Question

I just noticed that for vector push_back it is push back a reference to the element.

void push_back ( const T& x );

My question is does the memory layout changed after push_back?

For example, I have an array first which containing five elements and the layout is like this.

|          |          |          |          |          |              
|   A1     |     A2   |   A3     |    A4    |    A5    |

Now I have a vector v

v.push_back(A3)

Now, how does the memory look like?

How does the vector store the elements here?

How does the vector access the element?

Answer 1

A vector stores by value not by reference.

When you re-add the same element, a copy will be stored at the end. If you do not want to make a copy of the values you are inserting into the vector, then you should use pointers instead.

Example:

std::vector<std::string> v;
string s = "";
v.push_back(s);
s = "hi";
v.push_back(s);

v now contains 2 different elements, one with an empty string, and one with a string which contains "hi". Both strings in the vector remain independent from s.

Note: The internal implementation details of an STL container can vary, there is no guarantee that it will be implemented a certain way; however, the semantics of how an STL container works, will remain the same no matter what the internal implementation is.