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Question

我有以下课程

class node {
public:
    node() { }
    node(const node&);
    node(luint inID) { ID = inID; }
    ~node() { neighbors.clear(); }

    node& operator=(const node&);
    void addNeighbor(luint);
    void addNeighbor(const std::vector<luint>& );
    void setID(luint inID) { ID = inID; }
    luint getID() const { return ID; }
    std::vector<luint> & getNeighbors() const { return neighbors; }
protected:
    luint ID;
    std::vector<luint> neighbors;
};
void node::addNeighbor(const std::vector<luint>& inID) {
    for(int i = 0; i < inID.size(); i++)
        neighbors.push_back( inID[i] );
}
// etc..

现在我得到的错误是

graph.h: In member function 'std::vector<long unsigned int, std::allocator<long unsigned int> >& node::getNeighbors() const': In file included from main.cpp:10:
graph.h:28: error: invalid initialization of reference of type 'std::vector<long unsigned int, std::allocator<long unsigned int> >&' from expression of type 'const std::vector<long unsigned int, std::allocator<long unsigned int> >'
make[2]: *** [build/Debug/GNU-MacOSX/main.o] Error 1
make[1]: *** [.build-conf] Error 2
make: *** [.build-impl] Error 2

另一方面，如果我在函数“getNeighbors（）”的声明中删除“const”，我得到错误

graph.cpp: In member function 'void graph::addNode(const node*)':
graph.cpp:36: error: request for member 'addNeighbor' in '((graph*)this)->graph::nodeMap. std::map<_Key, _Tp, _Compare, _Alloc>::operator[] [with _Key = long unsigned int, _Tp = node*, _Compare = std::less<long unsigned int>, _Alloc = std::allocator<std::pair<const long unsigned int, node*> >](((const long unsigned int&)((const long unsigned int*)(& inNode-> node::getID()))))', which is of non-class type 'node*'
graph.cpp:36: error: passing 'const node' as 'this' argument of 'std::vector<long unsigned int, std::allocator<long unsigned int> >& node::getNeighbors()' discards qualifiers
make[2]: *** [build/Debug/GNU-MacOSX/graph.o] Error 1

有谁知道如何解决这个问题？

提前谢谢你，

更新

class node {
public:
    node() { }
    node(const node&);
    node(luint inID) { ID = inID; }
    ~node() { neighbors.clear(); }

    node& operator=(const node&);
    void addNeighbor(luint);
    void addNeighbor(const std::vector<luint>& );
    void setID(luint inID) { ID = inID; }
    luint getID() const { return ID; }
    std::vector<luint> & getNeighbors() { return neighbors; }
    std::vector<luint> const & getNeighbors() const { return neighbors; }
protected:
    luint ID;
    std::vector<luint> neighbors;
};

class graph {
public:
    graph() { }
    ~graph() { }

    void addNode(const node*);
    void addNode(const node&);
protected:
    std::map<luint, node*> nodeMap;
};

void node::addNeighbor(luint inID) {
    neighbors.push_back(ID);
}

void node::addNeighbor(const std::vector<luint>& inID) {
    for(int i = 0; i < inID.size(); i++)
        neighbors.push_back( inID[i] );
}

void graph::addNode(const node* inNode) {
    nodeMap[inNode->getID()] = new node(inNode->getID());
    nodeMap[inNode->getID()].addNeighbor( inNode->getNeighbors() );
}

void graph::addNode(const node& inNode) {
    nodeMap[inNode.getID()] = new node(inNode.getID());
}

编译错误：

graph.cpp: In member function 'void graph::addNode(const node*)':
graph.cpp:36: error: request for member 'addNeighbor' in '((graph*)this)->graph::nodeMap. std::map<_Key, _Tp, _Compare, _Alloc>::operator[] [with _Key = long unsigned int, _Tp = node*, _Compare = std::less<long unsigned int>, _Alloc = std::allocator<std::pair<const long unsigned int, node*> >](((const long unsigned int&)((const long unsigned int*)(& inNode-> node::getID()))))', which is of non-class type 'node*'
make[2]: *** [build/Debug/GNU-MacOSX/graph.o] Error 1
make[1]: *** [.build-conf] Error 2
make: *** [.build-impl] Error 2

Answer 1

如果我看对了它应该是这样的：

std::vector<luint> const & getNeighbors() const { return neighbors; }

Answer 2

由于您的方法是const，因此它必须使用const this指针，这意味着您无法修改当前实例的任何字段（就像它们都是{ {1}}）;这也意味着您无法返回对const的非const引用，因为您只有neighbors访问该字段。

要解决此问题，您应该为const版const返回const引用（当客户端具有getNeighbors引用或指针时使用您的类的实例，以便它不能通过引用修改const的内容）和该方法的非neighbors版本的“普通”引用。这称为const-overloading。

const

<小时/> 对于新错误：您使用点运算符而不是箭头（访问指针引用的对象的成员所需）。

// Non-const overload
std::vector<luint> & getNeighbors() { return neighbors; }
// const overload
std::vector<luint> const & getNeighbors() const { return neighbors; }

Answer 3

const限定成员函数承诺不修改对象的状态。这是要记住的关键。

现在，如果这样的成员函数允许返回对成员的引用，使得调用者可以修改该成员，则该函数声明保持该对象不可变的承诺。

修复涉及将返回类型作为'const'的引用（std :: vector const）

Answer 4

我通过显式转换修改了代码并且它可以正常工作

class node {
    friend class graph;
    friend ostream& operator<< (ostream&, const node&);
public:
    node() { }
    node(const node&);
    node(luint inID) { ID = inID; }
    ~node() { neighbors.clear(); }

    node& operator=(const node&);
    void addNeighbor(luint);
    void addNeighbor(const std::vector<luint>& );
    luint getID() const { return ID; }


protected:
    luint ID;
    std::vector<luint> neighbors;
};

class graph {
    friend ostream& operator<< (ostream&, graph&);
public:
    graph() { }
    ~graph() { }

    void addNode(const node& );
    void addNode(const node* );
    void readFile(const char * );
    void clearMap();
protected:
    std::map<luint, node*> nodeMap;
};

void graph::addNode(const node& inNode) {
    nodeMap[ (const luint)inNode.ID ] = new node( (const luint) inNode.ID );
    nodeMap[ (const luint)inNode.ID ]->neighbors.resize( (const luint) inNode.neighbors.size() );
    *nodeMap[ (const luint)inNode.ID ] = inNode;
}

Answer 5

阅读此C++ FAQ entry。如果要向成员变量返回非const引用，则不能将addNeighbours声明为const。

const冲突

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5 个答案: