I was looking at the C++ example of RAII on wikipedia, and I came across something that didn't make sense to me.
Here's the code snippet itself, all credit to wikipedia:
#include <string>
#include <mutex>
#include <iostream>
#include <fstream>
#include <stdexcept>
void write_to_file (const std::string & message) {
// mutex to protect file access
static std::mutex mutex;
// lock mutex before accessing file
std::lock_guard<std::mutex> lock(mutex);
// try to open file
std::ofstream file("example.txt");
if (!file.is_open())
throw std::runtime_error("unable to open file");
// write message to file
file << message << std::endl;
// file will be closed 1st when leaving scope (regardless of exception)
// mutex will be unlocked 2nd (from lock destructor) when leaving
// scope (regardless of exception)
}
That comment at the end says: "file will be closed first... mutex will be unlocked second...". I understand the concept of RAII, and I get what the code is doing. However, I don't see what (if anything) guarantees the order which that comment claims.
To end with a question mark: what guarantees that the file is closed before the mutex is unlocked?
what guarantees that the file is closed before the mutex is unlocked?
Because this is how C++ is designed: scoped objects (whatever the scope is: class, function, local block, ...) are destroyed in the reverse order in which they are initialized. There is really nothing more to say, it's just part of the spec.