[C++03] Temporaries passed to constructor yield invalid member references
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From: M Stefan on 29 May 2010 08:39 Hello. I am trying to write a class similar to this: template <class StringT> struct A { A (StringT const &str) : str(str) { } StringT const &str; }; The problem is that the constructor allows passing temporaries, which get destroyed as soon as the constructor finishes executing, not after the object is destroyed. For instance, constructing the class: A<std::string> a("hello"); would cause an implicit conversion from char const* to std::string const&, and the converted temporary will be passed to the ctor. The reference str of the class is then bound to this temporary, which gets destroyed before the object, thus leaving the object with an invalid reference. Then, I modified it so that no implicit conversions are allowed: template <class StringT> struct A { A (StringT const &str) : str(str) { } template <class U> explicit A (U) { BOOST_STATIC_ASSERT(false); } StringT const &str; }; This fixes the problem with the conversion from char const* to std::string const&, but still allows the following way of constructing the class: A<std::string> a( (std::string("a")) ); This constructs A with a temporary which again gets destroyed before the object, thus leaving an invalid ref behind. Discussing with some other people, we have come up with several solutions, none of which fully satisfy me: 1) Make the constructor take a non-const ref as opposed to a const ref. (Why not: breaks const correctness) 2) Add an additional level of indirection: (Why not: likely overhead) struct A { boost::any str; A () : str() {} template <class StringT> void set_str(StringT const &s) { str = s; } }; 3) Make the constructor take a StringT as opposed to a reference (Why not: overhead and additional memory) Please let me know if you have any better solutions to my problem. Note that compatibility with C++03 is a must here, and performance is kind of important. Best Regards, Stefan -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |