CODEWITHSUNDEEP
c++c++11vectorresize
Merni
Merni

Reputation: 2912

Which is most costly in C++, remove last element or resize of a vector?

Which operation is most costly in C++?
1. Resize of a vector (decrease size by 1)
2. Remove last element in vector

Upvotes: 1

Views: 1313

Answers (2)

Vlad from Moscow
Vlad from Moscow

Reputation: 310960

In my opinion they are equivalent. The both operations remove the last element and decrease the size.:)

According to the C++ Standard

void resize(size_type sz); 12 Effects: If sz <= size(), equivalent to calling pop_back() size() - sz times

So they are simply equivalent according to my opinion and the point of view of the Standard.:)

Also if to consider member function erase instead of pop_back (in fact they do the same in this case) then according to the same Standard

4 Complexity: The destructor of T is called the number of times equal to the number of the elements erased, but the move assignment operator of T is called the number of times equal to the number of elements in the vector after the erased elements.

As there are no move operations for the last element then the cost is the same.

Upvotes: 0

TemplateRex
TemplateRex

Reputation: 70526

From http://en.cppreference.com/w/cpp/container/vector which essentially quotes the Standard:

void pop_back();

Removes the last element of the container. No iterators or references except for back() and end() are invalidated.

void resize( size_type count );

Resizes the container to contain count elements. If the current size is greater than count, the container is reduced to its first count elements as if by repeatedly calling pop_back().

So in this case, calling resize(size() - 1) should be equivalent to calling pop_back(). However, calling pop_back() is the right thing to do as it expresses your intent.

NOTE: the answer is reflecting the changed interface of C++11's std::vector::resize(), which used to contain a hidden default argument which was being copied around (and which may or may not have been optimized away).

Upvotes: 11

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