How-to write a password-safe class?

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Yes, first define a custom allocator:

template <class T> class SecureAllocator : public std::allocator<T>
{
public:
    template<class U> struct rebind { typedef SecureAllocator<U> other; };

    SecureAllocator() throw() {}
    SecureAllocator(const SecureAllocator&) throw() {}
    template <class U> SecureAllocator(const SecureAllocator<U>&) throw() {}

    void deallocate(pointer p, size_type n)
    {
        std::fill_n((volatile char*)p, n*sizeof(T), 0);
        std::allocator<T>::deallocate(p, n);
    }
};

This allocator zeros the memory before deallocating. Now you typedef:

typedef std::basic_string<char, std::char_traits<char>, SecureAllocator<char>> SecureString;

However there is a small problem, std::string may use small string optimization and store some data inside itself, without dynamic allocation. So you must explicitly clear it on destruction or allocate on the heap with our custom allocator:

int main(int, char**)
{
    using boost::shared_ptr;
    using boost::allocate_shared;
    shared_ptr<SecureString> str = allocate_shared<SecureString>(SecureAllocator<SecureString>(), "aaa");

}

This guarantees that all the data is zeroed before deallocation, including the size of the string, for example.

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