无法获取标准向量中元素的指针

Question

I have a problem with std::vector

class myType
{
    int i;
};

int main()
{
    myType a1;
    myType a2;
    std::vector<myType> V;
    V.push_back(a1);
    myType* a1ptr = &V.back();
    V.push_back(a2);
}

it all works fine before I add a2, the ptr is point to some meaningful memory. but as soon as i added a2, the ptr is still point to the same place but with some garbage memory.

how is this happened?

Answer 1

When you add a2 into your vector, the vector must resize. This means new memory is allocated and the old vector copied into this new (larger) memory. The old memory is then deallocated. Your pointer is left dangling into this (old) deallocated memory.

If you construct your vector with a size, ie std::vector V(2), the vector won;t have to resize and your pointer will keep pointing to the value of a1.