Cannot inititialize std::function implicitly

Question

I\'ve written this functor to perform an and operation (&&):

// unary functor; performs \'&&\'
template 
         


        

Answer 1

The problem is that there's no way to deduce T in this context. Let's look at it from the compiler's perspective:

template 
AND And(function xx, function yy)

// Later, invoked as:
And( is_odd, is_big )

"Hm, no T is specified on the call, I will have to deduce it. What's the argument? is_odd, which is decayed to type int (*)(int). Now, I would have to instantiate std::function for all possible T values and see for which/if any it can be constructed from type int (*)(int). Well, there's infinitely many of them. Not gonna do this."

It would work if you specified the T tempalte argument explicitly:

return any_of( first, last, And( is_odd, is_big ) );

Live example

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Note that this holds even if you changed is_odd and is_big to match the function signature exactly (returning bool and taking const int&). It's the deduction that's the problem.