Why does c++ initialise a std::vector with zeros, but not a std::array?

Question

Isn't it a waste of time to initialize a vector with zeros, when you don't want it?

I try this code:

#include <iostream>
#include <vector>
#include <array>

#define SIZE 10

int main()
{
#ifdef VECTOR

  std::vector<unsigned> arr(SIZE);

#else

  std::array<unsigned, SIZE> arr;

#endif // VECTOR

  for (unsigned n : arr)
    printf("%i ", n);
  printf("\n");

  return 0;
}

and I get the output:

with vector

$ g++ -std=c++11 -D VECTOR test.cpp -o test && ./test 
0 0 0 0 0 0 0 0 0 0 

with an array

g++ -std=c++11  test.cpp -o test && ./test 
-129655920 32766 4196167 0 2 0 4196349 0 1136 0 

And I also try with clang++

So why zeros? And by the way, could I declare a vector without initializing it?

Answer 1

The more common way to declare a vector is without specifying the size:

std::vector<unsigned> arr;

This doesn't allocate any space for the vector contents, and doesn't have any initialization overhead. Elements are usually added dynamically with methods like .push_back(). If you want to allocate memory you can use reserve():

arr.reserve(SIZE);

This doesn't initialize the added elements, they're not included in the size() of the vector, and trying to read them is undefined behavior. Compare this with

arr.resize(SIZE);

which grows the vector and initializes all the new elements.

std::array, on the other hand, always allocates the memory. It implements most of the same behaviors as C-style arrays, except for the automatic decay to a pointer. This includes not initializing the elements by default.