Detlef Vollmann wrote:
Andrey Semashev wrote:
I think the performance cost of checking a reference or pointer for validity and checking some property in the error_code instance will be the same. After all, in both cases it comes down to a single comparison of two integers. Even if referencing the global default instance does add overhead, I bet it is negligible. No, it isn't. The null pointer will be in a register, while the global object might not even be in the cache. And this for functions that are called by the thousands per second on a busy server.
PS: This was one of the main reasons why the POSIX/C++ group is in favour of the pointer.
If the global object is used so often, it will likely be in cache. If not often, it doesn't matter. An whether the compiler can load a structure of size equal to int into a register is a question of the compiler's prosperity. Anyway, I'd prefer code clarity in this case instead of a possible tiny performance gain that will be swallowed by the surrounding code. Consider it in comparison with a file system operation cost, for example.