Is it possible to make a template variable in lambda signature generic?

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Suppose you have a function that takes a std::vector of any type and processes it in some way:

template<typename T> void foo(std::vector<T> &vec) {     // work with vec } 

Since C++14, we are able to achieve the same thing with lambdas. In this case, we call them generic lambdas, since we introduce a template-like deduction to them:

auto foo_lambda = [](std::vector<auto> &vec) {     // work with vec }; 

But our options seem quite limited to me. Suppose that I not only have to introduce a type deduction, but I also need to introduce template values. For example, let's change std::vector to std::array:

template<typename T, std::size_t size> void foo(std::array<T, size> &arr) {     // work with arr } 

When dealing with template functions, we are able to introduce a template value, which can be used to match argument's needs. Neat.

I wanted to achieve the same functionality with generic lambdas, but I was unable to do so.

Is there a way to introduce a similar, deduced value to a lambda expression so any std::arrays can be used with said lambda, similarily to the second version of the foo() function above?

EDIT: As stated in the comments by Evg, my vector<auto> syntax is non-standard GCC extension. For details see this answer referring to this document.


You can use some dedicated type trait:

#include <type_traits> #include <utility> #include <array>  template<typename x_Whatever> struct is_array: ::std::false_type {};  template<typename x_Item, ::std::size_t x_items_count> struct is_array<::std::array<x_Item, x_items_count>>: ::std::true_type {};  int main() {     auto Do_SomethingWithArray     {         [](auto & should_be_array)         {             static_assert             (                 is_array                 <                     ::std::remove_reference_t<decltype(should_be_array)>                 >::value             );                     }     };     ::std::array<int, 3> a{};     Do_SomethingWithArray(a); // Ok     int x{};     Do_SomethingWithArray(x); // error } 

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