use of undeclared identifier 'unique_ptr'

In C++, the error message "use of undeclared identifier 'unique_ptr'" typically occurs when the compiler does not recognize the 'unique_ptr' identifier. This error can be fixed by including the appropriate header file and using the correct namespace.

To resolve this issue, follow these steps:

  1. Check if the header file for 'unique_ptr' is included: Make sure you have included the <memory> header file at the beginning of your code. The 'unique_ptr' class is defined in this header file, so including it is necessary to use 'unique_ptr' in your code.

  2. Use the correct namespace: In C++, the 'unique_ptr' class is part of the 'std' namespace. You can either use the std::unique_ptr syntax every time you refer to 'unique_ptr', or you can add a using namespace std; statement at the beginning of your code to avoid using the 'std::' prefix.

Here's an example of how to fix the error:

#include <memory>

int main() {
  std::unique_ptr<int> myPtr(new int); // Correctly using unique_ptr

  // Rest of your code...

  return 0;
}

By including the <memory> header file and using the appropriate namespace, you should be able to resolve the "use of undeclared identifier 'unique_ptr'" error in C++.