Fast Buffer Access

#include <iostream>
#include <string>

// Forward declaration
class SPIBus;

class DisplayDriver {
public:
    // TODO: Implement fastWrite
    // It takes SPIBus& bus, int v1, int v2
    // It writes v1 to bus.buffer[0] and v2 to bus.buffer[1]
    void fastWrite(SPIBus& bus, int v1, int v2);
};

class SPIBus {
private:
    int buffer[2];

    // TODO: Declare DisplayDriver as a friend class
    // friend class ...
    friend DisplayDriver;

public:
    SPIBus() {
        buffer[0] = 0;
        buffer[1] = 0;
    }

    void dump() {
        std::cout << "Buffer: [ " << buffer[0] << " " << buffer[1] << " ]" << std::endl;
    }
};

// Implementation of fastWrite (defined after SPIBus so it sees the 'buffer' member)
void DisplayDriver::fastWrite(SPIBus& bus, int v1, int v2) {
    // TODO: Write to private member 'buffer'
    // bus.buffer[0] = v1;
    bus.buffer[0] = v1;
    bus.buffer[1] = v2;
}

int main() {
    SPIBus bus;
    DisplayDriver driver;
    int N;
    if (!(std::cin >> N)) return 0;

    for (int i = 0; i < N; ++i) {
        std::string cmd;
        std::cin >> cmd;

        if (cmd == "WRITE") {
            int v1, v2;
            std::cin >> v1 >> v2;
            driver.fastWrite(bus, v1, v2);
        } else if (cmd == "SHOW") {
            bus.dump();
        }
    }
    return 0;
}

Solving Approach