In C++, what is the purpose of the 'using namespace std;' declaration?

The declaration 'using namespace std;' is pivotal in C++ as it allows the programmer to directly access all the identifiers (such as functions, variables, and classes) defined in the standard namespace without needing to prefix them with 'std::'. For example, if you want to use the 'cout' stream from the standard library, without this declaration, you would need to write 'std::cout'. By including 'using namespace std;', you can simply use 'cout' directly.

This is particularly useful for simplifying code, especially in educational contexts or small projects where the potential for naming conflicts is minimal. It makes the code cleaner and easier to read, as it reduces the repetitiveness of writing 'std::' before every standard library call.

The other options do not accurately reflect the purpose of this declaration. It does not restrict the use of functions, nor does it define a new namespace or include a library; rather, it allows for more accessible use of the existing standard library functions defined in the standard namespace.