From 28ec4ea69984c823e18b06c235ecb8f789613724 Mon Sep 17 00:00:00 2001
From: vjacquet <vjacquet@flowgroup.fr>
Date: Sun, 26 Jun 2016 23:58:13 +0200
Subject: [PATCH] Differentiating exception type for Expects and Ensures

Expects throw std::logic_error when Ensures throws std::runtime_error.
Uses multiple inheritance so that the exception thrown is both the
standard and the gsl::fail_fast.
---
 include/gsl_assert.h      | 21 +++++++++++++++++----
 tests/assertion_tests.cpp | 28 +++++++++++++++++++++++++++-
 2 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/include/gsl_assert.h b/include/gsl_assert.h
index 51e8ab6..17a2a75 100644
--- a/include/gsl_assert.h
+++ b/include/gsl_assert.h
@@ -45,18 +45,31 @@
 
 namespace gsl
 {
-struct fail_fast : public std::runtime_error 
+struct fail_fast
 {
-	explicit fail_fast(char const* const message) : std::runtime_error(message) {}   
+    virtual char const* what() const = 0;
 };
+
+namespace details
+{
+    template<typename E>
+    struct contract_violation : public E, public fail_fast
+    {
+        explicit contract_violation(char const* const message) : E(message) {}
+        virtual char const* what() const override
+        {
+            return E::what();
+        }
+    };
+}
 }
 
 #if defined(GSL_THROW_ON_CONTRACT_VIOLATION)
 
 #define Expects(cond)  if (!(cond)) \
-    throw gsl::fail_fast("GSL: Precondition failure at " __FILE__ ": " GSL_STRINGIFY(__LINE__));
+    throw gsl::details::contract_violation<std::logic_error>("GSL: Precondition failure at " __FILE__ ": " GSL_STRINGIFY(__LINE__));
 #define Ensures(cond)  if (!(cond)) \
-    throw gsl::fail_fast("GSL: Postcondition failure at " __FILE__ ": " GSL_STRINGIFY(__LINE__));
+    throw gsl::details::contract_violation<std::runtime_error>("GSL: Postcondition failure at " __FILE__ ": " GSL_STRINGIFY(__LINE__));
 
 
 #elif defined(GSL_TERMINATE_ON_CONTRACT_VIOLATION)
diff --git a/tests/assertion_tests.cpp b/tests/assertion_tests.cpp
index acd381a..cdcd2e0 100644
--- a/tests/assertion_tests.cpp
+++ b/tests/assertion_tests.cpp
@@ -31,6 +31,19 @@ SUITE(assertion_tests)
     {
         CHECK(f(2) == 2);
         CHECK_THROW(f(10), fail_fast);
+        try { f(10); }
+        catch (fail_fast const& e) {
+            cstring_span<> prefix = ensure_z("GSL: Precondition failure");
+            cstring_span<> what = ensure_z(e.what()).first(prefix.length());
+            CHECK(prefix == what);
+        }
+        CHECK_THROW(f(10), std::logic_error);
+        try { f(10); }
+        catch (std::logic_error const& e) {
+            cstring_span<> prefix = ensure_z("GSL: Precondition failure");
+            cstring_span<> what = ensure_z(e.what()).first(prefix.length());
+            CHECK(prefix == what);
+        }
     }
 
     int g(int i)
@@ -44,7 +57,20 @@ SUITE(assertion_tests)
     {
         CHECK(g(2) == 3);
         CHECK_THROW(g(9), fail_fast);
-    }
+        try { g(9); }
+        catch (fail_fast const& e) {
+            cstring_span<> prefix = ensure_z("GSL: Postcondition failure");
+            cstring_span<> what = ensure_z(e.what()).first(prefix.length());
+            CHECK(prefix == what);
+        }
+        CHECK_THROW(g(9), std::runtime_error);
+        try { g(9); }
+        catch (std::runtime_error const& e) {
+            cstring_span<> prefix = ensure_z("GSL: Postcondition failure");
+            cstring_span<> what = ensure_z(e.what()).first(prefix.length());
+            CHECK(prefix == what);
+        }
+   }
 }
 
 int main(int, const char *[])