Add find_solid with implicit downcast and validity check

The `find_solid` that already exists, returns `G4VSolid*`. Sometimes you
want to access more specific methods which are only available in the
concrete subtypes, which requires a downcast and a pointer validity
check. This template absolves the user from writing lots of boilerplate.

Closes #151

doc/src/reference/headers.md

@@ -16,7 +16,7 @@ As your program grows and is split into multiple files, and you gain more experi
 
 + `<n4-main.hh>`
 
-   Groups together utilities that are required to write the `main` function of a `nain4` application. These are also available via the separate headers: 
+   Groups together utilities that are required to write the `main` function of a `nain4` application. These are also available via the separate headers:
    + `<n4-run-manager.hh>`: `nain4` interface for configuring the Geant4 run manager
    + `<n4-mandatory.hh>`: `nain4` utilities for concise implementation of the user-defined classes that must be registered with the run manager
 
@@ -44,3 +44,6 @@ As your program grows and is split into multiple files, and you gain more experi
   Utilities to help with writing automated tests for `nain4` applications. These are also available via the separate headers:
   + `<n4-defaults.hh>`: ready-made dummy application components
   + `<n4-geometry-iterators.hh>`: iterating over all sub-elements of a geometry
+
++ Other headers
+  + `<n4-exceptions.hh>`: exceptions relating to situations arising in nain4 code

nain4/src/meson.build

@@ -37,6 +37,7 @@ nain4_includes = [ 'n4-all.hh'
                  , 'n4-boolean-shape.hh'
                  , 'n4-constants.hh'
                  , 'n4-defaults.hh'
+                 , 'n4-exceptions.hh'
                  , 'n4-geometry-iterators.hh'
                  , 'n4-geometry.hh'
                  , 'n4-inspect.hh'

nain4/src/n4-all.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <n4-main.hh>
+#include <n4-exceptions.hh>
 #include <n4-geometry.hh>
 #include <n4-utils.hh>
 #include <n4-testing.hh>

nain4/src/n4-exceptions.hh

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <exception>
+#include <string>
+
+namespace nain4 {
+namespace exceptions {
+
+#define WRAP(NAME, FROM, MESSAGE)                              \
+  "\n\n"                                                       \
+  "########################################################\n" \
+  "[n4::exceptions::" + NAME + "]\n\n"                         \
+  "[raised by " + FROM + "]: " + MESSAGE + "\n"                \
+  "########################################################\n" \
+  "\n\n"
+
+struct exception : std::exception {
+  exception(const std::string& name, const std::string& from, const std::string& message)
+  : message(WRAP(name, from, message)) {}
+  const char* what() const noexcept { return message.c_str();}
+
+private:
+  std::string message;
+};
+
+#define EXCEPTION(NAME)                                     \
+struct NAME : exception {                                   \
+  NAME(const std::string& from, const std::string& message) \
+    : exception(#NAME, from, message) {}                    \
+};
+
+EXCEPTION(not_found)
+EXCEPTION(bad_cast)
+
+#undef N4_EXCEPTION
+#undef WRAP
+
+} // namespace exceptions
+} // namespace nain4
+
+namespace n4{ using namespace nain4; }

nain4/src/n4-inspect.hh

@@ -1,11 +1,14 @@
 #pragma once
 
+#include <n4-exceptions.hh>
 #include <n4-run-manager.hh>
 
 #include <G4LogicalVolumeStore.hh>
 #include <G4PhysicalVolumeStore.hh>
 #include <G4SolidStore.hh>
 #include <G4ParticleTable.hh>
+#include <stdexcept>
+#include <typeinfo>
 
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wshadow"
@@ -26,6 +29,20 @@ IA event_number  ()       { return n4::run_manager::get().here_be_dragons() -> G
 #undef NAME
 #undef NAME_VRB
 
+template<class DOWN>
+const DOWN* find_solid(const G4String& name) {
+
+  auto found = find_solid(name);
+  if (!found) {
+    throw n4::exceptions::not_found("find_solid", "solid " + name + " not found");
+  }
+  auto down = dynamic_cast<const DOWN*>(find_solid(name));
+  if (!down) {
+    throw n4::exceptions::bad_cast("find_solid", "solid " + name + " cannot be downcast to " + typeid(DOWN).name());
+  }
+  return down;
+}
+
 // Remove all, logical/physical volumes, solids and assemblies.
 inline void clear_geometry() { G4RunManager::GetRunManager() -> ReinitializeGeometry(true); }
 

nain4/test/test-nain4.cc

@@ -1,4 +1,5 @@
 #include <n4-defaults.hh>
+#include <n4-exceptions.hh>
 #include <n4-shape.hh>
 #include <n4-boolean-shape.hh>
 #include <n4-utils.hh>
@@ -37,6 +38,7 @@
 
 #include <cmath>
 #include <numeric>
+#include <stdexcept>
 #include <type_traits>
 
 #pragma GCC diagnostic push
@@ -1154,9 +1156,29 @@ TEST_CASE("nain find particle", "[nain][find][particle]") {
   auto convenient = nain4::find_particle(name);
   CHECK(convenient == pita);
   CHECK(convenient == solid);
- }
+}
+
+TEST_CASE("nain find solid downcast", "[nain][find][solid]") {
+  auto box  = n4::box ("box" ).cube(1)  .solid();
+  auto tubs = n4::tubs("tubs").r(1).z(2).solid();
+
+  auto found_box  = n4::find_solid<G4Box >("box" );
+  auto found_tubs = n4::find_solid<G4Tubs>("tubs");
+
+  CHECK(found_box  == box );
+  CHECK(found_tubs == tubs);
+}
 
 
+TEST_CASE("nain find solid downcast not found", "[nain][find][solid]") {
+  REQUIRE_THROWS_AS(n4::find_solid<G4Box>("does_not_exist"), n4::exceptions::not_found);
+}
+
+TEST_CASE("nain find solid downcast bad cast", "[nain][find][solid]") {
+  n4::box("box").cube(1).solid();
+  REQUIRE_THROWS_AS(n4::find_solid<G4Tubs>("box"), n4::exceptions::bad_cast);
+}
+
 TEST_CASE("nain clear_geometry", "[nain][clear_geometry]") {
   default_run_manager().run();