Error occurs, when you try to use a delegate as a parameter to a C linked function. ex: extern (C) void c_func(void delegate() dg); void d_func(void delegate() dg) { c_func(dg); } Error: function test.c_func (void delegate()) does not match parameter types (void delegate()) Error: cannot implicitly convert expression (dg) of type void delegate() to void delegate()
I don't think this bug is valid, something does need to be done about it though. The following works: ---- alias extern(C) void delegate() cdg; extern (C) void c_func(void delegate() dg); void d_func(cdg dg) { c_func(dg); } ---- So the problem is the linkage of the delegates doesn't match, there's no way to get around it without an alias though, you can't have extern(C) in the function parameters. Either the error needs improving, or there needs to be a way to specify the linkage in the function args.
So two questions: 1. Why the following is allowed: extern (C) void c_func(void delegate()); ? 2. What does mean extern (C) void delegate() ? Delegate is just a 'data structure which consists of two pointers' and anything more. Something like C delegate does not exists. Linkage is related to functions not data structures delegate is a data structure. So if D data structures are disallowed in C functions why the following is allowed? extern (C) void c_func(string, Foo); class Foo{...} void d_func() { scope foo = new Foo(); string str = "Hello World!"; c_func(str, foo); } string is a data structure as delegate, foo is a pointer to a data structure.
Error message fix: https://github.com/D-Programming-Language/dmd/pull/943
Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/6fec4c76494534bdbcd958eafea65c73b418deed Fix issue 4288 – Error on passing delegate to C linkage function. This commit makes TypeDelegate::toCBuffer2 reuse the same code as for function pointers, which solves the problem that the linkage type is not included in error messages. Previously, nonsensical messages like »cannot implicitly convert expression (dg) of type void delegate() to void delegate()« would be emitted. https://github.com/D-Programming-Language/dmd/commit/7b1ed6349f405e3fed46d7c7b1a7cc641d7e9a8a Merge pull request #943 from klickverbot/argument-calling-conv-error Issue 4288 – Error on passing delegate to C linkage function.