STL vector reserve() and copy()

I am trying to perform a copy from one vector (vec1) to another vector (vec2) using the following 2 abbreviated lines of code (full test app follows):

vec2.reserve( vec1.size() );
copy(vec1.begin(), vec1.end(), vec2.begin());

While the call to vec2 sets the capacity of vector vec2, the copying of data to vec2 seems to not fill in the values from vec1 to vec2.

Replacing the copy() function with calls to push_back() works as expected.

What am I missing here?

Thanks for your help. vectest.cpp test program followed by resulting output follows.

Compiler: gcc 3.4.4 on cygwin.

Nat

/**
 * vectest.cpp
 */

#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<int> vec1;
    vector<int> vec2;

    vec1.push_back(1);
    vec1.push_back(2);
    vec1.push_back(3);
    vec1.push_back(4);
    vec1.push_back(5);
    vec1.push_back(6);
    vec1.push_back(7);

    vec2.reserve( vec1.size() );
    copy(vec1.begin(), vec1.end(), vec2.begin());

    cout << "vec1.size()     = " << vec1.size() << endl;
    cout << "vec1.capacity() = " << vec1.capacity() << endl;

    cout << "vec1: ";
    for( vector<int>::const_iterator iter = vec1.begin(); iter < vec1.end(); ++iter ) {
        cout << *iter << " ";
    }
    cout << endl;

    cout << "vec2.size()     = " << vec2.size() << endl;
    cout << "vec2.capacity() = " << vec2.capacity() << endl;
    cout << "vec2: ";
    for( vector<int>::const_iterator iter = vec2.begin(); iter < vec2.end(); ++iter ) {
        cout << *iter << endl;
    }

    cout << endl;
}

Output:

vec1.size()     = 7
vec1.capacity() = 8
vec1: 1 2 3 4 5 6 7 
vec2.size()     = 0
vec2.capacity() = 7
vec2:

Solution

As noted in other answers and comments, you should just use vector's built-in functionality for this. But:

When you reserve() elements, the vector will allocate enough space for (at least?) that many elements. The elements do not exist in the vector, but the memory is ready to be used. This will then possibly speed up push_back() because the memory is already allocated.

When you resize() the vector, it will allocate enough space for those elements, but also add them to the vector.

So if you resize a vector to 100, you can access elements 0 - 99, but if you reserve 100 elements, they are not inserted yet, just ready to be used.

What you want is something like this:

vec2.reserve( vec1.size() );
copy(vec1.begin(), vec1.end(), std::back_inserter(vec2));

std::back_inserter is defined in <iterator>