C++ ambiguous overload for ‘operator ’

Question

I\'v read several posts here about this kind of errors, but I wasn\'t able to solve this one... Has soon I define the operator int and the function f, fails to compile. I teste

Answer 1

You have several options to fix your problem:

• Forbid implicit conversion from int to Fraccao: Make the constructor explicit.

• Forbid implicit conversion from Fraccao to int: Make the conversion operator explicit.

• Convert manually on the callers side, given Fraccao f; int i; either int(f)+i or f+Fraccao(i).

• Provide additional overloads to resolve the ambiguity:

Fraccao operator +(const Fraccao &a, const Fraccao &b);
Fraccao operator +(const int a, const Fraccao &b);
Fraccao operator +(const Fraccao &a, const int b);

The latter probably means you also want:

Fraccao & operator+=(const Fraccao &fra);
Fraccao & operator+=(const int i);

And finally, if you want the latter, you can use libraries like Boost.Operators or my df.operators to support you and avoid writing the same forwarders over and over again.

Answer 2

The compiler sees two ways to interpret a + 4. It can convert the 4 to an object of type Fraccao and use operator+(const Fraccao&, const Fraccao&) or it can convert a to type int with the member conversion operator, and add 4 to the result. The rules for overloading make this ambiguous, and that's what the compiler is complaining about. In general, as @gx_ said in a comment, this problem comes up because there are conversions in both directions. If you mark the operator int() with explicit (C++11) the code will be okay.

Answer 3

You don't seem to have posted the code that actually causes the error. I guess it looks something like

Fraccao a;
Fraccao b = a + 4;
Fraccao c = 4 + a;

The problem is that your class allows implicit conversions both to and from int; so a + 4 could be either

int(a) + 4

or

a + Fraccao(4)

with no reason to choose one over the other. To resolve the ambiguity, you could either:

• make the conversion explicit, as above; or
• declare either the constructor or the conversion operator (or even both) explicit so that only one (or even neither) conversion can be done implicitly.