重载抽象操作时锵链接错误=

Question

的VisualStudio的2013编译器处理下面的代码就好了，但铿锵5.0和6.2给了我一个链接错误：

#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; 
} 

下面是生成日志输出：

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&)在选自Base其中Derived继承所定义，并且是Derivedusing Base::operator=所以应被用于定义，未被默认赋值运算符覆盖。

我发现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，我想保留IBase::operator=方法。

Answer 1

的问题是，using声明不能算作一个用户声明赋值运算符[namespace.udecl]：

4 - [...] If an assignment operator brought from a base class into a derived class scope has the signature of a copy/move assignment operator for the derived class (12.8), the using-declaration does not by itself suppress the implicit declaration of the derived class assignment operator [...]

（在任何情况下，using Base::operator=为您提供了参数赋值运算符型Base const&，这是不参数类型资格作为拷贝赋值运算符之一[class.copy]/17 - T，T&，T const&等）

因为Derived没有用户声明的复制分配操作符，所以会自动生成一个，最终会调用IDerived::operator=，这会调用IBase::operator=。需要注意的是自动生成的拷贝赋值运算符调用子对象的拷贝赋值运算符忽略虚拟覆盖：

Each subobject is assigned in the manner appropriate to its type:

• if the subobject is of class type, as if by a call to operator= with the subobject as the object expression and the corresponding subobject of x as a single function argument (as if by explicit qualification; that is, ignoring any possible virtual overriding functions in more derived classes); [...]

一个解决将是写：

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

需要注意的是MSVC 2015年拒绝了您的代码，但上述工作修复：

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