由于类型删除的原因,我有一个模板 A<T>
可以保存任何数据类型。当 A
持有一个从 Derived
派生的多态类型 Base
并且我将它转换为 A<Base>
时,GCC 的未定义行为清理器会报告运行时错误:
#include <iostream>
struct I
{
virtual ~I() = default;
};
template<typename T>
struct A : public I
{
explicit A(T&& value) : value(std::move(value)) {}
T& get() { return value; }
private:
T value;
};
struct Base
{
virtual ~Base() = default;
virtual void fun()
{
std::cout << "Derived" << std::endl;
}
};
struct Derived : Base
{
void fun() override
{
std::cout << "Derived" << std::endl;
}
};
int main()
{
I* a_holding_derived = new A<Derived>(Derived());
A<Base>* a_base = static_cast<A<Base>*>(a_holding_derived);
Base& b = a_base->get();
b.fun();
return 0;
}
编译并运行
$ g++ -fsanitize=undefined -g -std=c++11 -O0 -fno-omit-frame-pointer && ./a.out
输出:
main.cpp:37:62: runtime error: downcast of address 0x000001902c20 which does not point to an object of type 'A'
0x000001902c20: note: object is of type 'A<Derived>'
00 00 00 00 20 1e 40 00 00 00 00 00 40 1e 40 00 00 00 00 00 00 00 00 00 00 00 00 00 21 00 00 00
^~~~~~~~~~~~~~~~~~~~~~~
vptr for 'A<Derived>'
#0 0x400e96 in main /tmp/1450529422.93451/main.cpp:37
#1 0x7f35cb1a176c in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2176c)
#2 0x400be8 (/tmp/1450529422.93451/a.out+0x400be8)
main.cpp:38:27: runtime error: member call on address 0x000001902c20 which does not point to an object of type 'A'
0x000001902c20: note: object is of type 'A<Derived>'
00 00 00 00 20 1e 40 00 00 00 00 00 40 1e 40 00 00 00 00 00 00 00 00 00 00 00 00 00 21 00 00 00
^~~~~~~~~~~~~~~~~~~~~~~
vptr for 'A<Derived>'
#0 0x400f5b in main /tmp/1450529422.93451/main.cpp:38
#1 0x7f35cb1a176c in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2176c)
#2 0x400be8 (/tmp/1450529422.93451/a.out+0x400be8)
Derived
live example on coliru
我有两个问题:
A<Derived>
到 A<Base>
的有效转换会是什么样子? 最佳答案
问题是 A<Base>
和 A<Derived>
彼此之间根本没有任何关系。它们的表示方式可能完全不同。对于您尝试执行的类型转换,A<Base>
是 A<Derived>
的基类是必要的,但显然并非如此。
看起来,您想要创建类似智能指针的东西,它的行为类似于值类型。顺便说一句,我不确定是否可以创建支持所有必要转换的值类型。如果在需要支持转换的类型组中有特定需求或已知的公共(public)基类,则可以实现相应的类。
关于c++ - 类型转换包含多态类型的模板时的未定义行为,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/34371058/