带有继承和强制转换的段错误

Question

我使用的是现有的C++，看起来像这样

class Geometry {
public:
enum {BOX, MESH} type;
std::string name;
};
class Mesh : public Geometry {
public:
std::string filename;
};
std::shared_ptr<Geometry> getGeometry() {
std::shared_ptr<Geometry> geom = std::make_shared<Geometry>();
geom->name = "geometry";
Mesh *mesh = new Mesh();
mesh->name = "name";
mesh->filename = "filename";
mesh->type = Geometry::MESH;
geom.reset(mesh);
return geom;
}

我需要扩展这些类提供的功能，所以我派生了这两个类

#include <memory>
#include <iostream>
class Geometry {
public:
enum {BOX, MESH} type;
std::string name;
};
class Mesh : public Geometry {
public:
std::string filename;
};
std::shared_ptr<Geometry> getGeometry() {
std::shared_ptr<Geometry> geom = std::make_shared<Geometry>();
geom->name = "geometry";
Mesh *mesh = new Mesh();
mesh->name = "name";
mesh->filename = "filename";
mesh->type = Geometry::MESH;
geom.reset(mesh);
return geom;
}
class GeometryDerived : public Geometry {
public:
void consumeGeometry() {
Geometry geom = static_cast<Geometry>(*this);
std::cout << geom.name << std::endl;
if(geom.type == Geometry::MESH) {
Mesh *mesh = static_cast<Mesh*>(&geom);
std::cout << mesh->filename << std::endl;
// mesh->get_output();
}
}
};
class MeshDerived : public Mesh {
public:
std::string get_output() {
return this->filename;        
}
};

int main(int argc, char** argv)
{
std::shared_ptr<Geometry> geom = getGeometry(); // this object is returned by the library
std::shared_ptr<GeometryDerived> geomDerived = std::static_pointer_cast<GeometryDerived>(geom);
geomDerived->consumeGeometry();
return 0;
}

我segfault一致(在我实际的代码,其垃圾字符串)

std::cout << mesh->filename << std::endl;

，但其他ints和doubles打印正确。甚至geom.name也打印正确。这里的弦是怎么回事?什么是正确的方式来投射对象在这里?两次强制转换对象看起来很难看。下面是GDB的输出

GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
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<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./cpp-example...
(gdb) run
Starting program: /home/user/cpp-example
name
Program received signal SIGSEGV, Segmentation fault.
__memmove_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:383
383 ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S: No such file or directory.
(gdb) bt
#0  __memmove_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:383
#1  0x00007ffff7c247b2 in _IO_new_file_xsputn (n=140737488346512, data=0xdb8bbc853a3b3400, f=<optimized out>) at fileops.c:1236
#2  _IO_new_file_xsputn (f=0x7ffff7d7e6a0 <_IO_2_1_stdout_>, data=0xdb8bbc853a3b3400, n=140737488346512) at fileops.c:1197
#3  0x00007ffff7c18541 in __GI__IO_fwrite (buf=0xdb8bbc853a3b3400, size=1, count=140737488346512, fp=0x7ffff7d7e6a0 <_IO_2_1_stdout_>) at libioP.h:948
#4  0x00007ffff7ed2824 in std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) ()
from /lib/x86_64-linux-gnu/libstdc++.so.6
#5  0x0000555555556858 in GeometryDerived::consumeGeometry (this=0x55555556def0) at ../downcasting.cpp:23
#6  0x0000555555556513 in main (argc=1, argv=0x7fffffffdec8) at ../downcasting.cpp:53

编辑1:下面的更改可以正常工作，但这不是我想要的

int main(int argc, char** argv)
{
std::shared_ptr<Geometry> geom = getGeometry();
if(geom->type == Geometry::MESH) {
std::shared_ptr<MeshDerived> meshDerived = std::static_pointer_cast<MeshDerived>(geom);
std::cout << meshDerived->filename << std::endl;
}
// std::shared_ptr<GeometryDerived> geomDerived = std::static_pointer_cast<GeometryDerived>(geom);
// geomDerived->consumeGeometry();
return 0;
}

Answer 1

你的代码有多个问题:

• 在GeometryDerived::consumeGeometry中，Geometry geom = static_cast<Geometry>(*this);通过切片当前对象创建了一个Geometry类型的新对象。稍后在该函数中执行Mesh *mesh = static_cast<Mesh*>(&geom);，它返回一个无效指针(因为geom不是Mesh)。通过该指针访问数据是UB。(由于大多数编译器使用的内存布局，它可能适用于大多数实现中在Geometry中声明的数据成员;没有定义良好的行为。)

• static_pointer_cast不检查指向对象是否确实是请求的类型。即使对象是不同的类型，强制转换也会成功，但是返回的指针无效。getGeometry函数返回一个指向Mesh对象的指针，该对象与GeometryDerived无关，因此强制转换返回一个无效指针。

---

对Edit 1的注释部分。不，这一行也是错误的:

std::shared_ptr<MeshDerived> meshDerived = std::static_pointer_cast<MeshDerived>(geom);

原因和我上面提到的一样:getGeometry返回一个指向Mesh的指针，而不是MeshDerived。这段代码可能会在大多数编译器上工作，因为MeshDerived与Mesh具有相同的内存布局，但这仍然不是一个有效的c++。

看起来你想要"添加方法"到运行时中的现有类。这在c++中是不可能的。要么你必须在主类(例如Mesh)中定义你需要的方法，要么使用访问者模式，即在一个单独的类中定义新功能，通过Mesh等的公共成员对其进行操作。