
{
buf = source.buf.dup;
}
void opAssign( const ref Foo source )
{
buf = source.buf.dup;
}
char[] buf;
}
Foo fun(const ref Foo foo, Foo foo2)
{
Foo bar = foo; // Error: cannot
implicitly convert expression (foo) of type const(Foo) to Foo
Foo baz = foo2;// Ok, No need for
constructors or opAssign
Foo baz2 = cast(const Foo)foo2; // Error: cannot
implicitly convert expression (Foo(null).this(foo2)) of type
const(Foo) to Foo
Foo bar2;
bar2 = foo; // uses opAssing( const
Foo ) / opAssign( const ref Foo )
Foo bar3;
bar3 = foo2;// uses opAssign( const
Foo ) / opAssign( Foo )
Foo bar4;
bar4 = cast(const Foo)foo2; // uses opAssing( const
Foo )
//Foo bar = Foo(foo); // This works provided there is
non-const opAssign defined.
//Foo bar = cast(Foo)foo; // This seems to work as well
return bar;
}
Foo foo;
foo = fun(foo, foo);
