C++ Constructor error when I separate the function prototype from the definition

Question

When I have both the prototype and definition in the same header file (as shown below), and I create an object in main (source.cpp) with Cube c1{}; I get no error and the default constructor works; c1's side will be defaulted to 0.0

class Cube {
    private:
        double side;
        static int counter; 
                                                   //this is cube.h       
    public:
        Cube(double s = 0.0) :side{ s } {  //constructor
            counter++;
        }
};

However, when I separate the interface from the implementation like this:

class Cube {
    private:
        double side;
        static int counter;  //static data
                                                   //this is cube.h
    public:
        Cube(double);
};

Its implementation:

#include <iostream>
#include "Cube.h"

int Cube::counter{ 0 };

                                                   //this is cube.cpp
Cube::Cube(double s = 0.0) :side{ s } {
    counter++;
}

And I go to the main function in source.cpp, Cube c1{}; now gives me the error:

no instance of constructor "Cube::Cube" matches the argument list

Note: When I gave c1 a value, like Cube c1{5}; it works in both cases.

Answer 1

You should put the default argument to the declaration Cube(double = 0.0);, not the definition. Otherwise the matching function cannot be found in other files.