Merge pull request #629 from borglab/feature/constexpr-symbolgenerator

SymbolGenerator: add chr() and made constexpr-capable

gtsam/inference/Symbol.h

@@ -167,10 +167,11 @@ inline Key Z(std::uint64_t j) { return Symbol('z', j); }
 /** Generates symbol shorthands with alternative names different than the
  * one-letter predefined ones. */
 class SymbolGenerator {
-  const char c_;
+  const unsigned char c_;
 public:
-  SymbolGenerator(const char c) : c_(c) {}
+  constexpr SymbolGenerator(const unsigned char c) : c_(c) {}
   Symbol operator()(const std::uint64_t j) const { return Symbol(c_, j); }
+  constexpr unsigned char chr() const { return c_; }
 };
 
 /// traits

gtsam/inference/tests/testKey.cpp

@@ -59,6 +59,12 @@ TEST(Key, SymbolGenerator) {
   EXPECT(assert_equal(a1, ddz1));
 }
 
+/* ************************************************************************* */
+TEST(Key, SymbolGeneratorConstexpr) {
+  constexpr auto Z = gtsam::SymbolGenerator('x');
+  EXPECT(assert_equal(Z.chr(), 'x'));
+}
+
 /* ************************************************************************* */
 template<int KeySize>
 Key KeyTestValue();