问题
What I read in the C++ standard about injected class names contradicts (as I see it) with the behavior of a sample program I will present shortly. Here's what I read:
From 3.4 (paragraph 3)
The injected-class-name of a class (clause 9) is also considered to be a member of that class for the purposes of name hiding and lookup.
From 9 (paragraph 2)
A class-name is inserted into the scope in which it is declared immediately after the class-name is seen. The class-name is also inserted into the scope of the class itself; this is known as the injected-class-name. For purposes of access checking, the injected-class-name is treated as if it were a public member name.
From these I understand that the following is a well-formed translation unit and it compiles successfully.
#include <vector>
class X: std::vector<int>
{
vector mem;
};
However, I would suppose that the following should have produced an error, but it doesn't
#include <vector>
class X: std::vector<int>, std::vector<char>
{
vector mem; //compiles OK... mem is apparently std::vector<int>
};
Since the name vector is injected into both std::vector<int> and std::vector<char> as as if a public member name, then it should be inherited by X and therefore the name vector in X should be ambiguous. Am I missing something?
P.S. I am using MSVC9.0
回答1:
I found it! It's right there in the standard! I was right! It should be ambiguous!
Clause 14.6.1 Paragraph
A lookup that finds an injected-class-name (10.2) can result in an ambiguity in certain cases (for example, if it is found in more than one base class). If all of the injected-class-names that are found refer to specializations of the same class template, and if the name is followed by a template-argument-list, the reference refers to the class template itself and not a specialization thereof, and is not ambiguous. [Example:
template <class T> struct Base { };
template <class T> struct Derived: Base<int>, Base<char>
{
typename Derived::Base b; // error: ambiguous typename
Derived::Base<double> d; // OK
};
—end example]
Bottom line: This is yet another Microsoft compiler BUG. Disabling language extensions doesn't help either.
回答2:
No, you are not missing anything, and your compiler seems to behave buggy. You can see how gcc handles it here: http://ideone.com/MI9gz
Its error message is:
prog.cpp:4:4: error: reference to 'vector' is ambiguous
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_vector.h:171:5: error: candidates are: class std::vector<char> std::vector<char>::vector
/usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/bits/stl_vector.h:171:5: error: class std::vector<int> std::vector<int>::vector
来源：https://stackoverflow.com/questions/7025054/ambiguous-injected-class-name-is-not-an-error