How to cleanse (overwrite with random bytes) std::string internal buffer?

Question

Consider a scenario, where std::string is used to store a secret. Once it is consumed and is no longer needed, it would be good to cleanse it, i.e overwrit

Answer 1

It is probably safe. But not guaranteed.

However, since C++11, a std::string must be implemented as contiguous data so you can safely access its internal array using the address of its first element &secretString[0].

if(!secretString.empty()) // avoid UB
{
    char* modifiable = &secretString[0];
    OpenSSL_cleanse(modifiable, secretString.size());
}

Answer 2

The standard explicitly says you must not write to the const char* returned by data(), so don't do that.

There are perfectly safe ways to get a modifiable pointer instead:

if (secretString.size())
  OpenSSL_cleanse(&secretString.front(), secretString.size());

Or if the string might have been shrunk already and you want to ensure its entire capacity is wiped:

if (secretString.capacity()) {
  secretString.resize(secretString.capacity());
  OpenSSL_cleanse(&secretString.front(), secretString.size());
}