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10 years, 2 months ago.
Creating Instances of a Class Within Another Class
Hi Mbed People,
Excuse me if this question seems a little rudimentary, I am relatively new to creating libraries and class structures. I have a piece of working code where one button-press class outputs data using another simple 7-segment display class. I got this to work by creating an instance of the 7-Segment class each time a button is pressed. Although this is operating in the desired manner, it feels like there could be a much more efficient way of doing it.
In buttonPressClass.cpp:
void channelSelect::plusButtonDown(){
currentChannel_ = currentChannel_ + 1;
sevenSeg display(p21,p22,p23,p24,p25,p27,p26); //instance of the sevenSeg Display Class
display.set(currentChannel_);
}
I tried a couple of things, including creating the instance of the 7-segment class globally, but couldn't get it to work.
As I said, the code is working, but any advice that will help me improve my class structures would be greatly appreciated!
Thanks, Tom.
2 Answers
10 years, 2 months ago.
I'm not sure if I understand you correctly.. But if I'm not mistaken then you are a bit confused about initializing the constructor of classes within classes.
Here is how I would have solved it:
cpp file
channelSelect::channelSelect() : display(p21,p22,p23,p24,p25,p27,p26) { //here we are running the constructor of the display class
//code here
}
header file
class channelSelect {
public:
channelSelect();
private:
sevenSeg display;
}
you could also solve the code this way by putting the pin names in the constructor of the channelSelect-class.
cpp file
channelSelect::channelSelect(PinName pin1, PinName pin2, PinName pin3, PinName pin4, PinName pin5, PinName pin6, PinName pin7)
: display(pin1,pin2,pin3,pin4,pin5,pin6,pin7) {
//code here
}
this way you declare the pins when calling the constructor of the channelSelect-class.
Other than that I think Davids method is a very good suggestion.
10 years, 2 months ago.
Hi Tom,
I'm closer to the lower end of the C++ skill set than many others - so I hope to see many responses. That said, I would ponder - does the display class serve any other need? For instance,
- Perhaps you have many buttons, but it is never used outside of the "channelSelect" class, or
- Perhaps some other methods completely outside of this might also want to access it?
In the first case, I might go with -
class channelSelect {
public:
channelSelect();
~channelSelect();
void plusButtonDown(void);
...
private:
int currentChannel_;
sevenSeg display(p21,p22,p23,...);
// Having written it, I don't like this here, because the port pin assignments are "buried",
// and if we pull them out to the channelSelect constructor, then we have two
// very unrelated concepts (button and display) being bound together in non-obvious ways...
};
channelSelect::channelSelect() {
currentChannel_ = 2; // some default
display.set(currentChannel_);
}
// but this does mean the constructor runs once, and it simplifies your code to
void channelSelect::plusButtonDown(){
currentChannel_ = currentChannel_ + 1;
display.set(currentChannel_);
}
For the second scenario, then you might pass a reference to channelSelect to the display class constructor.
class channelSelect {
public:
channelSelect(sevenSeg & display);
...
private:
sevenSeg & display; // reference instead of pointer [but this is an area where I remain "weak"]
...
};
channelSelect::channelSelect(sevenSeg & _display) : display(_display) {
currentChannel_ = 2;
display.set(currentChannel_);
}
// and this remains the same...
void channelSelect::plusButtonDown(){
currentChannel_ = currentChannel_ + 1;
display.set(currentChannel_);
}
one last disclaimer: I only mentally executed this.