If the construct automatically always leads to stack overflow errors, one could - as a third option - disallow it altogether.   Something like a default copy constructor may not be called from a derived class with a parameter passed by value.   So the programmer would have to solve it explicitly. This would be the better alternative than to make a surprising special case.   Can the compiler detect those cases at least most of the time?     -----Ursprüngliche Nachricht----- Von:David wang via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Mi 08.05.2024 08:05 Betreff:[std-proposals] Pass derived object by reference to single parameter delegated templated constructors An:C++ <std-proposals_at_[hidden]>; CC:David wang <wangjufan_at_[hidden]>; On Tue, 7 May 2024 10:36:25 +0200 Sebastian Wittmeier  wrote: >So in your case, you want to have a reference type deduced (by template argument deduction),  >but currently [expr.type]/1 disagrees (https://timsong-cpp.github.io/cppwp/expr.type#1): >If an expression initially has the type ?reference to T? ([dcl.ref], [dcl.init.ref]), the type is >adjusted to T prior to any further analysis. >The copy constructor is something special (compared to other constructors). >Why do you want to handle it by a template >in the first place?  Yes, I want to have a reference type deduced, but not  through the template parameter deduction mechanism PR87332.  The template parameter deduction mechanism leads to T, not T&. "template <typename T> base(T x) {}" prevents the compiler from calling the default copy constructor.   There are two ways to prevent it:  modifying function matching rules which seems to confuse the users and breaks existing rules or passing derived objects by reference which is easy to implement and test but violates [expr.type]/1. But this violation is just a special case.  Handling it by a template may be the user's choice which is provided by C++. Although users use it correctly, it still leads to bugs.   -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals