Translate exceptions by const reference instead of plain ref

docs/tostring.md

@@ -64,7 +64,7 @@ namespace Catch {
 By default all exceptions deriving from `std::exception` will be translated to strings by calling the `what()` method. For exception types that do not derive from `std::exception` - or if `what()` does not return a suitable string - use `CATCH_TRANSLATE_EXCEPTION`. This defines a function that takes your exception type, by reference, and returns a string. It can appear anywhere in the code - it doesn't have to be in the same translation unit. For example:
 
 ```cpp
-CATCH_TRANSLATE_EXCEPTION( MyType& ex ) {
+CATCH_TRANSLATE_EXCEPTION( MyType const& ex ) {
     return ex.message();
 }
 ```

src/catch2/catch_translate_exception.hpp

@@ -12,7 +12,7 @@ namespace Catch {
         class ExceptionTranslator : public IExceptionTranslator {
         public:
 
-            ExceptionTranslator( std::string(*translateFunction)( T& ) )
+            ExceptionTranslator( std::string(*translateFunction)( T const& ) )
             : m_translateFunction( translateFunction )
             {}
 
@@ -24,7 +24,7 @@ namespace Catch {
                     else
                         return (*it)->translate( it+1, itEnd );
                 }
-                catch( T& ex ) {
+                catch( T const& ex ) {
                     return m_translateFunction( ex );
                 }
 #else
@@ -33,12 +33,12 @@ namespace Catch {
             }
 
         protected:
-            std::string(*m_translateFunction)( T& );
+            std::string(*m_translateFunction)( T const& );
         };
 
     public:
         template<typename T>
-        ExceptionTranslatorRegistrar( std::string(*translateFunction)( T& ) ) {
+        ExceptionTranslatorRegistrar( std::string(*translateFunction)( T const& ) ) {
             getMutableRegistryHub().registerTranslator
                 ( new ExceptionTranslator<T>( translateFunction ) );
         }

tests/SelfTest/UsageTests/Exception.tests.cpp

@@ -122,15 +122,15 @@ TEST_CASE( "When unchecked exceptions are thrown, but caught, they do not affect
 }
 
 
-CATCH_TRANSLATE_EXCEPTION( CustomException& ex ) {
+CATCH_TRANSLATE_EXCEPTION( CustomException const& ex ) {
     return ex.getMessage();
 }
 
-CATCH_TRANSLATE_EXCEPTION( CustomStdException& ex ) {
+CATCH_TRANSLATE_EXCEPTION( CustomStdException const& ex ) {
     return ex.getMessage();
 }
 
-CATCH_TRANSLATE_EXCEPTION( double& ex ) {
+CATCH_TRANSLATE_EXCEPTION( double const& ex ) {
     return Catch::Detail::stringify( ex );
 }