使用clang或gcc(在macOS上)编译时，以下代码似乎运行良好，但使用MS Visual C++ 2017编译时，以下代码崩溃。 foo_clone->get_identifier()。

如果删除协变返回类型(所有克隆方法都返回IDO*)，删除std::enable_shared_from_this或将所有继承设为虚拟，则它在VC++上确实有效。

为什么它可以与clang/gcc一起使用，但不适用于VC++？

#include <memory>
#include <iostream>

class IDO {
public:
    virtual ~IDO() = default;

    virtual const char* get_identifier() const = 0;

    virtual IDO* clone() const = 0;
};

class DO
    : public virtual IDO
    , public std::enable_shared_from_this<DO> 
{
public:
    const char* get_identifier() const override { return "ok"; }
};

class D : public virtual IDO, public DO {
    D* clone() const override {
        return nullptr;
    }
};

class IA : public virtual IDO {};

class Foo : public IA, public D {
public:
    Foo* clone() const override {
        return new Foo();
    }
};

int main(int argc, char* argv[]) {
    Foo* foo = new Foo();
    Foo* foo_clone = foo->clone();
    foo_clone->get_identifier();
}

最佳答案

这似乎是VC++的错误编译。当enable_shared_from_this不存在时，它消失了。问题只是被掩盖了。

一些背景:解决C++中的重写函数通常是通过vtables进行的。但是，在存在多个虚拟继承和协变量返回类型的情况下，必须解决一些挑战以及满足这些挑战的不同方式。

考虑:

Foo* foo = new Foo();
IDO* ido = foo;
D* d = foo;

foo->clone(); // must call Foo::clone() and return a Foo*
ido->clone(); // must call Foo::clone() and return an IDO*
d->clone(); // must call Foo::clone() and return a D*

0: destructor
1: const char* get_identifier() const
2: IDO* clone() const

0: destructor
1: const char* get_identifier() const
2: IDO* clone() const
3: D* clone() const

0: destructor
1: const char* get_identifier() const
2: IDO* clone() const
3: D* clone() const
4: Foo* clone() const
5: Foo* clone() const

D* Foo::clone$D() const {
  Foo* real = clone();
  return static_cast<D*>(real);
}

foo->clone();

Foo* wtf = reinterpret_cast<Foo*>(
  reinterpret_cast<char*>(foo_clone) + 8);

class Foo   size(40):
    +---
 0  | +--- (base class IA)
 0  | | {vbptr}
    | +---
 8  | +--- (base class D)
 8  | | +--- (base class DO)
 8  | | | +--- (base class std::enable_shared_from_this<class DO>)
 8  | | | | ?$weak_ptr@VDO@@ _Wptr
    | | | +---
24  | | | {vbptr}
    | | +---
    | +---
    +---
    +--- (virtual base IDO)
32  | {vfptr}
    +---

Foo::$vbtable@IA@:
 0  | 0
 1  | 32 (Food(IA+0)IDO)

Foo::$vbtable@D@:
 0  | -16
 1  | 8 (Food(DO+16)IDO)

Foo::$vftable@:
    | -32
 0  | &Foo::{dtor}
 1  | &DO::get_identifier
 2  | &IDO* Foo::clone
 3  | &D* Foo::clone
 4  | &Foo* Foo::clone
 5  | &Foo* Foo::clone

Foo::clone this adjustor: 32
Foo::{dtor} this adjustor: 32
Foo::__delDtor this adjustor: 32
Foo::__vecDelDtor this adjustor: 32
vbi:       class  offset o.vbptr  o.vbte fVtorDisp
             IDO      32       0       4 0

class Foo   size(24):
    +---
 0  | +--- (base class IA)
 0  | | {vbptr}
    | +---
 8  | +--- (base class D)
 8  | | +--- (base class DO)
 8  | | | {vbptr}
    | | +---
    | +---
    +---
    +--- (virtual base IDO)
16  | {vfptr}
    +---

Foo::$vbtable@IA@:
 0  | 0
 1  | 16 (Food(IA+0)IDO)

Foo::$vbtable@D@:
 0  | 0
 1  | 8 (Food(DO+0)IDO)

Foo::$vftable@:
    | -16
 0  | &Foo::{dtor}
 1  | &DO::get_identifier
 2  | &IDO* Foo::clone
 3  | &D* Foo::clone
 4  | &Foo* Foo::clone
 5  | &Foo* Foo::clone

Foo::clone this adjustor: 16
Foo::{dtor} this adjustor: 16
Foo::__delDtor this adjustor: 16
Foo::__vecDelDtor this adjustor: 16
vbi:       class  offset o.vbptr  o.vbte fVtorDisp
             IDO      16       0       4 0

      mov         rcx,qword ptr [this]  
      call        Foo::clone ; the real clone  
      cmp         rax,0 ; null pointer remains null pointer
      je          fin
      add         rax,8 ; otherwise, add the offset to the D*
      jmp         fin
fin:  ret

mov         rax,qword ptr [foo]  
mov         rcx,rax  
mov         rax,qword ptr [rax] ; load vbptr  
movsxd      rax,dword ptr [rax+4] ; load offset to IDO subobject 
add         rcx,rax  ; add offset to Foo* to get IDO*
mov         rax,qword ptr [rcx]  ; load vtbl
call        qword ptr [rax+24]  ; call function at position 3 (D* clone)

mov         rax,qword ptr [foo_clone]  
mov         rcx,rax  
mov         rax,qword ptr [rax] ; load vbptr, loads null in the crashing case
movsxd      rax,dword ptr [rax+4] ; load offset to IDO subobject, crashes
add         rcx,rax  ; add offset to Foo* to get IDO*
mov         rax,qword ptr [rcx]  ; load vtbl
call        qword ptr [rax+8]  ; call function at position 1 (get_identifier)

关于c++ - 具有协变返回类型的方法在VC++上崩溃，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/47527366/