Re: Assigning a member function to signal callback function pointer
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.
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.