Why is this an issue?
Rvalue reference arguments allow to transfer ownership of objects.
When a function has an argument of type rvalue reference, it is expected to call std::move to take ownership of the corresponding
parameter when it is called.
If it does not do it or if it does it conditionally, ownership is unclear and this might lead to bugs.
This rule does not apply when the argument is a forwarding reference.
Noncompliant code example
enum class Shape {
empty, circle, square
};
class DrawingStore {
std::vector<Shape> store;
public:
void insertShape(Shape &&shape) {
store.emplace_back(shape); // Noncompliant, call to std::move is expected
}
void insertIfCircle(Shape &&shape) {
if (shape == Shape::circle) {
store.emplace_back(std::move(shape)); // Noncompliant, std::move is not always called, ownership of shape is not clear
}
}
};
Compliant solution
enum class Shape {
empty, circle, square
};
class DrawingStoreOk {
std::vector<Shape> store;
public:
void insertShape(Shape &&shape) {
store.emplace_back(std::move(shape));
}
void insertIfCircle(const Shape &shape) {
if (shape == Shape::circle) {
store.emplace_back(shape);
}
}
};
Resources
