digitalmars.D.learn - Delegate / Error: cannot implicitly convert expression...
- =?iso-8859-1?Q?Robert_M._M=FCnch?= (13/13) Jun 15 2019 Why does the follwing code give: Error: cannot implicitly convert
- Anonymouse (9/19) Jun 15 2019 By design, I think.
- =?iso-8859-1?Q?Robert_M._M=FCnch?= (19/29) Jun 15 2019 Hmm... but this here compiles:
Why does the follwing code give: Error: cannot implicitly convert
expression & myFunc of type void function(int a) to void delegate(int)
void myFunc(int a){return;}
void main()
{
void delegate(int) dg;
dg = &myFunc;
}
See: https://run.dlang.io/is/iTYo2L
--
Robert M. Münch
http://www.saphirion.com
smarter | better | faster
Jun 15 2019
On Saturday, 15 June 2019 at 15:54:00 UTC, Robert M. Münch wrote:
Why does the follwing code give: Error: cannot implicitly
convert expression & myFunc of type void function(int a) to
void delegate(int)
void myFunc(int a){return;}
void main()
{
void delegate(int) dg;
dg = &myFunc;
}
See: https://run.dlang.io/is/iTYo2L
By design, I think.
"delegate and function objects cannot be mixed. But the standard
function std.functional.toDelegate converts a function to a
delegate."
Your example compiles if the assignment is changed to dg =
toDelegate(&myFunc); (given appropriate imports).
https://tour.dlang.org/tour/en/basics/delegates
https://dlang.org/phobos/std_functional.html#.toDelegate
Jun 15 2019
On 2019-06-15 16:19:23 +0000, Anonymouse said:By design, I think: "delegate and function objects cannot be mixed. But the standard function std.functional.toDelegate converts a function to a delegate." Your example compiles if the assignment is changed to dg = toDelegate(&myFunc); (given appropriate imports). https://tour.dlang.org/tour/en/basics/delegates https://dlang.org/phobos/std_functional.html#.toDelegateHmm... but this here compiles: void main() { import std.stdio: write, writeln, writef, writefln; void foo(int a) {return; } void test() { void delegate(int) dg; dg = &foo; } } See: https://run.dlang.io/is/U7uhAX Is it because inside main() there is a stack frame? And with a global function there is none? I'm a bit confused... -- Robert M. Münch http://www.saphirion.com smarter | better | faster
Jun 15 2019
Alex <sascha.orlov gmail.com> On Saturday, 15 June 2019 at 16:34:22 UTC, Robert M. Münch wrote:On 2019-06-15 16:19:23 +0000, Anonymouse said:Correct. Inside main, foo is a delegate. https://dlang.org/spec/function.html#nestedBy design, I think: "delegate and function objects cannot be mixed. But the standard function std.functional.toDelegate converts a function to a delegate." Your example compiles if the assignment is changed to dg = toDelegate(&myFunc); (given appropriate imports). https://tour.dlang.org/tour/en/basics/delegates https://dlang.org/phobos/std_functional.html#.toDelegateHmm... but this here compiles: void main() { import std.stdio: write, writeln, writef, writefln; void foo(int a) {return; } void test() { void delegate(int) dg; dg = &foo; } } See: https://run.dlang.io/is/U7uhAX Is it because inside main() there is a stack frame? And with a global function there is none? I'm a bit confused...
Jun 15 2019
On Saturday, 15 June 2019 at 16:34:22 UTC, Robert M. Münch wrote:On 2019-06-15 16:19:23 +0000, Anonymouse said:yes. delegate is a function pointer + its context. At the global scope there's no context. Actually the following works fine: --- void foo(){writeln(__PRETTY_FUNCTION__);} void main(string[] args) { void delegate() dg; dg.funcptr = &foo; dg.ptr = null; // usually a "this" or a frame address dg(); } --- because dg.ptr would be used to retrieve the offset of the locals variable used in the parent stack or the address of a class instance (the this). But this is not required at all here. The opposite way would lead to various access violation.By design, I think: "delegate and function objects cannot be mixed. But the standard function std.functional.toDelegate converts a function to a delegate." Your example compiles if the assignment is changed to dg = toDelegate(&myFunc); (given appropriate imports). https://tour.dlang.org/tour/en/basics/delegates https://dlang.org/phobos/std_functional.html#.toDelegateHmm... but this here compiles: void main() { import std.stdio: write, writeln, writef, writefln; void foo(int a) {return; } void test() { void delegate(int) dg; dg = &foo; } } See: https://run.dlang.io/is/U7uhAX Is it because inside main() there is a stack frame? And with a global function there is none? I'm a bit confused...
Jun 15 2019
On Saturday, 15 June 2019 at 17:24:45 UTC, user1234 wrote:
---
void foo(){writeln(__PRETTY_FUNCTION__);}
void main(string[] args)
{
void delegate() dg;
dg.funcptr = &foo;
dg.ptr = null; // usually a "this" or a frame address
dg();
}
---
because dg.ptr would be used to retrieve the offset of the
locals variable used in the parent stack or the address of a
class instance (the this). But this is not required at all
here. The opposite way would lead to various access violation.
That only works because foo doesn't have any parameters. When you
add even just one, things will go wrong.
Jun 15 2019
On Saturday, 15 June 2019 at 17:42:04 UTC, ag0aep6g wrote:On Saturday, 15 June 2019 at 17:24:45 UTC, user1234 wrote:True if was forgetting that the context is a hidden parameter... Maybe it depends on the calling convention too ?--- void foo(){writeln(__PRETTY_FUNCTION__);} void main(string[] args) { void delegate() dg; dg.funcptr = &foo; dg.ptr = null; // usually a "this" or a frame address dg(); } --- because dg.ptr would be used to retrieve the offset of the locals variable used in the parent stack or the address of a class instance (the this). But this is not required at all here. The opposite way would lead to various access violation.That only works because foo doesn't have any parameters. When you add even just one, things will go wrong.
Jun 15 2019