C++ import external variable into private class variable

Question

I'm trying to get a variable declared in the main into the private variables of my class without passing it as an argument for the constructor. I need to link the interrupt controller to multiple hardware interrupts without re initializing the interrupt instance and thus overwriting it.

XScuGic InterruptInstance;

int main()
{
    // Initialize interrupt by looking up config and initializing with that config
    ConfigPtr = XScuGic_LookupConfig(INTERRUPT_DEVICE_ID);
    XScuGic_CfgInitialize(&InterruptInstance, ConfigPtr, ConfigPtr->BaseAddr);

    Foo foo;
    foo.initializeSpi(deviceID,slaveMask);

    return 0;
}

And the implementation of the class Foo:

class Foo
{
     // This should be linked to the one in the main
     XScuGic InterruptInstance;
public:
     // In here, the interrupt is linked to the SPI device
     void initializeSpi(uint16_t deviceID, uint32_t slaveMask); 
};

The deviceID and slaveMask are defined in a header which is included.

Is there some way to achieve this?

Answer 1

You can initialize a private class reference member with a constructor that uses the global variable, so then there is no need to pass it in the constructor:

XScuGic InterruptInstance_g; // global variable

class Foo {
private:
    const XScuGic& InterruptInstance; // This should be linked to the one in the main
public:
    Foo() : InterruptInstance{InterruptInstance_g} {}; // private variable is initialized from global variable
    void initializeSpi(uint16_t deviceID,uint32_t slaveMask); // In here, the interrupt is linked to the SPI device
};

int main()
{
    // Initialize interrupt by looking up config and initializing with that config
    ConfigPtr = XScuGic_LookupConfig(INTERRUPT_DEVICE_ID);
    XScuGic_CfgInitialize(&InterruptInstance,ConfigPtr,ConfigPtr->BaseAddr);

    Foo foo{}; // reference is not required as it will come from the global variable to initialize the private reference member
    foo.initializeSpi(deviceID,slaveMask);

    return 0;
}