Question

我想实现一个简单的二叉树，其中顶点包含lambda。

class Element {
public:
    Element() { }
    virtual int operator()();
};

class Node : public Element {
private:
    std::function<int(Element, Element)> fn;
    Element left;
    Element right;
public:
    Node(std::function<int(Element, Element)> _fn, Element _left, Element _right)
        : fn(_fn), left(_left), right(_right) { }
    int operator()() { return fn(left, right); }
};

class Leaf : public Element {
private:
    int value;
public:
    Leaf(int _value) : value(_value) { }
    int operator()() { return value; }
};

int main() {
cout << Node([](Element a, Element b) { return a() + b(); }, Leaf(2), Leaf(5))() << endl;
return 0;
}

以下是我得到的错误：

g++ -Wall -std=c++11 -Wextra -pedantic tree.cc -c
g++ -Wall -std=c++11 -Wextra -pedantic tree.o -o tree
tree.o: In function `main::{lambda(Element, Element)#1}::operator()(Element, Element) const':
tree.cc:(.text+0x18): undefined reference to `vtable for Element'
tree.cc:(.text+0x2a): undefined reference to `vtable for Element'
tree.o: In function `Element::Element()':
tree.cc:(.text._ZN7ElementC2Ev[_ZN7ElementC5Ev]+0xf): undefined reference to `vtable for Element'
tree.o: In function `Element::Element(Element const&)':
tree.cc:(.text._ZN7ElementC2ERKS_[_ZN7ElementC5ERKS_]+0x13): undefined reference to `vtable for Element'
tree.o: In function `Node::Node(std::function<int (Element, Element)>, Element, Element)':
tree.cc:(.text._ZN4NodeC2ESt8functionIFi7ElementS1_EES1_S1_[_ZN4NodeC5ESt8functionIFi7ElementS1_EES1_S1_]+0x4e): undefined reference to `vtable for Element'
tree.o:tree.cc:(.text._ZN4NodeC2ESt8functionIFi7ElementS1_EES1_S1_[_ZN4NodeC5ESt8functionIFi7ElementS1_EES1_S1_]+0x5a): more undefined references to `vtable for Element' follow
tree.o:(.rodata._ZTI4Leaf[_ZTI4Leaf]+0x10): undefined reference to `typeinfo for Element'
tree.o:(.rodata._ZTI4Node[_ZTI4Node]+0x10): undefined reference to `typeinfo for Element'
collect2: error: ld returned 1 exit status
make: *** [tree] Error 1

这是什么意思，是否存在类声明的问题，或者我不理解C ++中函数式编程的某些方面？

Answer 1

您尝试使用多态但没有指针/引用。这不起作用：Why doesn't polymorphism work without pointers/references?

有效

Element left;
Element right;

始终为Element类型（不是派生类型），您尝试在其上调用operator()。但是这个方法没有为Element实现，这是错误消息告诉你的。

Answer 2

您收到链接错误，因为您的基类Element缺少函数调用virtual int operator()();运算符重载函数的实现。

您设计的其他问题

对基类的所有引用都应该是const引用的'/ Element / const元素＆amp; /'
元素应该是ABC。函数调用重载应该是纯虚拟的。
函数调用重载应为const类型。

修改后的程序

#include <iostream>
#include <functional>
using namespace std;
class Element {
public:
    Element() { }
    virtual int operator()() const = 0;
};

class Node : public Element {
private:
    std::function<int(const Element&,const Element&)> fn;
    const Element& left;
    const Element& right;
public:
    Node(std::function<int(const Element&, const Element&)> _fn, const Element& _left, const Element& _right)
        : fn(_fn), left(_left), right(_right) { }
    int operator()() const { return fn(left, right); }
};

class Leaf : public Element {
private:
    int value;
public:
    Leaf(int _value) : value(_value) { }
    int operator()() const { return value; }
};

int main() {
    cout << Node([](const Element& a,const Element& b) { return a() + b(); }, Leaf(2), Leaf(5))() << endl;
    return 0;
}

链接错误 - 功能二叉树，派生类

2 个答案: