template_test.cpp

#include <iostream>

enum EntryIds {
  Entry1,
  NumberOfEntries
};

class EntryBase {
 public:
  EntryBase() : entry_id_(NumberOfEntries) {}
  EntryBase(int entry_id) : entry_id_(entry_id) {}
 private:
  int entry_id_;
};

template <int entry_id>
struct EntryT : public EntryBase {
  EntryT() : EntryBase(entry_id) {}
};

template <typename T>
struct EntryTypeT {
  typedef T type;
};

#ifdef DECLARE_ENTRY_
#error "The macro DECLARE_ENTRY_ is duplicated."
#else
#define DECLARE_ENTRY_(NAME, \
                       TYPE, \
                       DEFAULT_VALUE) \
  template <> \
  struct EntryT<NAME> { \
    typedef TYPE Type; \
    static const int ID = NAME; \
    static const Type getDefaultValue() { return DEFAULT_VALUE; } \
  }
#endif  // DECLARE_ENTRY_

DECLARE_ENTRY_(Entry1, unsigned int, 1024);

void getRawValue(unsigned int* out) {
  *out = 42;
}

template <int entry_id>
typename EntryTypeT<typename EntryT<entry_id>::Type>::type
getValue() {
  typedef EntryT<entry_id> Entry;
  typedef typename EntryTypeT<Entry::Type>::type EntryType;
  EntryType value = Entry::getDefaultValue();
  getRawValue(&value);
  return value;
}

int main() {
  unsigned int value = getValue<Entry1>();
  std::cout << value << std::endl;
  return 0;
}

cl compiles it.

g++ fails.
template_test.cpp: In function 'typename EntryTypeT<typename EntryT<entry_id>::Type>::type getValue()':
template_test.cpp:50:42: error: type/value mismatch at argument 1 in template parameter list for 'template struct EntryTypeT'
template_test.cpp:50:42: error:   expected a type, got 'Entry:: Type'
template_test.cpp:50:59: error: invalid type in declaration before ';' token
template_test.cpp: In function 'typename EntryTypeT<typename EntryT<entry_id>::Type>::type getValue() [with int entry_id = 0, typename EntryTypeT<typename EntryT<entry_id>::Type>::type = unsigned int]':
template_test.cpp:57:41:   instantiated from here
template_test.cpp:52:3: error: invalid conversion from 'EntryType* {aka int_}' to 'unsigned int_' [-fpermissive]
template_test.cpp:42:6: error:   initializing argument 1 of 'void getRawValue(unsigned int*)' [-fpermissive]

clang++ fails.
template_test.cpp:50:31: error: template argument for template type parameter must be a type; did you forget 'typename'?
  typedef typename EntryTypeTEntry::Type::type EntryType;
                              ^
template_test.cpp:21:20: note: template parameter is declared here
template
                   ^
template_test.cpp:50:45: error: expected a qualified name after 'typename'
  typedef typename EntryTypeTEntry::Type::type EntryType;
                                            ^
template_test.cpp:50:49: error: expected ';' at end of declaration
  typedef typename EntryTypeTEntry::Type::type EntryType;
                                                ^
                                                ;
template_test.cpp:51:3: error: use of undeclared identifier 'EntryType'; did you mean 'EntryTypeT'?
  EntryType value = Entry::getDefaultValue();
  ^~~~~~~~~
  EntryTypeT
template_test.cpp:22:8: note: 'EntryTypeT' declared here
struct EntryTypeT {
       ^
template_test.cpp:51:3: error: unknown type name 'EntryType'; did you mean 'EntryBase'?
  EntryType value = Entry::getDefaultValue();
  ^~~~~~~~~
  EntryBase
template_test.cpp:8:7: note: 'EntryBase' declared here
class EntryBase {
      ^
template_test.cpp:52:3: error: no matching function for call to 'getRawValue'
  getRawValue(&value);
  ^~~~~~~~~~~
template_test.cpp:42:6: note: candidate function not viable: no known conversion from 'EntryBase ' to 'unsigned int *'
      for 1st argument
void getRawValue(unsigned int out) {
     ^
6 errors generated.