Hi, I'd like to gauge whether there might be any interest in a new Boost library comprising a generic, non-intrusive, wrapper class for creating immutable/const values with sharing of the underlying object via internal reference counting. This new class would be called ValueRef (or possibly just Value), and it would effectively provide an assignable/swappable 'reference'/handle to shared, const values. It can be considered an effective alternative to the commonly used shared-pointer-to-const idiom, providing cleaner and more explicit value semantics, whilst also hiding calls to 'new' as an internal implementation detail. In a nutshell, ValueRef adapts a regular type T to give a new type ValueRef<T> with const T value semantics and uses reference counting of an underlying object on the heap internally to reduce copying overhead. ValueRef will spawn off a new copy of the wrapped object (if necessary) if the ValueRef is assigned another ValueRef or another object of type T. Const functions of T do not incur any overhead of counter access or copy and are invoked by either * or -> syntax. Forwarding constructors are provided which permit the wrapped value to be constructed in place, and conversion operator allows inter-operation with values of the unwrapped type. ValueRef also provides the common relational operators. I have so far implemented a rough (C++03-only so far) version of this (based on a stripped down version of Adobe::copy_on_write library) and a test suite and it seems to work okay. Example usage: /* type of ValueRef wrapped std::vector<int> */ typedef ValueRef<std::vector<int> > ValueRefVecType; /* create vector of 10 elements via forwarding constructor */ ValueRefVecType valueRefVector1(10,1); /* non-mutating/const op */ assert( valueRefVector1->size() == 10 ); /* non-mutating/const op */ assert( (*valueRefVector1)[0] == 1 ); /* error - compilation failure occurs calling a non-const op */ (*valueRefVector)[0] = 2; /* copy ctor */ ValueRefVecType valueRefVector2(valueRefVector1); /* test for equality */ assert( valueRefVector1 == valueRefVector2 ); /* test for equality (implicit conversion) */ assert( valueRefVector2 == std::vector<int>(10,1) ); /* valueRefVector1 and valueRefVector2 share same std::vector */ assert( valueRefVector1.identity(valueRefVector2) ); /* ref count is no longer one */ assert( !valueRefVector1.unique_instance() ); /* new value */ valueRefVector1 = std::vector<int>(5); /* non-mutating/const */ assert( valueRefVector1->size() == 5 ); /* test for equality - value semantics */ assert( !(valueRefVector1 == valueRefVector2) ); /* valueRefVector1 and valueRefVector2 do not share same std::vector */ assert( !valueRefVector1.identity(valueRefVector2) ); I am not aware of anything else current that provides this functionality. Regards, Tom