c++ - 对象字段返回对象本身，而不是存储在字段中的实例

Question

(不确定这个标题是否是最好的描述...)

大家好。我需要以下代码的帮助。在 process 函数中，我希望 cc 和 cc_sub (作为 cc 中的字段获取)显示不同内容(因为它们最初是这样制作的)但它们显示相同。 Storage 实例是在 makeObj 函数中创建的。有人可以告诉我出了什么问题以及如何解决这个问题吗？谢谢。

Int process(const Storage cc) {
    Storage cc_sub = (Storage&) cc.data;  // Here
    cout << ... << endl;         // Contents from cc and cc_sub are the same though they are not supposed to be.
}

class Storage {
    public:
        Storage() {
            this->data = NULL;
        }
        Storage(const Storage& obj) { //Copy Constructor. Result is the same even if I removed this entire clause.
            this->data = obj.data;
        }
        void* data;
        void setData(Storage dPtr) {
            this->data = &dPtr;
        }
};

Storage makeObj(std::string* s) { // I want to keep the return value as value instead of pointer, 
                       // since when it returns pointer the values pointed by reference behaved unexpectedly (guess it's because of temporary object type of error)... 
    Storage cc;
    :
    cc.setData(makeObj(&subString)); 
    return cc;
    :
}

Answer 1

您看到的奇怪行为是由于操纵临时 Storage 造成的变量，都在 setData 内和 makeObj .这种操作的结果在 C++ 中是未定义的。

尝试以下操作:

void process(const Storage cc) {
    Storage cc_sub = *((Storage*)cc.data);
    cout << ... << endl;
}

class Storage {
    public:
        Storage() {
            this->data = NULL;
        }

        Storage(const Storage& obj) {
            this->data = obj.data;
        }
        void* data;
        void setData(Storage* dPtr) {
            this->data = dPtr;
        }
};

Storage* makeObj(std::string* s) {
    // Allocate memory on the heap so that cc lives beyond this function call
    Storage* cc = new Storage;
    :
    // Populate the sub-storage with a heap-allocated variable
    cc->setData(makeObj(&subString));
    return cc;
}

请注意，当前编写的代码 makeObj如果调用将导致堆栈溢出，因为它无法结束其递归...

根据 cHao 的建议，这里有一个使用 shared_ptr 的更完整的程序(Microsoft Visual Studio 2008 版本 - 您可能需要更改 #include <memory> 和/或 std::tr1::shared_ptr，具体取决于您的编译器)。我已将数据类型从 void* 更改为至 Storage ( shared_ptr ) 并添加了一个字符串成员以更好地显示差异:

#include <string>
#include <iostream>
#include <memory>

using namespace std;
using namespace std::tr1;

class Storage {
    public:
        Storage() {
        }
        Storage(const Storage& obj) {
            this->data = obj.data;
        }
    string s;
    shared_ptr<Storage> data;
        void setData(shared_ptr<Storage> dPtr) {
            this->data = dPtr;
        }
};

void process(const shared_ptr<Storage> cc) {
    shared_ptr<Storage> cc_sub = cc.get()->data;
    cout << "Parent string: " << cc.get()->s << endl << 
            "Child string: " << cc_sub.get()->s << endl;
    cout << "Parent data pointer: " << cc.get()->data << endl << 
            "Child data pointer: " << cc_sub.get()->data << endl;
}

shared_ptr<Storage> makeObj(string* s) {
    shared_ptr<Storage> cc(new Storage);
    if (s->length() != 0) {
        string subString = s->substr(0, s->length()-1);
        cc->s = subString;
        cc->setData(makeObj(&subString));
    }

    return cc;
}


int main(int argc, char* argv[]) {
    string s = "hello";
    shared_ptr<Storage> storage = makeObj(&s);
    process(storage);
    return 0;
}