- Copy and Direct Initialization - 1 Update
- Onwards and upwards - 1 Update
| "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Aug 23 02:56PM +0200 On 22.08.2015 20:43, Stefan Ram wrote: > { > { beta b = alpha{}; /* > gamma g = alpha{}; */ } There is no direct way to construct a right hand side gamma from an alpha, because that would involve a user defined conversion. The compiler doesn't check such, it merely checks whether the actual argument matches the formal argument type of some gamma constructor. It /would/ match if alpha were derived from beta, but that's not a conversion, that's an IS-A relationship. > { beta b{ alpha{} }; > gamma g{ alpha{} }; }} Here possible conversions to the formal argument type, are considered, just as with any function call. > error message.) > But it /did/ use the sequence in the case of /direct/ > initialization. Well, there's only a single IMPLICIT conversion, namely from actual argument type alpha to formal argument type beta. But the expression gamma( alpha() ) is a conversion from alpha to gamma by way of beta. It involves two user defined conversions, but crucially, only a single implicit one. Here is a similar program that I think better illustrates how the limit on number of implicit user defined conversions can be reached, and then affects whether the code compiles or not: #include <iostream> using namespace std; struct alpha { alpha() { cout << "alpha()" << endl; } }; struct beta { beta() { cout << "beta()" << endl; } }; struct gamma { operator alpha() { cout << "gamma -> alpha" << endl; return alpha(); } gamma( beta ){ cout << "gamma( beta )" << endl; } gamma( alpha ) { cout << "gamma( alpha )" << endl; } gamma( gamma& ) { cout << "gamma( gamma& )" << endl; } }; int main() { //gamma g = beta(); // Over the limit. gamma g = gamma( beta() ); // OK, invoking `operator alpha()` } Output: beta() gamma( beta ) gamma -> alpha alpha() gamma( alpha ) This was the trick employed by old `std::auto_ptr`. Or to be precise, the trick that got standardized. It evolved a bit, as I recall. Cheers & hth., - Alf |
| woodbrian77@gmail.com: Aug 22 08:18PM -0700 > I guess the C++ Middleware Writer is the elephant in the > room. It's met with silence and denial by some, but we > continue working on the software and hardware. Horton Hears a Who. > Ebenezer Enterprises works to reward investments to 3 times the > original amount. So the investment would result in between > $0 and $3,600, depending on how things go for the company. Brian Ebenezer Enterprises - A person's a person no matter how small. http://webEbenezer.net |
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