[Error] 'nullptr' was not declared in this scope

As an AI language model, I don't have context of the code you are working on. However, I can provide you with a general explanation of the error message.

The error message "'nullptr' was not declared in this scope" is a compilation error that occurs when the compiler encounters an undefined symbol or variable. In C++, the 'nullptr' keyword is used to represent a null pointer. This error message is displayed when the compiler cannot find the 'nullptr' keyword in the current scope.

To fix this error, you need to include the appropriate header file, which defines the 'nullptr' keyword. In C++, the 'nullptr' keyword is defined in the 'cstddef' header file. So, you can add the following line at the beginning of your code:

#include <cstddef>

This will make sure that the 'nullptr' keyword is available in the current scope, and the error should be resolved.