在定义模板类、操作符和迭代器时遇到麻烦

Question

所以我试图定义一个模板类"TwoWayVector"one_answers"TwoWayVectorIterator"，但我遇到了很多问题。我想在TwoWayVector中定义==来返回一个引用，并将const TwoWayVector作为参数，这就是我定义其他操作符的方式，g++也没有抱怨，但由于某种原因，下面的代码产生了错误

TwoWayVector.cc: In member function ‘bool& TwoWayVector<T>::operator==(TwoWayVector<T>)         [with T = int]’:
Test.cc:10:   instantiated from here
TwoWayVector.cc:40: error: passing ‘const TwoWayVector<int>’ as ‘this’ argument of ‘T&       TwoWayVector<T>::operator[](int) [with T = int]’ discards qualifiers
Test.cc:10:   instantiated from here
TwoWayVector.cc:32: warning: reference to local variable ‘result’ returned

Test.cc

#include <iostream>
#include "TwoWayVector.cc"
int main(){
TwoWayVector<int> numbers;
TwoWayVector<int> numbers2;
numbers.push_back(3);
numbers.push_back(2);
numbers2.push_back(3);
numbers2.push_back(2);
cout << (numbers==numbers2);
cout << endl;
return 0;
}

TwoWayVector.cc

using namespace std;
#include "TwoWayVectorIterator.cc"
template <class T> class TwoWayVector{
public:
T* data;
int capacity;
int nextFree;
TwoWayVector(){
    capacity = 10;
    nextFree = 0;
    data = new T[capacity];
}
~TwoWayVector(){
    delete data;
}
T& operator[](const int index){
    if( index >= capacity || capacity + index < 0){
        string number = static_cast<ostringstream*>( &(ostringstream() << index) )->str();
        string error = "index " + number + " is out of bounds";
        throw error;
    }
    else if(index < 0){
        return data[nextFree+index];
    }
    return  data[index];
}
bool& operator==(const TwoWayVector vector2){
    bool result = true;
    if(capacity != vector2.capacity){
        result = false;
    }
    if(nextFree != vector2.nextFree){
        result = false;
    }
    for(int i=0; i<nextFree ; i++){
        if(data[i] != vector2[i]){
            result = false;
        }
    }
    return result;
}
 //memory leaks?
void push_back(T object){
    if(capacity <= nextFree){
        capacity = capacity*2;
        T* tmp = new T[capacity];
        for(int i=0; i<capacity; i++){
            tmp[i] = data[i];
        }
        delete data;
        data = tmp;
    }
    data[nextFree] = object;
    nextFree++;
}
T pop_back(){
    nextFree--;
    T result = data[nextFree];
    data[nextFree] = NULL;
    return result;
}
int size(){
    return nextFree;
}
TwoWayVectorIterator<T>* begin(){
    TwoWayVectorIterator<T>* i = new TwoWayVectorIterator<T>(0,this);
    return (i);
}
TwoWayVectorIterator<T>* end(){
    TwoWayVectorIterator<T>* i = new TwoWayVectorIterator<T>(nextFree,this);
    return(i);
}
};

TwoWayVectorIterator.cc

#include <sstream>
using namespace std;
template<typename T> class TwoWayVector;
template <class T> class TwoWayVectorIterator{
public:
TwoWayVector<T>* vector;
int currentPosition;
TwoWayVectorIterator(TwoWayVector<T>& vec){
    currentPosition = 0;
    vector = vec;
}
TwoWayVectorIterator( int pos , TwoWayVector<T>* vec){
    currentPosition = pos;
    vector = vec;
}
bool& operator==(const TwoWayVectorIterator vector2){
    bool contents, position;
    contents = (vector == vector2) ? true : false;
    position =(currentPosition == vector2->currentPosition) ? true : false;
    return (contents && position);
}
bool& operator!=(const TwoWayVectorIterator vector2){
    bool contents, position;
    contents = (vector == vector2) ? false : true;
    position=(currentPosition == vector2->currentPosition) ? false : true;
    return (contents || position);
}
TwoWayVectorIterator& operator++(){
    return *this;
    currentPosition = (currentPosition+1);
}
TwoWayVectorIterator& operator++(int){
    currentPosition = (currentPosition+1);
    return *this;
}
TwoWayVectorIterator& operator=(TwoWayVectorIterator* vector2){
    &vector = vector2;
    currentPosition = vector2->currentPosition;
    return *this;
}
TwoWayVectorIterator& operator+(int n){
    currentPosition = currentPosition+n;
    return *this;
}
TwoWayVectorIterator& operator-(int n){
    currentPosition = currentPosition-n;
    return *this;
}
bool& operator<(TwoWayVectorIterator* vector2){
    return (currentPosition<vector2->currentPosition);
}
T& operator*(){
    return vector[currentPosition];
}
};

如果我改变TwoWayVector中的==运算符定义。抄送至

bool operator==(TwoWayVector vector2){
    bool result = true;
    if(capacity != vector2.capacity){
        result = false;
    }
    if(nextFree != vector2.nextFree){
        result = false;
    }
    for(int i=0; i<nextFree ; i++){
        if(data[i] != vector2[i]){
            result = false;
        }
    }
    return result;
}

然后一切都编译了，但是当我运行它时，我得到

1
a.out(40908) malloc: *** error for object 0x7fe4f2c03b40: pointer being freed was not   allocated
*** set a breakpoint in malloc_error_break to debug
Abort trap: 6

任何想法?

Answer 1

在operator==内部调用operator[]，它是非const的，在const变量vector2上。

你应该添加一个操作符的只读版本:

const T & operator[](const int index) const;

但是你应该像这样使用object形参的引用:

bool operator==(const TwoWayVector &vector2) const;

否则，const关键字并没有真正做很多事情，因为它只是说对象的副本，它只是在函数的堆栈上创建的，不能修改，这并不重要。(因此，解决这个问题的最简单方法是从vector2中删除const关键字，但这并不完全正确。)

当然，不要返回bool值作为引用，因为它引用了变量result，一旦您离开函数，该变量将不再存在。

Answer 2

基本上由于缺乏复制构造函数而发生崩溃。参见三法则。当

bool operator==(TwoWayVector vector2){

在main中使用，创建vector的副本，保存numbers2的指针，然后在退出操作符==时删除副本，使numbers2具有指向已释放内存的指针，在退出main()时试图删除这些指针，这会导致错误。