QString conversion function

[Bziur]

Hello. I'm trying to make my work easier by overloading QString() conversion function:

private:
  QString* c_string;
public:
  operator QString() {
    if( c_type == STRING )
      return *c_string;
    else
      return toString();
  }

private:
  QString* c_string;
public:
  operator QString() {
    if( c_type == STRING )
      return *c_string;
    else
      return toString();
  }

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then I'm trying to use it like this

QString line(int i) const {
  return c_labels[i] + ": " + c_values[i]; // c_labels[i] returns QString, c_values[i] returns the object with the above conversion function.
}

QString line(int i) const {
  return c_labels[i] + ": " + c_values[i]; // c_labels[i] returns QString, c_values[i] returns the object with the above conversion function.
}

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and I'm getting error: call of overloaded 'QString(const IniParam&)' is ambiguous
I also tried putting ": " in QString(), same for c_values[i], <QString> before c_values[i] doesn't help either;
Any Solutions?

[ChrisW67]

We currently cannot see enough. Please post a small, complete example that fails to compile in this way.

[Bziur]

Ok so I figured out it happens when you overload some different conversion functions;
Any solution to that? I wanna be able to easily output to multiple formats.

convertable.h

#ifndef CONVERTABLE_H
#define CONVERTABLE_H
 
#include <QString>
 
class Convertable
{
private:
	int i;
	QString c_string;
public:
	Convertable(int in=0, const QString string = QString()):c_string(string),i(in) {}
 
	operator int()
	{
		return i;
	}
 
	operator QString()
	{
		return c_string;
	}
};
 
#endif // CONVERTABLE_H

#ifndef CONVERTABLE_H
#define CONVERTABLE_H

#include <QString>

class Convertable
{
private:
	int i;
	QString c_string;
public:
	Convertable(int in=0, const QString string = QString()):c_string(string),i(in) {}

	operator int()
	{
		return i;
	}

	operator QString()
	{
		return c_string;
	}
};

#endif // CONVERTABLE_H

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main.cpp

#include <QCoreApplication>
 
#include "convertable.h"
#include <QDebug>
 
QString toString(const QString &in1, Convertable &in2)
{
	return in1 + ": " + in2;
}
 
int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);
 
	QString A("Print");
	Convertable B("something");
	qDebug() << toString(A,B);
 
	return a.exec();
}

#include <QCoreApplication>

#include "convertable.h"
#include <QDebug>

QString toString(const QString &in1, Convertable &in2)
{
	return in1 + ": " + in2;
}

int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);

	QString A("Print");
	Convertable B("something");
	qDebug() << toString(A,B);

	return a.exec();
}

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[Radek]

Please specify what all is wrong now. At least, the following will not pass:

Convertable B("something");

Convertable B("something");

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You do not have a ctor for it. Try

Convertable B(0,"something");

Convertable B(0,"something");

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[Bziur]

I forgot to change that but it doesn't matter much as it's further down the issue list.
The error I'm getting:
C:\Test\main.cpp:8: error: ambiguous overload for 'operator+' (operand types are 'const QString' and 'Convertable')

return in1 + ": " + in2;
                  ^

return in1 + ": " + in2;
                  ^

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[Radek]

This can be caused by the int conversion of Convertable because there exists operator + ( const QString& str, char chr ). Try:

(1) Comment out the int conversion operator for now (only to see what's wrong).
(2) Make the 2nd param of toString() const Convertable&.

[Bziur]

added: const to Convertable & - same problem
then removed int conversion:

main.cpp:8: error: invalid user-defined conversion from 'const Convertable' to 'const QString&' [-fpermissive]

return in1 + ": " + in2;
                    ^

return in1 + ": " + in2;
                    ^

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main.cpp:3: In file included from ..\Test\main.cpp:3:0:
convertable.h:19: candidate is: Convertable::operator QString() <near match>
convertable.h:19: note: no known conversion for implicit 'this' parameter from 'const Convertable*' to 'Convertable*'
main.cpp:8: error: passing 'const Convertable' as 'this' argument of 'Convertable::operator QString()' discards qualifiers [-fpermissive]

return in1 + ": " + in2;
                    ^

return in1 + ": " + in2;
                    ^

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[Radek]

Okay, try adding

operator const QString() const
{
  return c_string;
}

operator const QString() const
{
  return c_string;
}

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[Bziur]

Sooo. It works.
But then again if I uncomment the int and add const to it. I get the same ambiguous overload warning

---

Added after 1 48 minutes:

So Lessons learned today
You shouldn't overload too many conversion functions as they lay confusion on c++ compilers when dealing with operators that accept multiple variable types. Stick to the necessary ones to avoid ambiguity and disarray.