why cant i initialize 3 default parameters at a time in c++

Question

class Complex 
{
private:

float rp, ip;
//const int a=10;
//static int b;
//static const int c = 50;
public:
Complex();
//Complex(float );
/*Complex(float,float);*/
Complex(float , float = 20, float = 30);
} 

The above code works fine but when i try to have 3 default parameters

class Complex
{
private:
float rp, ip;
//const int a=10;//static int b;//static const int c = 50;
public:
Complex();
//Complex(float );
/*Complex(float,float);*/
Complex(float =10  , float = 20, float = 30);
} 

I get the below error -

main.cpp(12): error C2668: 'Complex::Complex' : ambiguous call to
overloaded function complex.h(15): could be 'Complex::Complex(float,float,float)' complex.h(12): or 'Complex::Complex(void)'

Answer 1

This fails because the compiler cannot know which of the two different functions you mean when you write

Complex c;

Does that call

Complex();

or

Complex(float,float,float);

with the three default values?

You can get around this by removing the Complex() constructor and have default construction be handled by the Complex(float,float,float) constructor with the three default values.

Answer 2

The constructor with three default args conflicts with your no-arg constructor.

Answer 3

The constructor is ambigious. The compiler is unable to understand constructor you want to call if you construct Complex with no arguments: whether it is the default constructor

Complex();

or the constructor with three arguments all defaulted:

Complex(float =10  , float = 20, float = 30);

Both of these constructors would match if you do

Complex c;

or

new Complex();

Answer 4

The error message lays it out pretty clearly.

The problem is that because all 3 arguments are defaulted, the compiler has no idea how to deduce what function overload you're talking about.

// default constructor, or 3 defaulted floats?
Complex a;