Ambiguous Function Call?

[RicoJ]

My understanding for function call sequence is: exact matching function > template function.

However, below code yields "error: reference to ‘negate’ is ambiguous". Why is that?

int negate(int i) { return -i; }

template <class F>
auto negate(F f) { return -f(); }

int main()
{
    negate(1);
}

[JRManx]

Why do you define "negate()" as auto right after the template? Also, why are you returning "-f()" with a function specifier? You don't declare it as a function anywhere in the program. You are correct as to which function has priority (see code below).

#include <iostream>

int negate (int i)
{
	return -i;
}

template <class F>
// Defined return type as F because that is the generic type
// defined by the template
F negate (F num)
{
	// This subtracts one to test which function takes priority
	return -num - 1;
}

int main()
{
	int num = 2;
	
	int num2 = negate (num); // call to int function
	int num3 = negate<int> (num);  // call to template function
	
	std::cout << num2 << std::endl;  // outputs -2
	std::cout << num3 << std::endl;  // outputs -3
	
	return 0;
}

The int function took priority when the call was ambiguous. Was this what you were trying to do?

[George P]

Consider a C++20/2a version:

#include <iostream>

auto negate(auto f)
{
   return -f;
}

int main()
{
   std::cout << negate(1.1) << '\n';

   std::cout << negate(-2) << '\n';
}

-1.1
2

[Ganado]

RicoJ, what compiler/version are you using? Compiles for me. Maybe explicitly pick C++14 or beyond as the language standard.
When I compile with just C++11, I get

4:16: note: deduced return type only available with -std=c++1y or -std=gnu++1y

[thmm]

error: reference to ‘negate’ is ambiguous

Do you use using namespace std somewhere in your code ?

There is a function object for performing negation in the STL that might cause the ambiguity
https://en.cppreference.com/w/cpp/utility/functional/negate

[JRManx]

Hmm. When I used namespace std in Furry Guy's code, I got these errors:

test1.cc:5:13: error: 'auto' not allowed in function prototype
auto negate(auto f)
            ^~~~
test1.cc:12:12: error: no viable constructor or deduction guide for deduction of
      template arguments of 'negate'
   cout << negate(1.1) << '\n';
           ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/functional:639:29: note: 
      candidate template ignored: could not match 'negate<_Tp>' against 'double'
struct _LIBCPP_TEMPLATE_VIS negate : unary_function<_Tp, _Tp>
                            ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/functional:639:29: note: 
      candidate function template not viable: requires 0 arguments, but 1 was
      provided
test1.cc:14:12: error: no viable constructor or deduction guide for deduction of
      template arguments of 'negate'
   cout << negate(-2) << '\n';
           ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/functional:639:29: note: 
      candidate template ignored: could not match 'negate<_Tp>' against 'int'
struct _LIBCPP_TEMPLATE_VIS negate : unary_function<_Tp, _Tp>
                            ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/functional:639:29: note: 
      candidate function template not viable: requires 0 arguments, but 1 was
      provided
3 errors generated.

The first one is because my compiler doesn't support that particular C++20 functionality, but I'm not sure about the others.

Yeah, when I removed namespace std, it got rid of those last two errors. I think its a namespace std issue.

[seeplus]

This compiles OK as C++17/20 with VS2019:

int negate(int i) { return -i; }

template <class F>
auto negate(F f) { return -f(); }

int main()
{
	negate(1);
}

So does this:

#include <iostream>
using namespace std;

int negate(int i)
{
	return -i;
}

template <class F>
F negate(F num)
{
	return -num - 1;
}

int main()
{
	int num = 2;
	int num2 = negate(num); // call to int function
	int num3 = negate<int>(num);  // call to template function

	cout << num2 << std::endl;  // outputs -2
	cout << num3 << std::endl;  // outputs -3
}

and so does this as C++20:

#include <iostream>

auto negate(auto f)
{
	return -f;
}

int main()
{
	std::cout << negate(1.1) << '\n';

	std::cout << negate(-2) << '\n';
}

[mbozzi]

There is an unconstrained class template std::negate
https://en.cppreference.com/w/cpp/utility/functional/negate
This is why you don't say using namespace std, especially if you expect your code to compile on someone else's computer.

[JRManx]

Yup! What you can do instead:

using std::cout;
using std::endl;
using std::cin;
using std::string;
using std::vector;
using std::getline;
// etc etc etc 

Takes up a little more space than "using namespace std" (and TBH I'd just use std:: if my program had fewer than about 10 cout objects), but it won't cause you any errors!

[seeplus]

Using VS2019, this compiles:

#include <iostream>
#include <functional>
//using namespace std;

int negate(int i)
{
	return -i;
}

template <class F>
F negate(F num)
{
	return -num - 1;
}

int main()
{
	int num = 2;
	int num2 = negate(num); // call to int function
	int num3 = negate<int>(num);  // call to template function

	std::cout << num2 << std::endl;  // outputs -2
	std::cout << num3 << std::endl;  // outputs -3
}

whereas this doesn't with the using:

#include <iostream>
#include <functional>
using namespace std;

int negate(int i)
{
	return -i;
}

template <class F>
F negate(F num)
{
	return -num - 1;
}

int main()
{
	int num = 2;
	int num2 = negate(num); // call to int function
	int num3 = negate<int>(num);  // call to template function

	std::cout << num2 << std::endl;  // outputs -2
	std::cout << num3 << std::endl;  // outputs -3
}

This seems to be compiler dependent as to what other includes are also included with iostream etc.

[RicoJ]

Hi guys, Y'all have raised some very nice points! Yes, I did have

#include <functional>
using namespace std; 

Now I know the existence of std::negate. Thanks for bringing this up!