On Jun 9, 2012, at 6:03 PM, Howard Hinnant wrote:
On Jun 9, 2012, at 4:21 PM, Daniel Larimer wrote:
Generic code that expects traditional copy syntax is going to fail to compile when used with your convention:
template<class T> T min(initializer_list<T> t);
...
etc.
I suspect that you could create something to wrap the explicit-copy type if you really need to use it in such a case: template<typename T> class implicit_copyable { public: implicit_copyable( T e ) :val(std::move(e)){} implicit_copyable( implicit_copyable&& c ) :val(std::move(c.val)){} implicit_copyable( const implicit_copyable& c ) :val( copy(c.val) ){} operator T&(){ return val; } operator const T&()const { return val; } private: T val; } I suspect that generic algorithms that depend on copy syntax, would not be a good match or even be applicable for types that are implemented as 'explicit copyable only'. In my particular case I am passing 'messages' around and they only ever exist in one place at a time. I want the compiler to tell me when I try to make a copy. The effect is the same as using unique_ptr<Message> except that Message has value semantics and there is no 'extra' heap allocation for Message which is essentially a pointer to the real data. In theory I could 'copy' the message, but that would have to be a carefully considered decision.
On Jun 9, 2012, at 4:49 PM, Giovanni Piero Deretta wrote:
As per http://cpp-next.com/archive/2009/08/want-speed-pass-by-value/ , if the function is likely to consume or copy the parameter, the best solution is to pass by value. This means you'll only need to provide a single overload.
"The best solution" has an implicit "always" in it. Telling programmers they don't have to think, design or measure is a disservice to our industry. Pass-by-value is a good tool to have in the toolbox. It is not always the best solution. There is no solution that is always best.
....
For me this runs about 19% slower one way than the other. And I have no idea how portable that result is. Maybe for your application the 19% speed hit (more or less) is inconsequential. Maybe for someone else's application a 19% speed hit (more or less) is critical. <shrug> We the advise givers have no idea.
Pass-by-value is a good tool in the toolbox. It is not free. There are other good tools in the toolbox too. None of them are free either.
Good catch. Rule #1, always profile. I suspect that some guidelines on where to use pass-by-value vs my explicit-copy approach would be helpful. Obviously there is no 'always', but I hate having to think about it every time, so perhaps some rules of thumb are in order.
Howard
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