CODEWITHSUNDEEP
c++pointers
Pete
Pete

Reputation: 23

How to keep the initial value a pointer was pointing after a variable is changed?

I realize this code is silly, but it is a simplified version of my problem. The code below prints out 10 10's, what I would like it to do is print out the numbers 0 through 9, which were the values j was pointing at when j was added to the vector. How can I keep the pointer stored in the vector pointing to the original integer value it was pointing to when it was stored?

int main()
{
    std::vector<int*> test;
    int *j;
    for (int i = 0; i < 10; i++)
    {
        j = &i;
        test.push_back(j);
    }
    for (int i = 0; i < 10; i++)
    {
        std::cout << *test[i] << std::endl;
    }
        return 0;
}

Upvotes: 0

Views: 203

Answers (3)

bob
bob

Reputation: 41

You can point to elements of another container:

std::vector<int*> test;  // holding pointers
std::array<int, 10> ray; // holding values
std::iota(ray.begin(), ray.end(), 0) // 0, 1, 2, 3..
for (int i = 0; i < 10; i++)
{
    int *j = &ray[i];
    test.push_back(j);
}

Upvotes: 1

Tim Randall
Tim Randall

Reputation: 4145

I'm pretty sure you're looking for this

int main()
{
    std::vector<int> test;
    int *j;
    for (int i = 0; i < 10; i++)
    {
        j = &i;
        test.push_back(*j);
    }
    for (int i = 0; i < 10; i++)
    {
        std::cout << test[i] << std::endl;
    }
        return 0;
}

So.... I've changed 'test' to be a vector of integer values, not pointers, and I've dereferenced the pointer 'j' before storing it in the vector. And I've adjusted the print routine to print the value

Upvotes: 1

scohe001
scohe001

Reputation: 15446

For this simple example pointers aren't even necessary! We can simply do

std::vector<int> test;
for (int i = 0; i < 10; i++)
{
    test.push_back(i);
}

However, if you insist on using pointers like you are...you currently have Undefined Behavior.

After the loop ends, i goes out of scope and your pointers are pointing to memory you no longer own. What you want can be accomplished by using dynamically allocated memory. We can allocate a new integer for each value of the vector:

std::vector<int*> test;
int *j;
for (int i = 0; i < 10; i++)
{
    j = new int(i);
    test.push_back(j);
}

Just remember that you'll have to deallocate the memory yourself after you're done!

for (int i = 0; i < 10; i++)
{
    delete test.at(i);
}

Upvotes: 1

Related Questions