Updated cms::Exception using modern C++

- Added format option
- Use concepts to constrain operator<<

Author: Dr15Jones

FWCore/Utilities/interface/Exception.h

@@ -38,24 +38,17 @@
 #include <string>
 #include <exception>
 #include <type_traits>
+#include <string_view>
+#include <concepts>
+
+#include "fmt/format.h"
 
 #include "FWCore/Utilities/interface/thread_safety_macros.h"
 #include "FWCore/Utilities/interface/Likely.h"
 #include "FWCore/Utilities/interface/Visibility.h"
 
 namespace cms {
 
-  class Exception;
-  namespace detail {
-    // Used for SFINAE to control the instantiation of the stream insertion
-    // member template needed to support streaming output to an object
-    // of type cms::Exception, or a subclass of cms::Exception.
-    template <typename E>
-    using exception_type =
-        std::enable_if_t<std::is_base_of_v<Exception, std::remove_reference_t<E>>, std::remove_reference_t<E>>;
-
-  }  // namespace detail
-
   class dso_export Exception : public std::exception {
   public:
     explicit Exception(std::string const& aCategory);
@@ -118,13 +111,20 @@ namespace cms {
     //
 
     template <typename E, typename T>
-    friend typename detail::exception_type<E>& operator<<(E&& e, T const& stuff);
+      requires std::derived_from<std::remove_reference_t<E>, Exception>
+    friend E& operator<<(E&& e, T const& stuff);
 
     template <typename E>
-    friend typename detail::exception_type<E>& operator<<(E&& e, std::ostream& (*f)(std::ostream&));
+      requires std::derived_from<std::remove_reference_t<E>, Exception>
+    friend E& operator<<(E&& e, std::ostream& (*f)(std::ostream&));
 
     template <typename E>
-    friend typename detail::exception_type<E>& operator<<(E&& e, std::ios_base& (*f)(std::ios_base&));
+      requires std::derived_from<std::remove_reference_t<E>, Exception>
+    friend E& operator<<(E&& e, std::ios_base& (*f)(std::ios_base&));
+
+    template <typename... Args>
+    inline void format(fmt::format_string<Args...> format, Args&&... args);
+    inline void vformat(std::string_view fmt, fmt::format_args args);
 
     // This function is deprecated and we are in the process of removing
     // all code that uses it from CMSSW.  It will then be deleted.
@@ -152,20 +152,30 @@ namespace cms {
 
   // -------- implementation ---------
 
+  template <typename... Args>
+  inline void Exception::format(fmt::format_string<Args...> format, Args&&... args) {
+    ost_ << fmt::format(std::move(format), std::forward<Args>(args)...);
+  }
+
+  inline void Exception::vformat(std::string_view format, fmt::format_args args) { ost_ << fmt::vformat(format, args); }
+
   template <typename E, typename T>
-  inline typename detail::exception_type<E>& operator<<(E&& e, T const& stuff) {
+    requires std::derived_from<std::remove_reference_t<E>, Exception>
+  inline E& operator<<(E&& e, T const& stuff) {
     e.ost_ << stuff;
     return e;
   }
 
   template <typename E>
-  inline typename detail::exception_type<E>& operator<<(E&& e, std::ostream& (*f)(std::ostream&)) {
+    requires std::derived_from<std::remove_reference_t<E>, Exception>
+  inline E& operator<<(E&& e, std::ostream& (*f)(std::ostream&)) {
     f(e.ost_);
     return e;
   }
 
   template <typename E>
-  inline typename detail::exception_type<E>& operator<<(E&& e, std::ios_base& (*f)(std::ios_base&)) {
+    requires std::derived_from<std::remove_reference_t<E>, Exception>
+  inline E& operator<<(E&& e, std::ios_base& (*f)(std::ios_base&)) {
     f(e.ost_);
     return e;
   }

FWCore/Utilities/test/test_catch2_Exception.cc

@@ -37,6 +37,17 @@ namespace {
       "\n"
       "Gave up\n";
 
+  constexpr char expected_func4[] =
+      "An exception of category 'DataCorrupt' occurred.\n"
+      "Exception Message:\n"
+      "This is just a test: \n"
+      "double: 1.11111\n"
+      "float:  2.22222\n"
+      "uint:   75\n"
+      "string: a string\n"
+      "char*:  a nonconst pointer\n"
+      "char[]: a c-style array\n";
+
   void func3() {
     double d = 1.11111;
     float f = 2.22222;
@@ -77,10 +88,40 @@ namespace {
     }
   }
 
+  void func4() {
+    double d = 1.11111;
+    float f = 2.22222;
+    unsigned int i = 75U;
+    std::string s("a string");
+    char* c1 = const_cast<char*>("a nonconst pointer");
+    char c2[] = "a c-style array";
+    Thing thing(4);
+
+    //  throw cms::Exception("DataCorrupt")
+    cms::Exception e("DataCorrupt");
+    e.format(
+        "This is just a test: \n"
+        "double: {}\n"
+        "float:  {}\n"
+        "uint:   {}\n"
+        "string: {}\n"
+        "char*:  {}\n"
+        "char[]: {}\n",
+        d,
+        f,
+        i,
+        s,
+        c1,
+        c2);
+
+    throw e;
+  }
+
 }  // namespace
 
 TEST_CASE("Test cms::Exception", "[cms::Exception]") {
   SECTION("throw") { REQUIRE_THROWS_WITH(func1(), expected); }
+  SECTION("throw with format") { REQUIRE_THROWS_WITH(func4(), expected_func4); }
   SECTION("returnCode") {
     cms::Exception e1("ABC");
     REQUIRE(e1.returnCode() == 8001);