How to copy a class.

What is shallow copy?

When we copying class type, the default behavior is to copy the data members. When the data members contain data object like pointer to a array, we only copy the value of pointer(which is the address of the array) instead of the array itself. This behavior is so called shallow copy.

copy constructor vs copy assignment

When write copy constructor we have to:

Array(Array const& other)

• make sure to pass the reference and add const to it.
• allocate a new memory for the new object we create.
• copy the value one by one.

When write copy assignment we have to deal with the extra things include:

• destory the previous resource inside the object.
• return the reference to the current assigned object.

best pratice for writing copy assginment is to reuse the copy constructor and std::swap as follow:

Array& operator=(Array const& other)
{
// reuse copy constructor
Array copy { other };

// swap the members
std::swap(size, copy.size);
std::swap(data, copy.data);

// copy is destroyed which releases the previous data
return *this;
}

Move statement and where to use it

1 Array modify(Array array)
2 {
3 ++array[0];
4 return array;
5 }
6
7 int main()
8 {
9 Array a { create(1, 2, 3) };
10 Array b { modify(a) };
11 }

The 10th line of the code above call the copy constructor(which needs to allocate the dynamic memmory) a lot:

• When calling modify:
• 1. we have to copy a into parameter array
• When function modify return:
• 2. the local variable array will be destroyed so we have to copy array into a temporary object in main
• 3. To construct b we use the copy constructor again

We can modify the second steps here. Instead of copy, we can use move here where we pass rvalue reference as function parameter. Move will occur aotomatically whenever we are trying to copy an rvalue.

Here, rvalue identity can become a xvalue by binded to a rvalue reference.

relative material

• slide in tddd38:Fundamentals III