How to do portable 64 bit arithmetic, without compiler warnings

Question

I occasionally use 64 bit arithmetic in an open source C++ library of mine. I discovered that long long serves my purpose quite nicely. Even some 10 year old solari

Answer 1

When your library is provided as source, one option is to provide a "porting" header, in which it is your users' responsibility to provide a 64 bit type (you'd specify the name). It's then also naturally their responsibility to deal with any compiler warnings that their choice of type provokes, either avoid them, suppress them, or ignore them.

I guess this is what you call "messing around with #defines", but I don't think there's too much wrong with it. You can provide a default version which just uses long long directly and will work on your 10-year-old Solaris box and also on Windows, so most users would never need to go near the user-configurable part of your library.

Then for the pedantic users, you can provide a version for GCC which includes <sys/types.h> and uses int64_t instead of long long. This doesn't provoke any warning for me with g++ -pedantic. You could even do this in the default version by recognising GCC, which certainly is messing around with #defines, but again not in a way that's at all unusual for a multi-platform product.

If your library is also provided as binaries for certain platforms, then of course you have to decide what the 64 bit type is going to be. If it also appears in the library interface (and hence the header file), then you'll just have to choose one which will not provoke any warnings with reasonable compiler options. I think -pedantic is a reasonable compiler option, and apparently so do your users, so again that's int64_t on GCC.

Answer 2

You could replace your use of long long with one of the many C++ bigint libraries. I'm sure some of them avoid this compiler error. Personally, I'd rather stick with the error.

Answer 3

If you're unable to control the switches passed to gcc, you might be able to turn off the warning with a #pragma.

http://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html

Answer 4

You can silence the warning with -Wno-long-long (make sure it comes after -pedantic). 64-bit integers are required by C99 and I think also C++0x so compilers that don't have them are getting rare nowadays.

Answer 5

In GCC use the -Wno-long-long compiler option to suppress that particular warning.

You could also use -std=C++0x, but will probably reduce portability further.

Answer 6

You can also suppress the warning using gcc's "__extension__" feature, e.g.:

// No '-pedantic' warning/error.
__extension__ long long foo = 2;

// Exhibits '-pedantic' warning/error.
long long bar = 3

and the compile:

$ g++ -pedantic -fsyntax-only foo.cpp
foo.cpp:5: error: ISO C++ 1998 does not support 'long long'

Notice that only the last use of long long triggered the -pedantic error since no __extension__ was prepended. Regardless, I'd go with @Steve Jessop's suggestion of using int64_t instead.