resize versus push_back in std::vector : does it avoid an unnecessary copy assignment?

Question

When invoking the method push_back from std::vector, its size is incremented by one, implying in the creation of a new instance, and then the parameter you pass will be copied into this recently created element, right? Example:

myVector.push_back(MyVectorElement()); 

Well then, if I want to increase the size of the vector with an element simply using its default values, wouldn't it be better to use the resize method instead? I mean like this:

myVector.resize(myVector.size() + 1); 

As far as I can see, this would accomplish exactly the same thing but would avoid the totally unnecessary assignment copy of the attributes of the element.

Is this reasoning correct or am I missing something?

Answer 1

At least with GCC, it doesn't matter which you use (Results below). However, if you get to the point where you are having to worry about it, you should be using pointers or (even better) some form of smart pointers.. I would of course recommend the ones in the boost library.

If you wanted to know which was better to use in practice, I would suggest either push_back or reserve as resize will resize the vector every time it is called unless it is the same size as the requested size. push_back and reserve will only resize the vector if needed. This is a good thing as if you want to resize the vector to size+1, it may already be at size+20, so calling resize would not provide any benefit.

Test Code

#include <iostream> #include <vector>  class Elem{     public:         Elem(){             std::cout << "Construct\n";         }         Elem(const Elem& e){             std::cout << "Copy\n";         }         ~Elem(){             std::cout << "Destruct\n";         }    };   int main(int argc, char* argv[]){     {         std::cout << "1\n";         std::vector<Elem> v;         v.push_back(Elem());     }      {         std::cout << "\n2\n";         std::vector<Elem> v;         v.resize(v.size()+1);     } } 

Test Output

1 Construct Copy Destruct Destruct  2 Construct Copy Destruct Destruct 

Answer 2

I find myVector.push_back(MyVectorElement()); much more direct and easier to read.

The thing is, resize doesn't just resize the array and default-construct elements on those places; that's just what it defaults to. It actually takes a second parameter which is what each new element will be made a copy of, and this defaults to T(). In essence, your two code samples are exactly the same.