[local.fe] add check and not to FlatTop

dune/gdt/local/finite-elements/default.hh

@@ -180,7 +180,7 @@ public:
     // TODO: Double checked locking pattern is not thread-safe without memory barriers.
     if (fes_.count(key) == 0) {
       // the FE needs to be created, we need to lock
-      std::lock_guard<std::mutex> DXTC_UNUSED(guard)(mutex_);
+      std::lock_guard<std::mutex> DXTC_UNUSED(guard){mutex_};
       // and to check again if someone else created the FE while we were waiting to acquire the lock
       if (fes_.count(key) == 0)
         fes_[key] = factory_(geometry_type, order);

dune/gdt/local/finite-elements/flattop.hh

@@ -24,6 +24,9 @@ namespace GDT {
 
 
 /**
+ * Inspired by [Brenner, Davis, Sung, 2014, A partition of unity method for the displacement obstacle problem of clamped
+ *              Kirchhoff plates], section 2
+ *
  * \sa LocalFlatTop2dCubeFiniteElement
  * \sa LocalFlatTopFiniteElementFactory
  */
@@ -41,6 +44,9 @@ public:
   LocalFlatTop2dCubeFiniteElementBasis(const double& overlap = 0.5)
     : geometry_type_(Dune::GeometryTypes::cube(2))
   {
+    DUNE_THROW_IF(
+        !(overlap > 0.), Exceptions::finite_element_error, "Overlap has to be in (0, 1], is " << overlap << "!");
+    DUNE_THROW_IF(overlap > 1., Exceptions::finite_element_error, "Overlap has to be in (0, 1], is " << overlap << "!");
     // we cannot let L_ and R_ be members and define phi_L_ and phi_R_ in the ctor initializer list, as they will
     // copy/reference broken/empty/default L_ and R_
     const auto L_ = (1. - overlap) / 2.;