Interoperability of types in C and C++
c, c++, language-interoperability, language-lawyer
Solution
No.
There are widely used conforming compilers on x64 amd compatible cpus that treat `long`as 32 bit and others as 64 bit by default. So this is not even the case for two C++ compilers on the same system, let alone a C++ and C compiler.
Within one compiler, that is up to the compiler vendor if they are compatible. They usually (always) are. "one compiler" is a bit of a misnomer here: the C snd C++ compilers are different compilers, even if in the same binary by the same vendor, in a sense.
Problem
A very simple question: are there any guarantees that a C `int` is the same thing as a C++ `int`, on the same system? It goes without saying that this is, of course, a purely theoretical question. The C and C++ standards use the same language to define the fundamental types. But whereas Fortran 2003 makes it clear that ``` use ISO_C_BINDING integer(kind=c_int) :: i ``` declares an integer type which is compatible with the `int` type on a "companion C processor", I can't find any such assertion in the C++ stardard. It seems very odd that Fortran would provide stronger C interoperability guarantees than C++! The closest I can find is section 7.5 [dcl.link], paragraph 3 of the C++11 standard, which states that Every implementation shall provide for linkage to functions written in the C programming language But this little sentence doesn't (to me) seem strong enough to guarantee compatibility of fundamental types. Is there some other language in the C++ standard that I've overlooked which guarantees this, or is it just so obviously taken for granted that no-one has bothered to state it explicitly? EDIT: David Schwartz in the comments points out that I was imprecise when I said "the same system". I really meant the same "platform", i.e. hardware, OS, system libraries etc. It's really an ABI issue of course. In the quoted passage the C++ standard seems to want to indicate that you can call C functions with `extern "C"`, but I'm not sure if it provides enough other guarantees?