使用lambda和std :: function的C ++ 11类型推断

Question

I have the following snippet of code, that although entirely trivial, illustrates a pattern I am trying to use in more general code.

template<typename InT, typename ResT>
ResT unary_apply( InT val, std::function<ResT(InT)> fn )
{
    return fn(val);
}

I would like to be able to call unary_apply with function pointers, functors, lambdas etc: hence the use of std::function to abstract that all away.

When I try to use the above in the following way, C++ (g++ 4.7) is unable to perform the relevant type inference:

double blah = unary_apply( 2, []( int v ) { return 3.0 * v; } );

Failing with

src/fun.cpp:147:75: error: no matching function for call to ‘unary_apply(int, test()::<lambda(int)>)’
src/fun.cpp:147:75: note: candidate is:
src/fun.cpp:137:6: note: template<class InT, class ResT> ResT unary_apply(InT, std::function<ResT(InT)>)
src/fun.cpp:137:6: note:   template argument deduction/substitution failed:
src/fun.cpp:147:75: note:   ‘test()::<lambda(int)>’ is not derived from ‘std::function<ResT(double)>’

And I find that I have to explicitly specify the template parameters (in practice I believe it is just the return type that is not inferable):

double blah = unary_apply<int, double>( 2, []( int v ) { return 3.0 * v; } );

I am not that familiar with the type inference rules in C++11, but the above behaviour does seem reasonable (I can see that inferring via the internal mechanics of std::function is probably rather a big ask). My question is: is it possible to re-write the unary_apply function above to keep the same flexibility (in terms of the types of functions/functors etc that can be passed as a second parameter) whilst also giving more of a clue to type inference so I do not have to explicitly supply the template parameters at the point of call?

Answer 1

Going bit more duck-typey should work:

template <typename T, typename F>
auto unary_apply(T&& val, F&& func) -> decltype(func(val)) {
    return func(std::forward<T>(val));
}