重载abstract operator =时Clang链接器错误

Question

VisualStudio 2013编译器处理以下代码就好了，但是clang 5.0和6.2给我一个链接器错误：

#include <memory>

using namespace::std;

class IBase
{
public:
    virtual IBase& operator=(const IBase& other) = 0;
};

class Base : virtual public IBase
{
public:
    Base& operator=(const IBase& other) override
    {
        const Base& b = dynamic_cast<const Base&>(other);
        return *this = b;
    }

    virtual Base& operator=(const Base& other)
    {
        return *this;
    }
};

class IDerived : virtual public IBase
{
};

class Derived : public IDerived, public Base
{
public:
    using Base::operator=;

};

int main(int argc, const char * argv[]) {
    shared_ptr<Derived> d1 = make_shared<Derived>();
    shared_ptr<Derived> d2 = make_shared<Derived>();
    *d2 = *d1;
}

这是构建日志输出：

Ld /Users/Jennifer/Library/Developer/Xcode/DerivedData/Operator-bjjgcoxcziyegjgmazknrandutqz/Build/Products/Debug/Oper normal x86_64
    cd /Users/Jennifer/Documents/Operator
    export MACOSX_DEPLOYMENT_TARGET=10.9
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -L/Users/Jennifer/Library/Developer/Xcode/DerivedData/Operator-bjjgcoxcziyegjgmazknrandutqz/Build/Products/Debug -F/Users/Jennifer/Library/Developer/Xcode/DerivedData/Operator-bjjgcoxcziyegjgmazknrandutqz/Build/Products/Debug -filelist /Users/Jennifer/Library/Developer/Xcode/DerivedData/Operator-bjjgcoxcziyegjgmazknrandutqz/Build/Intermediates/Operator.build/Debug/Oper.build/Objects-normal/x86_64/Oper.LinkFileList -mmacosx-version-min=10.9 -stdlib=libc++ -Xlinker -dependency_info -Xlinker /Users/Jennifer/Library/Developer/Xcode/DerivedData/Operator-bjjgcoxcziyegjgmazknrandutqz/Build/Intermediates/Operator.build/Debug/Oper.build/Objects-normal/x86_64/Oper_dependency_info.dat -o /Users/Jennifer/Library/Developer/Xcode/DerivedData/Operator-bjjgcoxcziyegjgmazknrandutqz/Build/Products/Debug/Oper

Undefined symbols for architecture x86_64:
  "IBase::operator=(IBase const&)", referenced from:
      IDerived::operator=(IDerived const&) in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

IBase::operator=(IBase const&)在Base中定义， Derived继承自， Derived using Base::operator=因此它应该为Derived定义，而不是由默认赋值运算符覆盖。

我找到的一个解决方案是删除IBase::operator=方法，但这并不理想，因为它是任何继承类需要实现的方法。

有谁知道有什么区别以及如何解决它？ 如果可能的话，我想保留IBase::operator=方法。

Answer 1

问题是using声明不算作用户声明的赋值运算符[namespace.udecl] ：

4 - [...]如果从基类带入赋值类作用域的赋值运算符具有派生类（12.8）的复制/移动赋值运算符的签名，则using-declaration本身不会抑制隐式派生类赋值运算符的声明[...]

（在任何情况下， using Base::operator=为您提供一个赋值运算符，其参数类型为Base const& ，它不是一个有资格作为复制赋值运算符的参数类型[class.copy] / 17 - T ， T& ， T const&等）

因为Derived没有用户声明的复制赋值运算符，所以会自动生成一个，最终调用IDerived::operator=调用IBase::operator= 。 请注意，自动生成的复制赋值运算符会调用忽略虚拟覆盖的子对象复制赋值运算符：

每个子对象都以适合其类型的方式分配：

• 如果子对象是类类型，就好像通过调用operator=将子对象作为对象表达式而x的对应子对象作为单个函数参数（就像通过显式限定;即忽略任何可能的虚拟覆盖函数在更多派生类中）; [...]

修复就是写：

    Base& operator=(Derived const& other) { return Base::operator=(other); }

请注意，MSVC 2015会拒绝您的代码，但可以使用上述修复程序：

main.cpp(36): warning C4250: 'Derived': inherits 'Base::Base::operator =' via dominance
main.cpp(14): note: see declaration of 'Base::operator ='
main.obj : error LNK2019: unresolved external symbol "public: virtual class IBase & __thiscall IBase::operator=(class IBase const &)" (??4IBase@@UAEAAV0@ABV0@@Z) referenced in function "public: class IDerived & __thiscall IDerived::operator=(class IDerived const &)" (??4IDerived@@QAEAAV0@ABV0@@Z)
main.exe : fatal error LNK1120: 1 unresolved externals