Nrthugu

I'm developing a C++ library for an embedded device. I want to create classes to handle some hardware resources such as UART peripherals.

What I want to do is to offer some global variables, for example UART1 and UART2. But I also want to keep the constructor private, because it makes no sense that the user can create a new instance for an UART that doesn't exist, or even worse, duplicate one of the existing one and trying to access it from 2 different objects.

The problem is that I can't access to the private constructor when defining them outside the class, like this.

// uart.h

class Uart {

private:

   Uart(int uart_num);

}



extern Uart uart1, uart2;



// uart.c

Uart uart1(1);

Uart uart2(2);

What would be a good way to achieve this? I've been thinking about an static method like Uart &get_instance(int uart_num). This is not a bad option, but I like more the global variables for this, I see them cleaner and more elegant. If this is the best solution, should I return a pointer or a reference to the uart object?

I've also thought about static members of the class, but this makes too long strings to access the variables. My library is inside its own namespace so the user would have to access it writing mylibnamespace::Uart::uart1, instead of mylibnamespace::uart1. This is neither a bad solution, but I still prefer the global variables one.

I don't see why it is better for you to use global variables, I would use something more dynamic with pointeres or the factory solution suggested above. But, anyways, The only way I see to provide global variables with no constructor (or private) is creating from an inherit class using default copy constructor. Since this is not a pointer or a reference you should be very carefull, this UartPriv::UartPriv(int) constructor is just to fill Uart member variables, not additional methods or variables should be declared here.

// uart.h

class Uart{ 

};



Uart uart1, uart2;





// uart.c

class UartPriv : public Uart{

public:

    UartPriv( int uart )

    {

        //initialize Uart members here

    }

};



Uart Uart1 = UartPriv( 1 );

Uart Uart2 = UartPriv( 2 );

Let's create a full answer. I will avoid using friend declaration here.

class Uart {

public:

    static Uart& get(int uart_num);

   Uart(const Uart&) = delete;

   Uart(Uart&&) = delete;

private:

   Uart(int uart_num);

}



// This is the function you can use from the outside

Uart& get_Uart(int uart_num);



Uart Uart::get(int uart_num)

{

    Uart uart0(0);



    switch(uart_num):

    {

        case 0:

            return uart0;

    };

    throw std::out_of_range("no such UART");

}



Uart& get_Uart(uart_num)

{

    return Uart::get(uart_num);

}



Uart& foo = get_Uart(foo_num);