struct Element
{
    int int1;
    int int2;
    int int3;
};

std:vector<Element> elements_to_write;

size_t vec_size = elements_to_write.size();
out.write(reinterpret_cast<char*>(&vec_size), sizeof(vec_size));
for(int i = 0; i < vec_size; i++)
{
    Element& el = elements_to_write[i];
    out.write(reinterpret_cast<char*>(&el.int1), sizeof(el.int1));
    out.write(reinterpret_cast<char*>(&el.int2), sizeof(el.int2));
    out.write(reinterpret_cast<char*>(&el.int3), sizeof(el.int3));
}

std:vector<Element> elements_to_read;
size_t vec_size = 0;
in.read(reinterpret_cast<char*>(&vec_size), sizeof(vec_size));

elements_to_read.reserve(vec_size);

for(int i = 0; i < vec_size; i++)
{
    Element el;
    out.read(reinterpret_cast<char*>(&el.int1), sizeof(el.int1));
    out.read(reinterpret_cast<char*>(&el.int2), sizeof(el.int2));
    out.read(reinterpret_cast<char*>(&el.int3), sizeof(el.int3));
    elements_to_read.push_back(el);
}