Moving, Not Copying, Elements during Reallocation

Before we write the reallocate member, we should think a bit about what it must do. This function will

• Allocate memory for a new, larger array of strings

• Construct the first part of that space to hold the existing elements

• Destroy the elements in the existing memory and deallocate that memory

Looking at this list of steps, we can see that reallocating a StrVec entails copying each string from the old StrVec memory to the new. Although we don’t know the details of how string is implemented, we do know that strings have valuelike behavior. When we copy a string, the new string and the original string are independent from each other. Changes made to the original don’t affect the copy, and vice versa.

Because strings act like values, we can conclude that each string must have its own copy of the characters that make up that string. Copying a string must allocate memory for those characters, and destroying a string must free the memory used by that string.

Copying a string copies the data because ordinarily after we copy a string, there are two users of that string. However, when reallocate copies the strings in a StrVec, there will be only one user of these strings after the copy. As soon as we copy the elements from the old space to the new, we will immediately destroy the original strings.

Copying the data in these strings is unnecessary. Our StrVec’s performance will be much better if we can avoid the overhead of allocating and deallocating the strings themselves each time we reallocate.