为什么在虚函数中转换为基类会出现分段错误？
我想使用运算符<<打印出派生类。当我打印派生类时，我想首先打印它的基类，然后打印它自己的内容。
但我遇到了一些麻烦（请参阅下面的段错误）：
class Base {
public:
friend std::ostream& operator<<(std::ostream&, const Base&);
virtual void Print(std::ostream& out) const {
out << "BASE!";
}
};
std::ostream& operator<<(std::ostream& out, const Base& b) {
b.Print(out);
return out;
}
class Derived : public Base {
public:
virtual void Print(std::ostream& out) const {
out << "My base: ";
//((const Base*)this)->Print(out); // infinite, calls this fct recursively
//((Base*)this)->Print(out); // segfault (from infinite loop?)
((Base)*this).Print(out); // OK
out << " ... and myself.";
}
};
int main(int argc, char** argv){
Derived d;
std::cout << d;
return 0;
}
为什么我不能以其中一种方式进行转换？
((const Base*)this)->Print(out); // infinite, calls this fct recursively
((Base*)this)->Print(out); // segfault (from infinite loop?)
I want to print out a derived class using the operator<<. When I print the derived class, I want to first print its base and then its own content.
But I ran into some trouble (see segfault below):
class Base {
public:
friend std::ostream& operator<<(std::ostream&, const Base&);
virtual void Print(std::ostream& out) const {
out << "BASE!";
}
};
std::ostream& operator<<(std::ostream& out, const Base& b) {
b.Print(out);
return out;
}
class Derived : public Base {
public:
virtual void Print(std::ostream& out) const {
out << "My base: ";
//((const Base*)this)->Print(out); // infinite, calls this fct recursively
//((Base*)this)->Print(out); // segfault (from infinite loop?)
((Base)*this).Print(out); // OK
out << " ... and myself.";
}
};
int main(int argc, char** argv){
Derived d;
std::cout << d;
return 0;
}
Why can't I cast in one of these ways?
((const Base*)this)->Print(out); // infinite, calls this fct recursively
((Base*)this)->Print(out); // segfault (from infinite loop?)
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评论（4）
尝试 Base::Print(out)
Try Base::Print(out)
对于为什么的问题：
((Base)*this).Print(out);将当前实例切片到Base-类型临时。这会导致直接调用基类方法。((const Base*)this)->Print(out);通过指向基类的指针调用虚方法，该方法解析为子类方法，从而导致无限递归。((Base*)this)->Print(out);- 我很确定这是未定义的行为。To the question of why:
((Base)*this).Print(out);slices current instance toBase-typed temporary. That results in a direct call to base class method.((const Base*)this)->Print(out);calls virtual method through a pointer to base, which resolves to the child class method, which results in infinite recursion.((Base*)this)->Print(out);- I'm pretty sure this is undefined behavior.请改用
Base::Print。虚函数被派生类重写。本质上你是在一遍又一遍地调用 print
use
Base::Printinstead. Virtual functions are overriden by derived class.In essence you are calling print over and over
我建议阅读 C++ Primer 第 5 卷第 607 页“规避虚拟机制”，这是一个简短的部分，讨论您所犯的那种无限递归错误。
I suggest reading C++ Primer 5th page 607 "Circumventing the Virtual Mechanism" which is a short section talking about the kind of infinite-recursion error you made.