# Memory container

A major concern with mixing modern multi-user OSes and cryptographic code is that at any time the code (including secret keys) could be swapped to disk, where it can later be read by an attacker, or left floating around in memory for later retrieval.

For this reason the library uses a `std::vector` with a custom allocator that will zero memory before deallocation, named via typedef as `secure_vector`. Because it is simply a STL vector with a custom allocator, it has an identical API to the `std::vector` you know and love.

Some operating systems offer the ability to lock memory into RAM, preventing swapping from occurring. Typically this operation is restricted to privileged users (root or admin), however some OSes including Linux and FreeBSD allow normal users to lock a small amount of memory. On these systems, allocations first attempt to allocate out of this small locked pool, and then if that fails will fall back to normal heap allocations.

The `secure_vector` template is only meant for primitive data types (bytes or ints): if you want a container of higher level Botan objects, you can just use a `std::vector`, since these objects know how to clear themselves when they are destroyed. You cannot, however, have a `std::vector` (or any other container) of `Pipe` objects or filters, because these types have pointers to other filters, and implementing copy constructors for these types would be both hard and quite expensive (vectors of pointers to such objects is fine, though).