Challenge 2: Reading a Moved-From std::string — Possible Solution
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#include <iostream>
#include <string>

int main() {
    std::string original = "Hello, world!";
    std::string moved_to = std::move(original);

    std::cout << "moved_to: " << moved_to << std::endl;
    std::cout << "original: " << original << std::endl;

    return 0;
}

Typical output (the exact second line is not guaranteed by the
standard):

    moved_to: Hello, world!
    original:

Explanation: moved_to correctly received the string's content, since
std::move selected std::string's own move constructor/assignment,
transferring its internal buffer efficiently. original, however, is
now in std::string's own "valid but unspecified" moved-from state --
in practice, libstdc++/libc++ typically leave it as an empty string
(as shown above), but the C++ standard does NOT guarantee this exact
result; a different standard library implementation could leave it in
some other valid-but-different state.

Why this is a logic bug rather than undefined behavior: per the
chapter, a moved-from object remains genuinely SAFE to use in a
limited way -- it's still a valid, well-formed std::string object,
safe to destroy or reassign a new value to. Reading its CONTENT for any
purpose beyond that, as this program does, isn't memory-unsafe or
UB-triggering the way c1-7's dangling pointer would be -- it simply
reads a real, valid string whose actual content the program has no
right to assume anything specific about. The mistake is purely at the
LOGIC level (assuming original still held "Hello, world!"), not a
memory-safety violation.

WHY THIS WORKS AS AN ANSWER
------------------------------
This shows a realistic (though not standard-guaranteed) output,
explicitly flags that the specific empty-string result is an
implementation detail rather than a guarantee, and correctly
distinguishes the logic-bug classification (reading unspecified but
still valid, memory-safe content) from undefined behavior, exactly the
precise distinction the chapter draws.