CODEWITHSUNDEEP
c++visual-studioc++11move-semanticsmove-constructor
lightrek
lightrek

Reputation: 981

Can move constructor take arguments other than the class itself?

Basically, the argument of the move constructor is the class itself.

However, if I want to construct an object of the class from a lvalue without copy operation can I do like this?

class A{
   A(const LargeDataType& inData):data(inData) {} // constructor 1
   A(LargeDataType&& inData):data(std::move(inData)) {} // constructor 2
   private:
        LargeDataType data;
};

To use it:

Method 1:

LargeDataType outData = 100;
A objA(std::move(outData)); // call constructor 2

Method 2 (If constructor 2 was not implemented):

LargeDataType outData = 100;
A objA(std::move(outData)); // call constructor 1

In this way, there is no copy operation when constructing the objA. My questions are:

  1. Is this legal to create a move constructor like this?

  2. This is more efficient than traditional constructor because no copy needed during objA creation?

  3. Whether method 2 could be better and has the same efficiency as the method 1?

Thanks a lot!

Upvotes: 1

Views: 515

Answers (1)

R Sahu
R Sahu

Reputation: 206717

What you have is not a move constructor. It is just a constructor that takes rvalue references as arguments. The are perfectly fine constructors but they are not move constructors.

From the C++11 Standard (12.8/3):

A non-template constructor for class X is a move constructor if its first parameter is of type X&&, const X&&, volatile X&&, or const volatile X&&, and either there are no other parameters or else all other parameters have default arguments (8.3.6).

Only

A(A&& a) { ... }
A(const A&& a) { ... }
A(volatile A&& a) { ... }
A(volatile const A&& a) { ... }

maybe called move constructors.

If you have parameters with default values in addition to the above, they also qualify as move constructors. E.g.

A(A&& a, T arg = {}) { ... }

Upvotes: 2

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