vendredi 9 janvier 2015

Simple way to selectively disable template member in C++11

Let's start with a code (please note that is's shortened to show the problematic parts only).



#include <type_traits>
#include <utility>
#include <list>
#include <forward_list>
#include <functional>

template<typename Container, typename Compare>
class SortedContainer
{
public:

template<typename... Args>
auto erase(Args&&... args) -> decltype(std::declval<Container>().erase(std::forward<Args>(args)...))
{
return container_.erase(std::forward<Args>(args)...);
}

decltype(std::declval<const Container>().size()) size() const
{
return container_.size();
}

private:

/// container used for keeping elements
Container container_;
};

SortedContainer<std::list<int>, std::less<int>> list;
SortedContainer<std::forward_list<int>, std::less<int>> forward_list;

int main()
{

}


I'd like to selectively disable some functions from the SortedContainer template in case the underlying Container doesn't have the required function with matching signature.


The example above fails, because std::forward_list doesn't have erase() and size() functions:



$ g++ -std=c++11 test.cpp
test.cpp: In instantiation of ‘class SortedContainer<std::forward_list<int>, std::less<int> >’:
test.cpp:30:57: required from here
test.cpp:13:7: error: ‘class std::forward_list<int>’ has no member named ‘erase’
auto erase(Args&&... args) -> decltype(std::declval<Container>().erase(std::forward<Args>(args)...))
^
test.cpp:18:51: error: ‘const class std::forward_list<int>’ has no member named ‘size’
decltype(std::declval<const Container>().size()) size() const
^


I cannot use std::enable_if with std::is_member_function_pointer, because erase() is overloaded. Maybe some clever casting would help with that?


I hoped that it would "just work", as this is very similar to the last solution from this answer http://ift.tt/1I0trqG , but it doesn't, even if I give type (typename Container::iterator()) after the comma in the decltype - the error message is identical.


I was hoping that there is some nice solution that doesn't require implementing a dozen of has_*() constexpr functions to check whether or not this particular function exists. I couldn't find any example of "check whether this member with this arguments exists" template that was flexible, so it could work like:



has_function<&Container::erase(std::forward<Args>(args)...)>::value
has_function<&Container::size()>::value


without need to implement separate has_erase() and has_size().


I'd also prefer NOT to specialize the SortedContainer class for std::forward_list, because I don't care what the actual type is, just whether or not it has the functions needed for forwarding wrappers.


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