In C++, the noexcept keyword is used to declare that a function is guaranteed not to throw exceptions. This guarantee allows the compiler to optimize code more effectively and provides clearer expectations about a function's behavior, especially in contexts like move operations within standard library containers.

When applying noexcept, it's important to differentiate between exceptions that occur within the function body and those that happen during parameter passing. The noexcept specification only applies to the function's internal operations. For instance, in the provided code, the constructor of B is marked as noexcept, which is safe because any potential exceptions from constructing the default parameter A(true) occur before the constructor body is executed and do not violate the noexcept guarantee of B's constructor itself.

Best practices for using noexcept include ensuring that all operations within a function are exception-free before marking it as noexcept. This is particularly recommended for move constructors and move assignment operators to enhance the performance of standard library containers like std::vector. Additionally, using conditional noexcept in templates can provide flexibility by allowing exception guarantees to depend on template parameters. However, incorrectly marking a function that might throw exceptions as noexcept can lead to unexpected program termination through std::terminate, so careful consideration and thorough testing are essential.