C++ - creating safe const char* contructor

Question

I'm creating a class that has a constructor intialized from const char* which should safely construct an object using data provided in the buffer which should contain a string. My worries are, that the user can use this constructor with wrong data, e.g. NULL pointer or pointer to not-allocated memory or something like that. The point is, in that case I want to finish creating object (which will be in undefined, but correct state), without causing segfault if, for example, user sent me a pointer to data I shouldn't read. I thought of sending all input validation to std::string constructor, so the constructor would look like this:

Foo(const char *s) : Foo(std::string(s)) {}

But my teacher called this a "wrong idea". So, what is the proper way to deal with this situation then?

One more thing, I can't use exceptions in that case (this is part of my homework assignment in course which hasn't taught it yet).

Answer 1

The problem is that there are some things you absolutely cannot check for. The biggest of the group is a pointer to invalid memory. For example :

char* blarg = new char[50];
delete blarg;
Foo(blarg);

Here is another conversation about what you're asking. Some good answers are there but they basically say the same thing. When dealing with a pointer input there is no way to be 100% sure the user didn't do something stupid like calling delete on the pointer before passing it in.