There are plenty of systems where errno is #define'd to a more complex expression in order to provide per-thread errors. From glibc's bits/errno.h on Linux:

There are plenty of systems where errno is #define'd to a more complex expression in order to provide per-thread errors. From glibc's bits/errno.h on Linux: # ifndef __ASSEMBLER__ /* Function to get address of global `errno' variable. */ extern int *__errno_location (void) __THROW __attribute__ ((__const__)); # if !defined _LIBC || defined _LIBC_REENTRANT /* When using threads, errno is a per-thread value. */ # define errno (*__errno_location ()) # endif # endif /* !__ASSEMBLER__ */ #endif /* _ERRNO_H */ I wouldn't expect it to be common to see a macro with parameters being an lvalue, though. It wouldn't phase a C++ programmer, since references make it possible and even idiomatic to return lvalues. Where you normally see this is: x["foo"] = "bar"; is just syntactic sugar for // operator[] could be any "normal" function name! x.operator[]("foo") = "bar";