I started using c++ std::unique_ptr
s and I stumbled across the following problem:
I want to pass a reference of my unique pointer to a function and assign a new value to the pointer in this function. Intuitively, I think the function needs to take a lvalue
reference to be able to modify the pointer. But, in the following code example both options ( lvalue
reference and rvalue
reference + std::move
) do the same thing (I suppose).
#include <memory>
#include <utility>
struct Widget {
Widget(int) {}
};
void reseat_RValue(std::unique_ptr<Widget>&& uniqPtr) {
auto tempUniqPtr = std::make_unique<Widget>(2003);
uniqPtr = std::move(tempUniqPtr);
}
void reseat_LValue(std::unique_ptr<Widget>& uniqPtr) {
auto tempUniqPtr = std::make_unique<Widget>(2010);
uniqPtr = std::move(tempUniqPtr);
}
int main() {
auto uniqPtr = std::make_unique<Widget>(1998);
reseat_RValue(std::move(uniqPtr)); // (1)
reseat_LValue(uniqPtr); // (2)
}
I don't really understand, what's the difference between (1)
and (2)
or rather which one I should use for my use-case. So maybe someone could explain what is going on with each of this functions and which one I should use.
Yes they do the same thing. From your intent, ie passing-by-reference and modifying it in the function, passing-by-lvalue-reference would be better. It makes your intent more clear (as @bitmask suggested); the function expects an lvalue and would modify it inside the function. And for passing-by-rvalue-reference you need to use std::move
for lvalues to be passed to the function, and it allows to pass rvalues like temporaries which seems meaningless. eg
// meaningless; the temporary `std::unique_ptr` would be destroyed immediately
reseat_RValue(std::make_unique<Widget>(1998));
Actually std::unique_ptr::reset does the same thing for you in function with lvalue reference - it is simpler. IMO, the rvalue semantics fits best for huge objects, not for pointers. But yes, unique_ptr uses it to achieve a sole pointer count.
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