How to calculate a hash of const char* with given size and content examination by using standard tools

I have a sub_string struct and I would like to use standard tools to calculate a hash of it:

struct sub_string
{
    const char* ptr;
    size_t size;
};

Here are some restrictions:

• It has to be done by standard tools provided by c++11.
• const char* doesn't have to be NUL-terminated.
• I don't want to use std::hash<const char*> specialization, since it computes a hash based on a pointer without content examination.
• I don't want to create a new std::string{ptr, size} just to compute a hash.
• I would like to have a sub_string hash value to be the same value as std::string{ptr, size} hash.
• I don't want to touch implementation details of std::string.

How to do it?