> For the complete documentation index, see [llms.txt](https://ref.gitbook.io/notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ref.gitbook.io/notes/c-c++/untitled.md).

# Containers

The `std::list<std::array<unsigned char, 69>>` is how you do it. Previously, I did it wrong by using `std::vector` instead of `std::list`. The problem with `vector` is that it loses the abstraction. Yes, a vector of array works, but the layout is all flat and you cannot use `push_back` this command cannot save the array into the vector. The only that works is `std::memcpy` like i said, all is flat.

Whereas, the `std::list` or list of array works as I needed. Each array is an individual element of the list. This is not the case with vector!

Here is the snippet that works:

```cpp
void list_of_array()
{
    unsigned char *buf;
    int n;

    unsigned char key1[32];
    unsigned char key2[32];
    unsigned char key3[32];

    randombytes_buf(key1, 32);
    randombytes_buf(key2, 32);
    randombytes_buf(key3, 32);

    int vcount = 3;
    unsigned char *verification_keys = new unsigned char[crypto_sign_PUBLICKEYBYTES*3];
    
    std::memcpy(
        verification_keys, 
        key1, 
        crypto_sign_PUBLICKEYBYTES);

    std::memcpy(
        verification_keys + crypto_sign_PUBLICKEYBYTES, 
        key2, 
        crypto_sign_PUBLICKEYBYTES);

    std::memcpy(
        verification_keys + crypto_sign_PUBLICKEYBYTES*2, 
        key3, 
        crypto_sign_PUBLICKEYBYTES);

    std::list<std::array<unsigned char, crypto_sign_PUBLICKEYBYTES>> trustedkeys;
    std::array<unsigned char, crypto_sign_PUBLICKEYBYTES> public_key;

    unsigned char *VKEYS = new unsigned char[crypto_sign_PUBLICKEYBYTES*3];

    for (int i = 0; i < vcount; i++) {
        // copy a C array slice to C++ std::array
        std::copy(
            verification_keys + i * crypto_sign_PUBLICKEYBYTES,
            verification_keys + i * crypto_sign_PUBLICKEYBYTES + crypto_sign_PUBLICKEYBYTES ,
            std::begin(public_key));

        buf = public_key.data();
        n = public_key.size();

        std::memcpy(VKEYS + i * crypto_sign_PUBLICKEYBYTES, buf, n);
        trustedkeys.push_back(public_key);
    }

    for (auto &key : trustedkeys) {
        buf = key.data(); 
    }

    delete[] verification_keys;
    delete[] VKEYS;
}
```

**Create a list of jagged array in C++:**

```cpp
int qlen;
unsigned char* buffe;
std::list<std::vector<unsigned char>> LIST;
std::vector<unsigned char> ELEM;
unsigned char elem1[64];
unsigned char elem2[60];
unsigned char elem3[69];

randombytes_buf(elem1, sizeof elem1);
randombytes_buf(elem2, sizeof elem2);
randombytes_buf(elem3, sizeof elem3);

ELEM.insert(ELEM.end(), &elem1[0], &elem1[64]);
LIST.push_back(ELEM);

ELEM.clear(); // needed

ELEM.insert(ELEM.end(), elem2, elem2 + sizeof elem2);
LIST.push_back(ELEM);

ELEM.clear(); // needed

ELEM.insert(ELEM.end(), &elem3[0], &elem3[69]);
LIST.push_back(ELEM);

for (auto& elem : LIST) {
    qlen = elem.size();
    buffe = elem.data();
    std::cout << qlen << std::endl;
}
```


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