v.reserve(10);
std::cout << "After reserve" << std::endl;
std::cout << "Size: " << v.size() << std::endl;

Here you will se that after the reserve, the vector do not contain any
object. So of course you can not access any objects that don't exist.

Phlip wrote:

Thomas wrote:

Does [reserve()] default-construct each element?

No! It only reserves the vector for a number of elements, but do not
create any objects in it. Se the member funtions size() and capacity().

So are these behaviors undefined?

vector<int> v;
v.reserve(1);
v[0]; // <-- ?
int q;
q; // <-- ?

Yes, I think so too. Can't imagine that the vector should hava
different behavior with objects that POD's.

I am fairly certain q causes undefined behavior, because an integer might
contain garbage with an illegal bit pattern. Treating it as an rvalue, even
without using it, is undefined.

If so, is v[0] undefined?

If the vector instead contained objects with important default constructors,
v[0] would be undefined too, right?

Yes! That's true!
Try this:

#include <iostream>
#include <vector>
class foo_t
{
public:
foo_t(void)
{
std::cout << "foo_t(void)" << std::endl;
}
foo_t(const foo_t&)
{
std::cout << "foo_r(const foo_t)" << std::endl;
}
~foo_t(void)
{
std::cout << "~foo_t(void)" << std::endl;
}
};

int main(int /*argc*/, char** /*argv*/)
{

std::vector<foo_t> v;
std::cout << "Before reserve" << std::endl;
v.reserve(10);
std::cout << "After reserve" << std::endl;
std::cout << "Size: " << v.size() << std::endl;
std::cout << "Capacity: " << v.capacity() << std::endl <<
std::endl;;

std::cout << "Starting std::vector<foo_t> v2(10)" << std::endl;
std::vector<foo_t> v2(10);
std::cout << "Ending std::vector<foo_t> v2(10)" << std::endl;
std::cout << "Size(v2): " << v2.size() << std::endl;
std::cout << "Size(v2): " << v2.capacity() << std::endl <<
std::endl;

std::cout << "Ending main" << std::endl;

return 0;

}

Here you will se that after the reserve, the vector do not contain any
object. So of course you can not access any objects that don't exist.

std:vector<> has a constructor that takes an int, an this creates
default constructed objects. It actually creates a, default constructed
object and copies this a number of times as you will se i you run my
example. std::vector<>, also has a constrcutor sthat takes an int and
an object of std:vector<>::vector_type. This creates a vector with a
number of copies of this object.Ex:
std::vector<std::string> v(10, "Hello");
std::cout << v.at(2) << std::endl;//will print "Hello"