Universal Reference And Perfect Forwarding
Perfect fowarding in C++ preserves the type and value categories of an input argument.
Refresher - Value categories:
- lvalue
- rvalue
In the following snippet, we have a few interesting aspects
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template<typename Func>
void profile_and_call(Func&& func){
std::forward<Func>(func)();
}
Universal Reference
std::forward calls a callable be it a lvalue, or an rvalue. When a type is deduced in a template, if it’s in the form &&, it’s a universal reference, Func&& in this case is the universal reference to func. So if it’s:
- an lvalue:
profile_and_call(another_function),another_functionisanother_function&, andFunc&&is deduced toanother_function& &&and becomesanother_function& - an rvalue:
profile_and_call(std::move(another_function)),another_functionisanother_function&&, andFunc&&is deduced toanother_function&& &&and becomesanother_function&&
Specifically, this preserves the value category of the function, specifically when the function inside has move semantics. If it’s a simple const Func& func argument, we will not be able to move. Also, if the function is a captured lambda, it’s a temporary object that will be best used if it’s “moved”
On the other hand, note that T&& is a universal reference only when T is a deduced template parameter. (so it doesn’t happen outside of the template scenario).
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void func(int&& arg); // rvalue reference only
