Cosmetic improvements to Sampler and DynamicSampler

src/graph/generation/dynamic_sampler.hh

@@ -105,8 +105,9 @@ public:
         else
         {
             pos = _free.back();
-            _items[_idx[pos]] = v;
-            _valid[_idx[pos]] = true;
+            auto i = _idx[pos];
+            _items[i] = v;
+            _valid[i] = true;
             _tree[pos] = w;
             _free.pop_back();
         }
@@ -160,7 +161,7 @@ public:
         return _items[i];
     }
 
-    bool is_valid(size_t i)
+    bool is_valid(size_t i) const
     {
         return (i < _items.size() && _valid[i]);
     }

src/graph/generation/sampler.hh

@@ -134,7 +134,7 @@ private:
 // uniform sampling from containers
 
 template <class Iter, class RNG>
-auto& uniform_sample(Iter begin, const Iter& end, RNG& rng)
+auto&& uniform_sample(Iter begin, const Iter& end, RNG& rng)
 {
     auto N = end - begin;
     std::uniform_int_distribution<size_t> i_rand(0, N - 1);
@@ -143,7 +143,7 @@ auto& uniform_sample(Iter begin, const Iter& end, RNG& rng)
 }
 
 template <class Container, class RNG>
-auto& uniform_sample(Container& v, RNG& rng)
+auto&& uniform_sample(Container& v, RNG& rng)
 {
     return uniform_sample(v.begin(), v.end(), rng);
 }