Question

I’m getting an error on this operator overload. Here is the error that I’m getting:

Angle.cpp:466: error: ambiguous overload for ‘operator<<‘ in
‘(+out)->std::basic_ostream<_Elem, _Traits>::operator<< [with
_Elem = char, _Traits = std::char_traits](((const Angle*)this)->Angle::
getDegrees()) << “\37777777660″‘

Here is my class to overload the << operator

#ifndef OUTPUTOPS_H
#define OUTPUTOPS_H 1

#include <ostream>
#include "BoolOut.h"

// Prints the output of an object using cout. The object
// must define the function output()
template< class T >
std::ostream& operator<<(std::ostream& out, const T& x)
{
    x.output(out);

    return out;
}

#endif // OUTPUTOPS_H

The problem happens here:

void Angle::output(std::ostream& out) const
{
    out << getDegrees() << "°";
}

Which oddly enough happens not from getDegrees() but from the string. I tried changing the string to something like, “hello” just to make sure that it wasn’t the symbol but I received the same error.

Here is the rest of the code excluding unrelated code:

// Angle.h

#include "OutputOps.h"
// End user added include file section

#include <vxWorks.h>
#include <ostream>

class Angle {

public:
    // Default constructor/destructor
    ~Angle();

    // User-defined methods
    //
    // Default Constructor sets Angle to 0.
    Angle();
    //
    ...
    // Returns the value of this Angle in degrees.
    double getDegrees() const;
    ...
    // Prints the angle to the output stream as "x°" in degrees
    void output(std::ostream& out) const;

};

#endif // ANGLE_H


//  File Angle.cpp

#include "MathUtility.h"
#include <math.h>
// End user added include file section

#ifndef Angle_H
#include "Angle.h"
#endif


Angle::~Angle()
{
    // Start destructor user section
    // End destructor user section
}

//
// Default Constructor sets Angle to 0.
Angle::Angle() :
radians( 0 )
{
}

...
//
// Returns the value of this Angle in degrees.
double Angle::getDegrees() const
{
    return radians * DEGREES_PER_RADIAN;
}

//
// Returns the value of this Angle in semicircles.
...

//
// Prints the angle to the output stream as "x°" in degrees
void Angle::output(std::ostream& out) const
{
    out << getDegrees() << "°";
}

Answer 1

It’s because you use template in your overloaded operator<<, but this overload is not in class, so you can’t set typename T. In other words, you have to overload operator<< for every type of variable which you would like to use or overload this operator in class which also is a template. For example:

std::ostream& operator<<(std::ostream& out, const Angle& x)
{
    x.output(out);

    return out;
}

This error means that compiler can’t predict what kind of variables will be used there.

---

You overload this operator for all possible data, so when you pass getDegrees() function, which returns double then i don’t think that x.output(out); will work 😉 (hint x will be double)