C++ vector push_back()

Question

I just saw in a tutorial someone who used in the same file both:

myVector.back().push_back();
myVector.push_back();

What's the difference?

Answer 1

The first had to be something like

vector<vector<T>>

Otherwise it would not work. back() returns the element at the back of the vector. When you say

myVector.back().push_back();

it would be accessing the last vector<T>, then calling push_back() on that inner vector

If it is the case that myVector is a vector<vector<T>>, then

myVector.push_back();

would be pushing back an empty vector<T> whereas

myVector.back().push_back();

would be pushing back a default T onto the last vector<T> in myVector.

• user3728078: In my case myVector is a vector containing a class
• Cory Kramer: In that case, back() would return an instance of your class that was the last element in the vector. So unless your class has a push_back() method, then the first version will not compile.
• user3728078: So if I got it right, back() points the last item in the vector and then the push_back() adds something to the end of this item.

Answer 2

myVector may be a std::vector<std::vector<T>>