This is more of a c++ standards question. Consider the following code:
template <typename T>
class has_Data
{
typedef char one;
typedef long two;
template <typename C> static one test( typeof(&C::Data) ) ;
template <typename C> static two test(...);
public:
enum { value = sizeof(test<T>(0)) == sizeof(char) };
};
class MyClass {
private:
struct Data {
};
};
void function(bool val = has_Data<MyClass>::value) {}
The above code works with gcc (GCC) 4.4.3
However with clang version 3.3 (2545b1d99942080bac4a74cda92c620123d0d6e9) (2ff97832e593926ea8dbdd5fc5bcf367475638a9)
it gives this error:
test_private_data.cpp:7:54: error: 'Data' is a private member of 'MyClass'
template <typename C> static one test( typeof(&C::Data) ) ;
^
/devshared/home/rhanda/test_private_data.cpp:7:37: note: while substituting explicitly-specified template arguments into function template 'test'
template <typename C> static one test( typeof(&C::Data) ) ;
^
/devshared/home/rhanda/test_private_data.cpp:21:26: note: in instantiation of template class 'has_Data<MyClass>' requested here
void function(bool val = has_Data<MyClass>::value) {}
^
1 error generated.
Which one is right?
From standard document (n3485), I found a statement which seems to agree with clang more than gcc.
Access control is applied uniformly to all names, whether the names are referred to from declarations or expressions.
I would assume that GCC is right.
The first thing to note is that no non-friend
code should be able to positively report the existence of a given private member. So if that is what you try to do, you have to modify your design. A class can do anything with its private members, and other code (excepting friends) should have no way to know about it. That's by design.
However, there is the SFINAE principle: substitution failure is not an error. Since MyClass::Data
is private, the code in has_Data
should – in my opinion – act as if there was no C::Data
member at all. Hence the first function would lead to a substitution failure, which gets silently ignored, and the second function is the one used. Adding a bit more code, my GCC 4.7.2 compiles this without issues and with has_Data<MyClass>::value
evaluating to false
. Correct SFINAE in my opinion.
Trying to back this opinion up with a quotation from the document you referred to, I found the following in section 14.8.2 paragraph 8:
Note: Access checking is done as part of the substitution process.
This is a non-normative note in the standard, but to me appears to be a very readable and clear indication that SFINAE should in fact apply in this situation, just the way GCC handles it.
Edit: As @hvd pointed out in a comment, the above is only true for C++11. In older versions of the standard, the situation used to be different. Issue 1170: Access checking during template argument deduction has details on that change.
GCC will not compile this code with -std=c++03
or -std=c++11
due to the fact that typeof
is a GNU extension. The fact that -std=gnu++03
still compiles the code might perhaps be considered inappropriate, but since the way forward is using the C++11 semantics, I wouldn't bother filing a report about this.
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