push_back()/emplace_back() on array invalidates pointers to existing array items

[samwcoulter]

I am serializing some internal structures to Json and would like to populate an array with null json values, storing a pointer to each null value and then later filling that value with the appropriate data. Eg:

    json arr = json::array();
    arr.emplace_back(nullptr);
    auto* ptr0 = &arr.at(0);
    arr.emplace_back(nullptr);
    auto* ptr1 = &arr.at(1);
    arr.emplace_back(nullptr);
    auto* ptr2 = &arr.at(2);
    //Some time later:
    *ptr0 = 0;
    *ptr1 = 1;
    *ptr2 = 2;

However, this apparently yields an array where the first two elements are null, and only the last element in the array contains the actual data (an integer == 2).

I am not sure if I am simply using the API incorrectly, or there is an actual issue that needs to be addressed. I would very much appreciate any help, thanks!

[nickaein]

By default, JSON library uses std::vector data structure for storing JSON arrays. Therefore calling std::vector::emplace_back() or std::vector::push_back invalidates the iterators (and pointers in your case) when the capacity() is less or equal than size().

You can:

1. Get reference to the items after emplace_back calls are done.
2. Reserve the vector capacity beforehand. Since the JSON library API doesn't expose reserve(), you should first get a pointer/reference to the underlying std::vector and then call reserve() on it. This is discussed at resize(...)? #1157 and json array: initialize with prescribed size and resize method. #1057 for resize() method and you can adapt it for reserve().

[samwcoulter]

The sized constructor from #1057 suited my needs perfectly, thank you.