print() doesn't work with custom GFX class

[Spongman]

v1.4.4 on rp2040, arduino, platformio.

#include <Arduino_GFX_Library.h>

void setup()
{
	class MyGFX : public Arduino_GFX
	{
		int foo;

	public:
		MyGFX(int16_t w, int16_t h) : Arduino_GFX(w, h)
		{
		}

		virtual bool begin(int32_t speed = GFX_NOT_DEFINED) { return true; }

		virtual void writePixelPreclipped(int16_t x, int16_t y, uint16_t color)
		{
			Serial.printf("writePixelPreclipped %i, %i, %x\n", x, y, color);
		}
	};

	MyGFX buffer(240, 135);

	Serial.println(F("start"));
	buffer.startWrite();

	buffer.drawPixel(0, 0, RGB565(255, 255, 0));

	buffer.setCursor(10, 10);
	buffer.setTextColor(RGB565_GREEN);
	buffer.print("text");

	buffer.endWrite();
	Serial.println(F("done"));
}

for some reason, buffer.print() isn't doing anything, and I can't tell why. all i get is

start
writePixelPreclipped 0, 0, ffe0
done

If I comment out the int foo; line, then it works:

start
writePixelPreclipped 0, 0, ffe0
writePixelPreclipped 10, 12, 7e0
writePixelPreclipped 11, 12, 7e0
writePixelPreclipped 12, 10, 7e0
writePixelPreclipped 12, 11, 7e0
writePixelPreclipped 12, 12, 7e0
...

[moononournation]

you did not mention your method should override parent class

[Spongman]

not sure what you mean. the writePixelPreclipped() method above does override the one in Arduino_GFX

[moononournation]

you may refer other class how can override it

[Spongman]

you may refer other class how can override it

sorry, i don't understand what you're saying.

#include <iostream>

struct Base
{
	virtual void Method() { std::cout << "base\n"; }
};

struct Derived : public Base
{
	virtual void Method() { std::cout << "derived\n"; }
};

int main()
{
	Base *base = new Derived();
	base->Method();
}

prints "derived"

https://godbolt.org/z/M3dKhM397

[Spongman]

can you suggest a fix?

[moononournation]

no idea

[Spongman]

ok, so this

you did not mention your method should override parent class

isn't the cause ?

[moononournation]

You can see all the child classes have override keyword in header files

[Spongman]

the override keyword is optional. I clearly illustrated above that Derived::method() overrides Base::Method(). can you explain adding the override keyword does ?

this issue has nothing to do with the override keyword and its presence or absence.

[Spongman]

why is this not an issue now?

[Spongman]

can you show how adding/removing the override keyword changes the behavior of either of the examples I have given?

[Spongman]

this:

#include <iostream>

struct Base
{
	virtual void Method() { std::cout << "base\n"; }
};

struct Derived : public Base
{
	void Method() override { std::cout << "derived\n"; }
};

int main()
{
	Base *pb = new Derived();
	pb->Method();
}```

does _exactly_ the same as the example without `override`.

[moononournation]

Sorry, I did not have related teaching materials.

[Spongman]

there's no teaching required here. however, you seem to be implying that the override keyword is necessary when overriding a method in c++. this is not the case. the override keyword is 100% optional.

[Spongman]

but since you seem to be stuck on that misconception, here's the original example with the override keyword added:

#include <Arduino_GFX_Library.h>

void setup()
{
	class MyGFX : public Arduino_GFX
	{
		int foo;

	public:
		MyGFX(int16_t w, int16_t h) : Arduino_GFX(w, h)
		{
		}

		bool begin(int32_t speed = GFX_NOT_DEFINED) override { return true; }

		void writePixelPreclipped(int16_t x, int16_t y, uint16_t color) override
		{
			Serial.printf("writePixelPreclipped %i, %i, %x\n", x, y, color);
		}
	};

	MyGFX buffer(240, 135);

	Serial.println(F("start"));
	buffer.startWrite();

	buffer.drawPixel(0, 0, RGB565(255, 255, 0));

	buffer.setCursor(10, 10);
	buffer.setTextColor(RGB565_GREEN);
	buffer.print("text");

	buffer.endWrite();
	Serial.println(F("done"));
}

it exhibits exactly the same behavior as I described.

please can you explain how that is not an issue?

[Spongman]

this is from chatgpt:

User: is the override keyword required when overriding a virtual method in c++ ?

ChatGPT
No, the override keyword is not strictly required when overriding a virtual method in C++. However, it is considered good practice to use it.

Using the override keyword explicitly tells the compiler that you intend to override a virtual function from a base class. It helps catch errors at compile time, such as misspellings in the function signature or accidental creation of a new function instead of overriding an existing one.

Without the override keyword, the compiler will still allow the function to override the virtual function if the signatures match, but it won't provide any indication that the function is intended to override a base class function. Therefore, using override improves code readability and helps prevent mistakes.

[Spongman]

would you like me to provide more teaching materials?