Re: Assigning a member function to signal callback function pointer
On Dec 25, 9:21 pm, Rolf Magnus <ramag...@t-online.de> wrote:
James Kanze wrote:
Which still isn't a valid function to pass to a C
interface, the linkage is wrong.
You may mean "calling convention", not "linkage", although
in all compilers that I know of that let you specify the
calling convention, it's part of the type (e.g. __stdcall
vs. __cdecl with MSVC).
The standard specifies a means of specifying the calling
convention; it's called a linkage specification.
No, I should have said "Linkage specification". Nothing to do
with calling conventions.
Linkage specification is the C++ syntax for specifying the
calling conventions: "C++", "C", "Fortran", etc. ("C++" and
"C" are required; any others are implementation defined.)
Well, "linkage" contains everything that is involved in being
able to call functions defined in the other language. That
includes calling conventions as well as name mangling.
And anything else that might be needed. I've usually heard
"calling conventions" used for all of it. But of course, that
was before type safe name mangling was used: the "calling
convention" was typically something along the lines of
prefixing a '_' to the name.
Passing a pointer to a C++ function to a function expecting
a pointer to a C function is an issue, although many
compilers will ignore the difference. Others aren't so
lax.
The linkage is part of the type. Using the address of a
function with C++ linkage to initialize a pointer to a
function with C linkage is a diagnosable error; a C++
compiler is required to emit a diagnostic. Some compilers
have bugs, however, and don't. (And why Microsoft decided
to invent a completely different mechanism, when the
standard already defined one, I don't know. Logically,
instead of new keywords, I would have expected ``extern
"stdcall"'', etc. Not that it makes much difference in
practice.)
My guess is that this is simply because that C++ feature
doesn't exist in C and because the Microsoft variant is older
than the C++ mechanism.
The C++ mechanism dates to well before Microsoft had a C++
compiler, or were even developing one.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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