#Using C from C++
3 messages in this thread
I am new to C++, so this may be a dumb questions, but what the heck…
I have some C functions that I have used for eons to handle tasks I need
frequently. I am now getting into my first TCL program, and figured I'd just
bring along my standard "support.c" file.
Everything compiled fine. However, when I placed a call to a function in that
file, my link said that the function was undefined.
I solved the problem simply by changing the name of the file from "support.c"
to "support.cp" and typecasting a few places where C++ says that it can't
implicitely convert from (void *) to (char *).
My questions are:
1) Why does changing the file to *.cp fix the problem? I know that this makes
it use the C++ compiler instead of the C compiler, but why does that make any
difference to the linker here?
2) Why can't C++ convert from (void *) to (char *) without typecasting? I
thought that void matched any type, or is that no longer true in C++? Is there
another way to indicate "a pointer of any type"?
– Burt
The reason changing the extension from .c to .cp made it work is the functions
are now C++ functions, complete with mangled names. That's where you were
running into trouble before. To inhibit the name mangling, put 'extern "C"' in
front of the prototype, like so:
extern "C" void *myFunc (int arg1, float arg2);
// or
extern "C"
{
void *myFunc (int arg1, float arg2);
int *myFunc1 (int arg1, float arg2); // also a C function
}
Now, for the pointer conversion. Let me see if I understand what you're
saying. You want to do something like this in C++?
void *vPtr;
char *cPtr;
// assume pointers are initialized & what-not
cPtr = vPtr; // or equivalent in a function call
In other words, you want to put the contents of a void pointer into a char
pointer? C++ lets you assign any pointer type to variable of type void *, but
NOT the other way around; C++ considers such things "unsafe." Typecasting,
however, works, as in
(char *)vptr = cPtr;
Joe
>> C++ lets you assign any pointer type to variable of type void *, but NOT the
other way around; C++ considers such things "unsafe." Typecasting, however,
works
Yes, I knew that typecasting worked, and was what I did to make that code work.
I was just surprised that going _both_ ways is OK in C but not in C++ and
wanted to verify that I was understanding what I saw. Thanks for verifying
that I wasn't _too_ far off the deep end. 🙂
– Burt