c++ - g++编译错误 "... is protected from within this context"，而clang没有错误

Question

我有以下代码:

#include <iostream>

class BaseClass {
 protected:
 static int x;
};

int BaseClass::x;

class DerivedA: public BaseClass {
 public:
     DerivedA() {
        x = 3;
     }
};    

class DerivedB: public BaseClass {
 public:
     DerivedB() {
        std::cout << DerivedA::x;
     }
};

int main(int argc, char* argv[]) {
        DerivedB b;
}

使用 g++ 编译 (g++ classtest.cpp) 我收到以下错误:

classtest.cpp: In constructor ‘DerivedB::DerivedB()’:
classtest.cpp:9:5: error: ‘int BaseClass::x’ is protected
int BaseClass::x;
^ classtest.cpp:25:32: error: within this context
std::cout << DerivedA::x;

当我使用 clang++ (clang++ classtest.cpp) 编译时没有错误。

为什么 g++ 返回编译错误？

我使用 g++ 5.1.0 版和 clang++ 3.6.1 版

Answer 1

GCC 错误。 [class.access.base]/p5:

A member m is accessible at the point R when named in class N if

• m as a member of N is public, or
• m as a member of N is private, and R occurs in a member or friend of class N, or
• m as a member of N is protected, and R occurs in a member or friend of class N, or in a member of a class P derived from N, where m as a member of P is public, private, or protected, or
• there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B.

N 是 DerivedA，m 是 x，R 是构造函数DeriveddB。存在 DerivedA 的基类 BaseClass，可在 R 访问，x 在类 中命名BaseClass(即，BaseClass::x)可以在 R 上清楚地访问，所以到第四个要点，DerivedA::x 可通过 R 访问。