CODEWITHSUNDEEP
c++templates
skluzada
skluzada

Reputation: 153

std::function with function as default parameter

I have this method header: 

void Find(const _T & v, int max, const function<bool(const _E &)> & filter)

I'm using std::function because I need to accept function pointer, functor or lambda expression. I would like the filter to be optional, and default to function that always return true (nothing is filtered out) as default parameter. I've tried something like this: 

#include <functional>

template <typename E>
bool alwaystrue(const E & e){ return true; }

template <typename T, typename E>
void Find(const T & v, int max,
          const std::function<bool(const E &)> & filter = alwaystrue);

int main()
{
    Find<int, int>(1, 2);
}

But this doesn't compile:

50016981.cpp: In function ‘void Find(const T&, int, const std::function<bool(const E&)>&) [with T = int; E = int]’:
50016981.cpp:11:24: error: cannot resolve overloaded function ‘alwaystrue’ based on conversion to type ‘const std::function<bool(const int&)>&’
     Find<int, int>(1, 2);
                        ^

I've also tried having the function inside my class but got a similar error.

Is there a problem combining with std::function with templates? And if so, could you suggest how to do what I want? I want to avoid overloading the Find() function (if possible) so I don't have duplicate code.

Upvotes: 0

Views: 2217

Answers (1)

Toby Speight
Toby Speight

Reputation: 30824

You need to indicate which instantiation of alwaystrue you want to use as your default, i.e. alwaystrue<E>:

template <typename T, typename E>
void Find(const T& v, int max,
          const std::function<bool(const E&)>& filter = alwaystrue<E>);

Upvotes: 2

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