Standard overloaded std::abs doesn't match std::function<double (double)> - c++

Question

I'm getting the following error
min.cpp:17:30: error: no viable conversion from '<overloaded function type>' to 'Container::UnaryFun' (aka 'function<double (double)>')
this->addFunction("abs", abs);
when trying to compile the following code:
#include <cmath>
#include <string>
#include <functional>
class Test
{
public:
using UnaryFun = std::function<double (double)>;
Test()
{
this->addFunction("abs", abs);
}
auto addFunction(const std::string& name, UnaryFun fun) -> void
{
// ...
}
};
auto main() -> int {
Test eval;
return 0;
}
I've tried to check the declaration of std::abs for argument double and return type double and looks like this:
inline _LIBCPP_INLINE_VISIBILITY double abs(double __lcpp_x) _NOEXCEPT {
return __builtin_fabs(__lcpp_x);
}
in /usr/local/Cellar/llvm/15.0.7_1/include/c++/v1/stdlib.h.
It is accesible specifically for the double type. I've checked this by adding:
double a = 5;
double b = std::abs(a);
and this compiles without problems or conversion warnings.
I've tried to declare my own abs function like so:
inline double xabs(double val)
{
return val < 0 ? -val : val;
}
and then change the following code like so to use this new xabs instead of std::abs
this->addFunction("abs", xabs);
and after this change, the code compiles.
Any ideas why the code with std::abs doesn't compile?
My environment:
OS: Mac OS 12.6
Compiler:
Apple clang version 14.0.0 (clang-1400.0.29.202)
Target: x86_64-apple-darwin21.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
Command to compile: g++ -std=c++2a -o min min.cpp
Update based on comments
I dug a bit deeper, and it seems that there is a problem with how std::function is declared, which led to the problem above.
If I declare addFunction like so, without std::function, the problem disappears.
auto addFunction(const std::string& name, double (*fun)(double)) -> void
{
}
This means that the compiler cannot figure out the matching abs if std::function is used but it can identify the matching overload if the type of the function is described directly without std::function.

Answer 1

The problem is that, since it has multiple overloads, std::abs doesn't have a single type. That means that the compiler can't select a std::function constructor to use to convert it since it can't deduce a type for the constructor's template parameter.
There are a couple of ways to get around that:
Use a cast:
addFunction("abs", std::static_cast<double(*)(double)>(std::abs));
Wrap it in a lambda:
addFunction("abs", [](double d) { return std::abs(d); });
As you've done, wrap it in a non-overloaded function