Szilárd Greszler

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6 years, 6 months ago.

Error: No default constructor exists for class "mbed::DigitalOut"

Hi,

I have a problem using DigitalOut in my own class.

See my header and cpp file below:

lights.h

#ifndef lights_H
#define lights_H

#include "mbed.h"

class Lights {
public:
    Lights(PinName pIL, PinName pIR);// constructor
    ~Lights();// destructor

private:
    DigitalOut do_indexLeft;
    DigitalOut do_indexRight;
};
    
#endif

lights.cpp

#include "lights.h"

Lights::Lights(PinName pIL, PinName pIR) : do_indexLeft(pIL), do_indexRight(pIR) {}

So I would like to initialize the DigitalOuts from my main() function by calling:

main.cpp

#include "lights.h"

Lights lights(PTA1, PTB9);

The problem is that compiling the lights.cpp in web based IDE returns: Error: No default constructor exists for class "mbed::DigitalOut" in "lights.cpp", Line: 4, Col: 2

What can be the problem? I'm quite new for mbed and also used C++ a long time ago.

Thanks for your help in advance!

BR, Szilárd

I slept a bit on this problem, and found that the problem is not in the cpp file, where the compiler complains, but in the header file:

DigitalOut do_indexLeft; // this variable declaration causes the error printout, as it would invoke the default constructor

The solution is using pointers:

DigitalOut* do_indexLeft;

Then creating the object in the constructor:

do_indexLeft = new DigitalOut(pIL);
posted by Szilárd Greszler 17 Oct 2017

Hello Szilárd,

Both methods should work. I was not able to reproduce such error when testing your code. Using the online compiler the following main.cpp (plus a copy of your code with a defined destructor) compiled with no errors:

include "lights.h"

Lights      lights(PTA1, PTB9);
DigitalOut  led(LED1);

int main()
{
    while(1){
        led = !led;
        wait(0.5);
    };
}
posted by Zoltan Hudak 22 Nov 2018

Hi Zoltán,

Thanks for your comment!

In my original code I used declarations using the default constructor (without parameters). Maybe in actual version of mbed OS this is supported, but back then it was not. This is why I've modified my code to use pointers and dynamic allocation of these objects, which was a solution.

UPDATE: I've checked and default constructors are still not supported in newest release, so you probably used pointer version of my code?

BR, Szilárd

posted by Szilárd Greszler 23 Nov 2018

2 Answers

Szilárd Greszler
poster
5 years, 4 months ago.

Finally I can transform my answer in a comment to a real answer:

I slept a bit on this problem, and found that the problem is not in the cpp file, where the compiler complains, but in the header file:

DigitalOut do_indexLeft; // this variable declaration causes the error printout, as it would invoke the default constructor

The solution is using pointers:

DigitalOut* do_indexLeft;

Then creating the object in the constructor:

do_indexLeft = new DigitalOut(pIL);

Default constructor for classes like DigitalIn, DigitalOut, I2C, etc. (so where instantiating the class doesn't make sense without pin assignments) is not supported, and it is good as it is. Solution is instantiating these classes in the code only when the pins to be used are already decided.

Accepted Answer
David Möschwitzer
5 years, 4 months ago.

I had the same problem. Your soulution works also with I2C and DigitalIn. Thanks!

Hi David,

Welcome! Nice to get a feedback that my post was helpful for others!

BR, Szilárd

posted by Szilárd Greszler 23 Nov 2018

Assigned to Szilárd Greszler 5 years, 4 months ago.

This means that the question has been accepted and is being worked on.