rvalue-reference

Overload resolution between value, rvalue reference, const lvalue reference

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What are the rules here? As there is only one parameter, the rule is that one of the three viable parameter initializations of that parameter must be a better match than both the other two. When two initializations are compared, either one is better than the other, or neither is better (they are indistinguishable). Without … Read more

C++11 rvalues and move semantics with return statement

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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

Passing/Moving parameters of a constructor in C++0x

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You take each one by value, like this: struct foo { foo(std::string s, bar b, qux q) : mS(std::move(s)), mB(std::move(b)), mQ(std::move(q)) {} std::string mS; bar mB; qux mQ; }; The initialization of the function parameters by the argument will either be a copy-constructor or move-constructor. From there, you just move the function parameter values into … Read more

What’s a use case for overloading member functions on reference qualifiers?

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In a class that provides reference-getters, ref-qualifier overloading can activate move semantics when extracting from an rvalue. E.g.: class some_class { huge_heavy_class hhc; public: huge_heavy_class& get() & { return hhc; } huge_heavy_class const& get() const& { return hhc; } huge_heavy_class&& get() && { return std::move(hhc); } }; some_class factory(); auto hhc = factory().get(); This does … Read more

Is it useless to declare a local variable as rvalue-reference, e.g. T&& r = move(v)?

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No, AnyTypeMovable&& r = move(v); here is not useful at all. Consider the following code: #include <iostream> #include <vector> class MyMovableType { int i; public: MyMovableType(int val): i(val){} MyMovableType(MyMovableType&& r) { this->i = r.i; r.i = -1; } MyMovableType(const MyMovableType& r){ this->i = r.i; } int getVal(){ return i; } }; int main() { std::vector<MyMovableType> … Read more

What is going on: C++ std::move on std::shared_ptr increases use_count?

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What is going on, here? On MacOS, it seems that you must explicitly enable move-sematics with -std=c++11 (or later standards)¹. Otherwise, the example happens to compile (i.e., std::shared_ptr from the related library implementation is usable) but doesn’t work correctly as the required language features aren’t enabled. This results in actual copies being made instead of … Read more

Does the C++ standard guarantee that a failed insertion into an associative container will not modify the rvalue-reference argument?

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Explicit and unequivocal NO. Standard doesn’t have this guarantee, and this is why try_emplace exists. See notes: Unlike insert or emplace, these functions do not move from rvalue arguments if the insertion does not happen, which makes it easy to manipulate maps whose values are move-only types, such as std::map<std::string, std::unique_ptr<foo>>. In addition, try_emplace treats … Read more

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