Question

I define below data type:

typedef int MyInt;

Then I hope to define a new data type based on size of MyInt, something like below code, but I can’t find a solution for it.

#if sizeof(MyInt) == 2
    typedef long MyLong;
#else
    typedef short MyLong;
#endif

Could anybody help?

Answer 1

It’s not the preprocessor’s job to evaluate sizeof, that’s done by the compiler which is a later stage in the process. Evaluating sizeof needs deep C knowledge that the preprocessor simply doesn’t have.

You could (in theory) consider the preprocessor as a separate step, that does text only transforms, in effect converting “foo.c” to “foo-preprocessed.c”. The latter file won’t have any #include or #ifs left, they’re all evaluated and replaced by the preprocessor. The actual compiler never sees them.

You should consider using <stdint.h> and the known-precision types (uint16_t and friends).