Making at work for container with not random access iterators

include/gsl.h

@@ -89,7 +89,13 @@ template <class T, size_t N>
 T& at(std::array<T, N>& arr, size_t index) { fail_fast_assert(index < N); return arr[index]; }
 
 template <class Cont>
-typename Cont::value_type& at(Cont& cont, size_t index) { fail_fast_assert(index < cont.size()); return cont[index]; }
+typename Cont::value_type& at(Cont& cont, size_t index)
+{
+    fail_fast_assert(index < cont.size());
+    auto it = cont.begin();
+    std::advance(it, index);
+    return *it;
+}
 
 
 //

tests/at_tests.cpp

@@ -17,6 +17,7 @@
 #include <UnitTest++/UnitTest++.h> 
 #include <gsl.h>
 #include <vector>
+#include <list>
 
 using namespace std;
 using namespace Guide;
@@ -52,6 +53,16 @@ SUITE(at_tests)
 
         CHECK_THROW(at(a, 4), fail_fast);
     }
+
+    TEST(StdList)
+    {
+        std::list<int> a = { 1, 2, 3, 4 };
+
+        for (int i = 0; i < 4; ++i)
+            CHECK(at(a, i) == i+1);
+
+        CHECK_THROW(at(a, 4), fail_fast);
+    }
 }
 
 int main(int, const char *[])