AMDG 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.
Detlef
PS: This was one of the main reasons why the POSIX/C++ group is in favour of the pointer.
Why does the global object need to be read? static error_code throws_; error_code& throws() { return(throws_); } void f(error_code& ec = throws()) { if(&ec == &throws()) { throw ...; } else { .... } } This should not actually load anything except the address of the global error_code. The only advantage of using a null pointer is that it can be checked with a slightly simpler instruction. In Christ, Steven Watanabe