c中的空对象

概述我试图检查船对象是否为空,但我收到一条错误消息 Crane.cpp:18: error: could not convert ‘((Crane*)this)->Crane::ship.Ship::operator=(((const Ship&)(& Ship(0, std::basic_string, std::allocator >(((const char*)”arrive”), ((cons 我试图检查船对象是否为空,但我收到一条错误消息

Crane.cpp:18: error: Could not convert ‘((Crane*)this)->Crane::ship.Ship::operator=(((const Ship&)(& Ship(0,std::basic_string,std::allocator >(((const char*)”arrive”),((const std::allocator&)((const std::allocator)(& std::allocator())))),std::allocator >(((const char)”Ship”),((const std::allocator&)((const std::allocator*)(& std::allocator()))))))))’ to ‘bool’

Crane::Crane(int craneID,int cranestatus,bool free,Ship ship){    setCraneID(craneID);    setCranestatus(cranestatus);    setFree(free);    setShip(ship);}Crane::Crane(){}Crane::~Crane(){}voID Crane::print(){    cout << "Crane ID: " << craneID << endl;    cout << "Crane Status: " << cranestatus << endl;    cout << "Crane is free: " << free << endl;    if (ship = NulL) //this is the problem    {        cout << " " << endl;    }    else    {        ship.print();//i have another print method in the Ship class    }}

我试过了

if (ship == NulL)

但我收到此错误消息

Crane.cpp:18: error: no match for ‘operator==’ in ‘((Crane*)this)->Crane::ship == 0’

怎么做对吗？

解决方法 那是因为ship不是Ship的指针,即Sh​​ip *,但它本身就是Ship对象;那么你不能将它转换为0,这是…指针的空地址.

如果你想要一个指向Ship的指针你应该这样做

ShiP* ship = new Ship;// Catch a std::bad_alloc exception if new fails

然后,如果您将指针作为函数参数获取,则可以测试它是否为null：

voID foo(ShiP* ship_pointer){    if(ship_pointer == 0)        // Oops pointer is null...    else        // Guess it's OK and use it.}

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