Passing arithmetic function objects into map

[Ganado]

There was this question here, asking about how to make a particular piece of code shorter: http://cplusplus.com/forum/beginner/282476/

Just for fun, I was trying to something like the following:

#include <iostream>
#include <map>
#include <functional>

int main()
{
    using FuncPtr = double *(double, double);
    
    std::map<char, FuncPtr> ops {
        { '+', std::plus<>{} },
        { '-', std::minus<>{} },
        { '*', std::multiplies<>{} },
        { '/', std::divides<>{} }
    };
    
    std::cout << ops['+'](3.0, 2.0) << '\n';
}

That is, let a map decide whether to call std::plus, std::minus, etc.
https://en.cppreference.com/w/cpp/utility/functional/plus

But I get compiler errors,

In file included from /usr/include/c++/4.9/bits/stl_algobase.h:64:0,
                 from /usr/include/c++/4.9/bits/char_traits.h:39,
                 from /usr/include/c++/4.9/ios:40,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from 1:
/usr/include/c++/4.9/bits/stl_pair.h: In instantiation of 'struct std::pair<const char, double*(double, double)>':
14:5:   required from here
/usr/include/c++/4.9/bits/stl_pair.h:102:11: error: field 'std::pair<const char, double*(double, double)>::second' invalidly declared function type
       _T2 second;                /// @c second is a copy of the second object
           ^
 In function 'int main()':
14:5: error: no matching function for call to 'std::map<char, double*(double, double)>::map(<brace-enclosed initializer list>)'
14:5: note: candidates are:
In file included from /usr/include/c++/4.9/map:61:0,
                 from 2:
/usr/include/c++/4.9/bits/stl_map.h:270:9: note: template<class _InputIterator> std::map<_Key, _Tp, _Compare, _Alloc>::map(_InputIterator, _InputIterator, const _Compare&, const allocator_type&)
         map(_InputIterator __first, _InputIterator __last,
         ^
/usr/include/c++/4.9/bits/stl_map.h:270:9: note:   template argument deduction/substitution failed:
14:5: note:   cannot convert '{'*', std::multiplies<void>{}}' (type '<brace-enclosed initializer list>') to type 'const std::less<char>&'

I tried alternative syntaxes such as using parentheses instead of {}, or not using <> or (), or using <double> instead, but no luck.

Any idea how to get something like this to work? Preferably without having to make my own struct, but it's fine if that's the only way.

[DizzyDon]

Try

    using FuncPtr = std::function<double(double, double)>;

[Ganado]

Yep, that works. Thank you.

Apparently lambdas work, too: http://www.cplusplus.com/forum/beginner/282476/#msg1222630 if one wanted to avoid std::function.