On Jul 6, 2004, at 11:37 AM, David B. Held wrote:
"Howard Hinnant" <hinnant@twcny.rr.com> wrote in message news:D131A1F4-CEE6-11D8-ACE3-003065D18932@twcny.rr.com...
[...] I initially viewed call_traits as a way to optimize whether a type should be passed by const ref or by value. But I was mistaken. The real value was in passing/returning reference types without bumping into the reference-to-reference problem.
So are you saying that call_traits is *not* useful for said optimization? If so, why not?
For it to make a significant difference in performance, say for passing a parameter, the function would have to be very short - short enough that it should be inlined. On Metrowerks Pro 9.2 / Mac I've just run the following experiment: inline int foo1(int i) { return i; } inline int foo2(const int& i) { return i; } int main() { int i = 1; return foo1(i); } I've looked at the generated (optimized) assembly of main, using both foo1 and foo2. The assembly is identical in both cases. No doubt I could come up with examples where the assembly is different. But I'm having more trouble coming up with a situation where the performance difference is enough that I'd want to start lacing my generic code with call_traits for this purpose. However if you have such examples, I'm interested in pursuing this further. -Howard