Create member object using constructor arguments, or pass reference to object via constructor

Question

I'm not entirely sure about the proper C++ terminology, so I'll just illustrate what I'm trying to achieve by giving a Java example since it should be pretty obvious:

public class LcdDecorator {
    private HD44780Lcd lcd;

    public LcdDecorator(int rows, int cols, int i2cAddress) {
        this.lcd = new HD44780Lcd(rows, cols, i2cAddress);
    }
}

or... alternatively...

public class LcdDecorator {
    private HD44780Lcd lcd;

    public LcdDecorator(HD44780Lcd lcd) {
        this.lcd = lcd;
    }
}

For what it's worth, there's no real concern about scope... LcdDecorator is a Singleton that gets created in setup(), then lives forever and gets used by loop() until the power gets shut off.

Here's what I've attempted so far:

LcdFacade.h:

#ifndef CORECONTROLLER_LCDFACADE_H
#define CORECONTROLLER_LCDACADE_H

#include <stdint.h>
#include "../.pio/libdeps/megaatmega2560/HD44780_LCD_PCF8574/src/HD44780_LCD_PCF8574.h"

class LcdFacade {

public:
    LcdFacade(uint8_t rows, uint8_t cols, uint8_t i2cAddress) ;
};

#endif //CORECONTROLLER_LCDFACADE_H

LcdFacade.cpp:

#include "LcdFacade.h"
#include "../.pio/libdeps/megaatmega2560/HD44780_LCD_PCF8574/src/HD44780_LCD_PCF8574.h"

HD44780LCD lcd;

LcdFacade::LcdFacade (uint8_t rows, uint8_t cols, uint8_t i2cAddress) : lcd{rows, cols, i2cAddress} {
    // ... the rest of the constructor goes here ...
}

note: I also tried using parentheses instead of braces for lcd's constructor:

LcdFacade::LcdFacade(uint8_t rows, uint8_t cols, uint8_t i2cAddress) : lcd(rows, cols, i2cAddress) {

In CLion, the 'lcd{rows, cols, i2caddress}' portion of the constructor in LcdFacade.cpp is red-underlined, and reports member initializer 'lcd' does not name a non-static data member or base class

Meanwhile, gcc(?) reports error: no matching function for call to 'HD44780LCD::HD44780LCD() on line 4 of LcdFacade.cpp.

Any ideas?

Edgar Bonet · Accepted Answer · 2022-06-23 07:41:26Z

When you define

HD44780LCD lcd;

at global scope, you are creating a global variable that is initialized before main() is called. In this case, it is initialized with the default constructor HD44780LCD::HD44780LCD(), which does not exist, hence the error. You could avoid this error by calling a suitable constructor in the initialization, e.g.

HD44780LCD lcd(4, 20, 0x23);

but this may be inconvenient to you.

If you want to initialize lcd in the constructor of LcdFacade, then it must be a non-static member of the class:

// In LcdFacade.h:
class LcdFacade {
public:
    LcdFacade(uint8_t rows, uint8_t cols, uint8_t i2cAddress);
private:
    HD44780LCD lcd;  // <- non-static member
};

// In LcdFacade.cpp:
LcdFacade::LcdFacade(uint8_t rows, uint8_t cols, uint8_t i2cAddress)
: lcd(rows, cols, i2cAddress) {
    ...
}

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