CODEWITHSUNDEEP
c++pointersmove
mcAngular2
mcAngular2

Reputation: 309

When do I need move-operations , c++

Imagine the following code

 struct Vehicle { virtual void drive() = 0; };
 struct Car: public Vehicle { 
   void drive() override { std::cout <<   "driving a car\r\n"; } 
 };
 struct Cycle : public Vehicle { 
   void drive() override { std::cout << "driving a cycle\r\n"; } 
 };
 Vehicle* makeVehicle(const std::string& type) {
   if (type == "car") { return new Car(); } 
   else if (type == "cycle") { return new Cycle(); } 
   else { return nullptr; } 
 }

Do I need do implement moving-operations concerning "return new Car()"? Or are they senseless because of the use of pointers? In which cases are the default moving-operation sufficient?

Cheers!

Upvotes: 0

Views: 128

Answers (1)

Nir Friedman
Nir Friedman

Reputation: 17704

If you have move, you have C++11, and if you have C++11, you have unique_ptr. So you actually want to write your code like this:

std::unique_ptr<Vehicle> makeVehicle(const std::string& type {
    if (type == "car") { return std::make_unique<Car>(); }
    ...
}

unique_ptr indicates that the person calling makeVehicle now owns the returned object and will automatically ensure that it gets destroyed and memory returned. unique_ptr implements move operations automatically, that's why this code can compile (if it wasn't movable or copyable, it wouldn't be possible to return it from a function).

So basically, for polymorphic types, typically you will not need to implement moving operations. But unique_ptr doesn't provide copying operations so you still may want to implement the oldie-but-goldie clone function (if you really need it).

I recommend reading more about move semantics and in particular the "Rule of Zero"; it's generally better to avoid implementing copy/move by hand where possible. See: http://en.cppreference.com/w/cpp/language/rule_of_three.

Upvotes: 3

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