如何将带有指针的结构插入 map？

Question

I'm trying to insert a C struct from a third-pary library into a map. The problem is, the struct contains a pointer to other structs as a member field and when inserting this into my_map, the S2.arr field seems to be getting over written and I'm not sure why.

I've been able to reproduce this have have it running here.

http://coliru.stacked-crooked.com/a/7c47a5c7c7294a49

I know the shared_ptr's may seem to confuse things, but I've left my main containers in place as it's how my larger app is accessing things. Here's what I've pieced together to reproduce the problem I'm having.

typedef struct {
    int index;
    int value;
} S1;

typedef struct {
    int index;
    int num;
    S1 *arr;
} S2;

// populate new_vec with S1 if idx equals S1.index.
void find(int idx, 
    std::shared_ptr<std::vector<S1>> vec, 
    std::shared_ptr<std::vector<S1>> new_vec)
{
    for (const auto& s1 : *vec) {
        if (idx == s1.index) {
            new_vec->push_back(s1);
        }
    }
}

int main()
{
    bool found = false;

    std::shared_ptr<std::vector<S1>> array;
    std::shared_ptr<std::map<int, S2>> my_map = std::make_shared<std::map<int, S2>>();
    std::shared_ptr<std::vector<S1>> vec = std::make_shared<std::vector<S1>>();
    std::shared_ptr<std::vector<S1>> new_vec = std::make_shared<std::vector<S1>>();

    // initialize vec
    for (int i = 0; i < 3; ++i)
        vec->push_back(S1 { 1, i});

    for (int i = 0; i < 4; ++i) 
        vec->push_back(S1 { 2, i});

    for (const auto& s1 : *vec) {
        found = false;

        array = std::make_shared<std::vector<S1>>();

        // populate array with S1.indices that equal 1.
        find(s1.index, vec, array);

        my_map->insert(std::pair<int, S2>(s1.index, 
            S2 {
                s1.index,
                static_cast<int>(array->size()), 
                std::move(array->data())
            } 
        ));
    }

    for (const auto& [key, val] : *my_map) {
        std::cout << "key: " << key << std::endl;

        for (int i = 0; i < val.num; ++i) {
            S1 s = val.arr[i];
            std::cout << "  index: " << s.index << ", val: " << s.value << std::endl;
        }
    }
}

My intention is to have this print the following:

key: 1
  index: 1, val: 0
  index: 1, val: 1
  index: 1, val: 2
key: 2
  index: 2, val: 0
  index: 2, val: 1
  index: 2, val: 2
  index: 2, val: 3

Yet what I'm getting is this:

key: 1
  index: 2, val: 0
  index: 2, val: 1
  index: 2, val: 2
key: 2
  index: 2, val: 0
  index: 2, val: 1
  index: 2, val: 2
  index: 2, val: 3

Update: Made some edits as maps only contain unique keys.

Answer 1

array = std::make_shared<std::vector<S1>>();

On each iteration of the for loop that has this statement, this overwrites the previous value of array that was created on the last iteration of this for loop.

Since this is the only and the last smart pointer reference to the vector, the previous array always gets destroyed. On each iteration of the for-loop, because this std::shared_ptr does not wind up anywhere else, the vector created on the previous iteration of the loop will go to the big bit bucket in the sky.

But before this happens, with each reincarnation of std::vector , later in this for loop we have:

 std::move(array->data())

Two problems here:

1) This std::move accomplishes absolutely nothing useful whatsoever, here. But, more importantly:

2) As you know, data() returns an internal pointer to the vector's underlying storage. But on the next iteration of the loop, this array gets destroyed, as I explained above, leaving a dangling pointer to the contents of the destroyed vector, behind.

As such, subsequent usage of the pointer becomes undefined behavior. With or without std::move (which means nothing here).

To fix this, you need to take into greater consideration what smart pointers are; and how they work; what std::vector is, what its data() method returns; object lifetime and scoping in C++; then combine this together in a way that makes it work for your program correctly.