Switching between std::to_string() and .toString() depending upon template type

I'm currently programming my own PriorityQueue data structure in C++ which I have made into a template class with typename T.

The toString() member function of my class is defined as:

/**
 * @brief Gives a std::string representation of this queue structure
 *
 * The priority queue is returned as a string in the following format:
 *
 * \code{.cpp}
 * Data Item        Priority
 * [Item]           [P1]
 * [Item]           [P2]
 * [Item]           [P3]
 * \endcode
 *
 * where P1 < P2 < P3.
 *
 * @return String representation of this priority queue
 */
std::string toString() const {

    std::string tempString = "";

    // initialise temporary node to front of priority queue
    PQNode<T>* tempNode = front;

    // append string with headers
    tempString += "Data Item \t\t Priority\n";

    // while tempNode is not null, continue appending queue items to string
    while (tempNode != nullptr) {

        tempString += std::to_string(tempNode->head) + "\t\t\t " + std::to_string(tempNode->priority) + "\n";

        // shift tempNode to tail of current tempNode (moving along queue)
        tempNode = tempNode->tail;

    }

    return tempString;

}

How could I code this such that std::to_string() is used within this method if the type of the template is a primitive such as an int or double and .toString() (calling the toString method of the passed class) is called if the type of the template is a class with its own toString member function?

I presume there is a way to do it with #ifndef clauses however I don't have much experience using these.