将临时对象作为对抽象的引用传递给构造函数 [英] Passing temporary object as a reference to an abstract to a constructor
问题描述
我对C ++有点生疏,遇到了一些麻烦。
I'm a bit rusty on my C++ and have run into a bit of a snag.
我有一个类,例如 user
为其构造函数引用一个抽象类。理想情况下,我希望该类支持以下语法:
I have a class, in the example user
which takes a reference to an abstract class for its constructor. Ideally, I'd like for the class to support the following syntax:
User u( Sub("") );
u.work();
我有一个抽象类 super
基本类称为 sub
。 user
类在其构造函数中引用了super。在运行时,用户
传递了对 sub
的临时引用。
I have an abstract class super
with a base class called sub
. The user
class takes a reference to super in its constructor. At runtime, user
is passed a temporary reference to sub
.
调用 u.work()
时出现异常-因为临时目录已被解构。 (奇怪的是,如果删除 super
和 sub
上的析构函数,我会得到不同的行为。)
When u.work()
is called I get an exception - because the temporary has been deconstructed. (Oddly enough I get a different behaviour if I remove the destructors on super
and sub
).
下面的示例代码!
是否可以解决此问题?
我知道如果我接受更改签名并在呼叫站点更改语法的方式很多,但是我更喜欢保留调用签名:
I know there are lots of ways if I accept to change the signature and change the syntax at the call site, but I'd prefer to hold on to the calling signature:
User u( Sub("") );
u.work();
示例代码
class Super {
public:
Super()
{
std::cout << __FUNCTION__ << std::endl;
};
virtual ~Super()
{
std::cout << __FUNCTION__ << std::endl;
}
virtual void func() = 0;
};
class Sub : public Super
{
public:
Sub(char* name) :
name_(name)
{
std::cout << __FUNCTION__ << std::endl;
};
virtual ~Sub()
{
std::cout << __FUNCTION__ << std::endl;
}
virtual void func()
{
std::cout << __FUNCTION__ << std::endl;
}
char* name_;
};
class User
{
public:
User(Super& s) :
super_(s)
{
std::cout << __FUNCTION__ << std::endl;
}
~User()
{
std::cout << __FUNCTION__ << std::endl;
}
void work()
{
std::cout << __FUNCTION__ << std::endl;
super_.func();
}
Super& super_;
};
int main()
{
User u( Sub("") );
u.work();
return 0;
}
推荐答案
临时对象的生存期就像您的 Sub()
终止于它所在的完整表达式的末尾,即分号。
The lifetime of a temporary object like your Sub("")
ends at the end of the full-expression in which it appears, i.e. at the semicolon.
C ++不允许您将此类临时对象绑定到可变引用,但是Microsoft的编译器允许这样做。但是,从这样的临时值初始化类成员引用会立即产生一个悬空引用,您永远不能再使用它。
C++ doesn't allow you to bind such temporary objects to mutable references, but Microsoft's compiler does allow that. Nonetheless, initializing a class member reference from such a temporary value immediately produces a dangling reference, which you must never ever use again; evaluating it outside the constructor body results in undefined behaviour.
解决此问题的方法是不在类中存储引用!
The way to fix this is to not store references in your class!
例如,如果要拥有一个多态值,则可以使用唯一的指针:
For example, if you wanted ownership of a polymorphic value, you could use a unique pointer:
#include <memory>
class User
{
std::unique_ptr<Super> s_;
public:
User(std::unique_ptr<Super> s) : s_(std::move(s)) {}
void work() { s_->func(); }
};
User u(std::make_unique<Sub>("Hello"));
u.work();
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