const ref rvalues

[Andrei Alexandrescu]

Eldar Insafutdinov wrote:
> dsimcha Wrote:
> 
>> I mentioned this deep in another thread, but I think it deserves its own
>> thread.  Can we get something like:
>>
>> void doStuff(T)(const ref T val) {
>>     // do stuff.
>> }
>>
>> T getVal() {
>>     return someValue;
>> }
>>
>> void main() {
>>     doStuff(getVal());  // Doesn't currently work.
>> }
>>
>> For non-const ref parameters, it's understandable that passing rvalues in
>> doesn't work because this is likely a bug that the compiler should catch.
>> However, for const ref parameters, can't the compiler just implicitly put the
>> value on the caller's stack frame and convert it to an lvalue rather than
>> forcing the programmer to write the boilerplate to do this manually?  This
>> would result in:
>>
>> doStuff(getVal()); -->
>>
>> auto __temp = getVal();
>> doStuff(__temp);
> 
> My vote for it.

Binding rvalues to const references was probably the single most hurtful 
design decisions for C++. I don't have time to explain now, but in short 
I think all of the problems that were addressed by rvalue references, 
and most of the aggravation within the design of rvalue references, are 
owed by that one particular conversion.

If we ever allow automatic conversion of an rvalue to a ref, we should 
know very well what we're doing.


Andrei