How do I fix an "ambiguous" function call?

c++ ambiguous-call
Moshe picture Moshe · Sep 26, 2011 · Viewed 64.3k times · Source

I'm working on a C++ program for class, and my compiler is complaining about an "ambiguous" function call. I suspect that this is because there are several functions defined with different parameters.

How can I tell the compiler which one I want? Aside from a case-specific fix, is there a general rule, such as typecasting, which might solve these kinds of problems?

Edit:

In my case, I tried calling abs() inside of a cout statement, passing in two doubles.

cout << "Amount is:" << abs(amountOrdered-amountPaid);

Edit2:

I'm including these three headers:

#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

Edit3:

I've finished the program without this code, but in the interest of following through with this question, I've reproduced the problem. The verbatim error is:

Call to 'abs' is ambiguous.

The compiler offers three versions of abs, each taking a different datatype as a parameter.

Answer

Matt K picture Matt K · Sep 26, 2011

What's happened is that you've included <cstdlib> (indirectly, since it's included by iostream) along with using namespace std;. This header declares two functions in std with the name abs(). One takes and returns long long, and the other returns long. Plus, there's the one in the global namespace (that returns int) that comes from <stdlib.h>.

To fix: well, the abs() that takes double is in <cmath>, and that will actually give you the answer you want!

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