vector<unique_ptr<T>> takes more than three times as much memory as vector<T> in u-CodePudding

As far as I understand unique_ptr<T> is not supposed to have such a huge overhead.

What do I wrong?

size_t t = sizeof(DataHelper::SEQ_DATA); // t = 12

std::vector<std::vector<std::unique_ptr<DataHelper::SEQ_DATA>>> d(SEQ_00_SIZE   1); // SEQ_00_SIZE = 4540
for (unsigned int i = 0; i < d.size();   i) {
    for (unsigned int k = 0; k < 124668;   k) {
        std::unique_ptr<DataHelper::SEQ_DATA> sd = std::make_unique<DataHelper::SEQ_DATA>();
        d[i].push_back(std::move(sd));
    }
}

takes about ~21GB of ram.

std::vector<std::vector<DataHelper::SEQ_DATA>> d(SEQ_00_SIZE   1);
for (unsigned int i = 0; i < d.size();   i) {
    for (unsigned int k = 0; k < 124668;   k) {
        DataHelper::SEQ_DATA sd;
        d[i].push_back(sd);
    }
}

takes about ~6,5GB of ram.

Additional information:

struct SEQ_DATA {
    uint16_t id = 0;
    uint16_t label = 0;
    float intensity = 0.0f;
    float z = 0.0f;
};

I just want to have a single vector<vector<T>> which holds my 4540 * 124668 objects as efficient as possible. I read values from binary files. Since the number of elements within the binary files varies, I cannot initialize the inner vector with the correct number (i.e. 124668 is only true for the first file).

gcc 9.3.0, c 17

CodePudding user response：

"std::unique_ptr doesn't have huge overhead" means that it doesn't have huge overhead compared to a bare pointer to dynamic allocation:

{
    auto ptr = std::make_unique<T>();
}
// has comparable cost to, and has exception safety unlike:
{
    T* ptr = new T();
    delete ptr;
}

std::unique_ptr doesn't make the cost of dynamic allocation cheaper.

I just want to have a single vector<vector<T>> which holds my 4540 * 124668 objects as efficient as possible.

The most efficient way to store 4540 * 124668 objects is a flat array:

std::vector<DataHelper::SEQ_DATA> d(4540 * 124668);

However, the benefit of this isn't necessarily significant given that the inner vectors aren't very small.

(i.e. 124668 is only true for the first file).

If you don't need all 124668 elements, then it may be a waste of memory to have the unused elements in the vector.