error: ‘boost::mutex’ has not been declared

This error occurs when the compiler is unable to find the declaration for the Boost mutex class. The Boost library provides a set of C++ libraries that extend the functionality of the language. The mutex class is used for thread synchronization and is part of the Boost.Thread library.

To fix this error, make sure that you have included the necessary header files in your code. You need to include the Boost.Thread library header file, which contains the declaration for the mutex class.

The header file you need to include is:

#include <boost/thread.hpp>

Make sure that you have installed the Boost library on your system and that the library is properly linked to your project. You may also need to specify the Boost library path in your project settings.