They are occasionally useful. The draft C++0x itself uses them in a few places, for example: template <class T> void ref(const T&&) = delete; template <class T> void cref(const T&&) = delete; The above two overloads ensure that the other ref(T&) and cref(const T&) functions do not bind to rvalues (which would otherwise be possible). … Read more
This is not how std::make_pair is intended to be used; you are not supposed to explicitly specify the template arguments. The C++11 std::make_pair takes two arguments, of type T&& and U&&, where T and U are template type parameters. Effectively, it looks like this (ignoring the return type): template <typename T, typename U> [return type] … Read more
Generalized lambda capture in C++14 In C++14 we will have the so called generalized lambda capture. This enables move capture. The following will be legal code in C++14: using namespace std; // a unique_ptr is move-only auto u = make_unique<some_type>( some, parameters ); // move the unique_ptr into the lambda go.run( [ u = move(u) … Read more
Beta_ab&& Beta::toAB() const { return move(Beta_ab(1, 1)); } This returns a dangling reference, just like with the lvalue reference case. After the function returns, the temporary object will get destructed. You should return Beta_ab by value, like the following Beta_ab Beta::toAB() const { return Beta_ab(1, 1); } Now, it’s properly moving a temporary Beta_ab object … Read more
Is bar an rvalue or an lvalue? The question answers itself. Whatever has a name is an lvalue(1). So bar is an lvalue. Its type is “rvalue reference to string“, but it’s an lvalue of that type. If you want to treat it as an rvalue, you need to apply std::move() to it. If you … Read more
First example std::vector<int> return_vector(void) { std::vector<int> tmp {1,2,3,4,5}; return tmp; } std::vector<int> &&rval_ref = return_vector(); The first example returns a temporary which is caught by rval_ref. That temporary will have its life extended beyond the rval_ref definition and you can use it as if you had caught it by value. This is very similar to … Read more
It declares an rvalue reference (standards proposal doc). Here’s an introduction to rvalue references. Here’s a fantastic in-depth look at rvalue references by one of Microsoft’s standard library developers. CAUTION: the linked article on MSDN (“Rvalue References: C++0x Features in VC10, Part 2”) is a very clear introduction to Rvalue references, but makes statements about … Read more
You have to understand the forwarding problem. You can read the entire problem in detail, but I’ll summarize. Basically, given the expression E(a, b, … , c), we want the expression f(a, b, … , c) to be equivalent. In C++03, this is impossible. There are many attempts, but they all fail in some regard. … Read more
I’d say the Rule of Three becomes the Rule of Three, Four and Five: Each class should explicitly define exactly one of the following set of special member functions: None Destructor, copy constructor, copy assignment operator In addition, each class that explicitly defines a destructor may explicitly define a move constructor and/or a move assignment … Read more