C++ : What is wrong with my class constructors ? (no default constructor exists &/or conflicting declaration)?

Question

Trying to write a simple exponential filter class, self contained in a .h file. I'm getting these errors:

1. no default constructor exists for class "ExpoFilter"
2. conflicting declaration 'ExpoFilter alpha_temp'

I don't understand the problem, I have 2 constructors, one for an int type parameter and one for a float type parameter. Isn't that enough to be considered 2 different function signatures?

I am also calling the first constructor from within the second one, which I've done successfully in the past. Also tried proper full .cpp + .h file without success.

So what is wrong?

Here is the code:

    /*
      Exponential filter, a.k.a exponential moving average, object Class
    */
    
    #ifndef ExpoFilter_h
    #define ExpoFilter_h
    
    
    class ExpoFilter
    {
      // user-accessible "public" interface
      public:
      // Constructors
      ExpoFilter::ExpoFilter(float alpha){
        _alpha = alpha;
        _alpha2 = 1.0f - _alpha;
      }
    
      ExpoFilter::ExpoFilter(int N){
        float alpha_temp = 2.0f / (N + 1);
        ExpoFilter(alpha_temp);
      }
    
      ExpoFilter::operator float(){
          return _value;
      }
      
      float ExpoFilter::operator=(float newValue)
      {
          return NewValue(newValue);
      }
    
      float ExpoFilter::NewValue(float newValue){
        _value = newValue * _alpha + _value * _alpha2;
        return _value;
      }
    
      // library-accessible "private" interface
       private:
       float _alpha;
       float _alpha2;
       float _value;
    
    };
    
    #endif

Answer 1

The problem is that since you have a parameterized constructor for your class ExpoFilter, the compiler will not synthesize the default constructor ExpoFilter::ExpoFilter() by itself. This means that if you want to create/construct an object of class ExpoFilter using the default constructor, say by writing ExpoFilter alpha_temp;, then the compiler will produce error because there is no default constructor.

To solve this you must add a default constructor for your class as shown below:

class ExpoFilter
{
    //other code here as before 
    //default constructor that uses constructor initializer list
    ExpoFilter() : _alpha(0), _alpha2(0), value(0)
    {
    }
}

Now, when you wrote:

ExpoFilter(alpha_temp); //this creates a temporary object named alpha_temp using the default constructor

The above statement creates an object of type ExpoFilter named alpha_temp using the default constructor. But since the compiler did not synthesize a default ctor, you will get the mentioned error.

If your intention was to call the parameterized constructor instead of default constructor then you could change the above statement to:

ExpoFilter(2.0f / (N + 1));

Answer 2

"Default constructor" in this context means a constructor that takes no arguments, i.e.

ExpoFilter() {...}

You have a constructor that takes an int argument, and a constructor that takes a float argument, but no constructor that takes no arguments. So code like this:

ExpoFilter alpha_temp;

... can't compile because the compiler doesn't know how to construct the object with no argument supplied.

Btw, this line:

ExpoFilter(alpha_temp);

.... is not calling a constructor-method on your object; rather it is constructing a second (temporary/anonymous) ExpoFilter object that is created on the stack and then immediately destroyed -- probably not what you intended. (in particular, that line won't change the state of your original ExpoFilter object!). If you want to delegate to another constructor-method from within your constructor (and you are compiling with C++11 or newer) you'll need to call the other-constructor from your constructor's initializer-list instead:

ExpoFilter::ExpoFilter(int N)
    : ExpoFilter(2.0f / (N + 1))
{
   // ...
}