Explain C++ code

What this C++ sample shows

templates

largest is written once for any T that can be copied and compared with >. largest(std::vector<int>{3, 9, 4}) makes the compiler generate an int version, which returns 9.

RAII

Document prints closing in its destructor. The destructor runs on its own when the unique_ptr that owns the document is destroyed at the end of main, so cleanup can't be forgotten.

move semantics

std::move(open.front()) hands ownership to current. The pointer left in the vector is now null, so the last line prints true notes.txt.

pointers

std::unique_ptr is a pointer that owns its object: one owner at a time, deleted automatically, and it can be moved but not copied.

Good to know

• items.at(0) throws std::out_of_range on an empty vector, where items[0] would be undefined behavior.
• explicit stops a plain std::string from turning into a Document by accident, for example when passed to a function that takes a Document.
• The vector still holds one element after the move. It is an empty unique_ptr, and dereferencing it is undefined behavior.

Watch out: closing notes.txt prints once, after everything else. The document is destroyed exactly once, by whichever unique_ptr owns it when main ends, here current.