c++ array declaration in a header

class Test {
int a[];
public:
Test(int size);
};

Test::Test(int size) {
a[size];   // this is wrong, but what can i do here?
}

class Test {
    int *a;
public:
    Test(int size) {
       a = new int[size];
    }
    ~Test() { delete [] a; }
private:
    Test(const Test& other);
    Test& operator=(const Test& other);
};

class Test
{
    std::vector<int> a;
    public:
        Test(std::size_t size):
            a(size)
        {}
};

template <int N> class Test {
    int a[N];
public:
    Test() { }
};

Test<5> test;
Test<40> biggertest;

std::vector<char> vec(10);
memset(&vec[0], 0, vec.size());