# c++ – How do I write a 2D vector to a binary file?

## Question:

Suppose I have some kind of 2D vector:

``````std::vector<std::vector<int>> main_vector;
std::vector<int> temp_vector;

for (std::size_t i = 0; i < 5; i++) temp_vector.push_back(i);
for (std::size_t i = 0; i < 5; i++) main_vector.push_back(temp_vector);
``````

How do I write it to a binary file? Accordingly, the dimension is unknown. I know that it won't work in a standard way as with arrays.

``````bin_file.open("test.txt", std::ios::out | std::ios::binary);
bin_file.write((char *)&main_vector, sizeof(main_vector)); //не сработает
``````

For greater generalization, we will assume that vectors inside a vector can have different sizes.

Then approximately (I don't compile – just a sketch!)

``````// Пишем размер внешнего вектора
size_t sz = main_vector.size();
stream.write(&sz,sizeof(sz));

for(size_t i = 0; i < sz; ++i)
{
// То же самое для каждого внутреннего:
vector<int>& v = main_vector[i];
size_t sz = v.size();
stream.write(&sz,sizeof(sz));
// Пишем данные
for(size_t j = 0; j < sz; ++j)
{
int n = v[j];
stream.write(&n,sizeof(n));
}
// Или, поскольку в векторе данные лежат подряд -
// просто stream.write(v.data(), sizeof(int)*sz);
}
``````

Read – in reverse order. First – the total number of vectors inside the main one, then a loop through them – reading the number of int'ov and reading and inserting the corresponding number of values ​​(or, again, creating a vector with a known number of elements and reading directly into the internal buffer …)

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