我在C 中创建一个队列类,并且难以使前部功能正常工作。它应该在队列中打印第一个节点的值。我的queue.cpp课程在这里
#include "queue.h"
#include <stdlib.h>
#include <iostream>
using namespace std;
queue::queue()
{
front_p = NULL;
back_p = NULL;
current_size = 0;
}
void queue::enqueue(int item)
{
node newnode = node(item, NULL);
if (front_p == NULL) //queue is empty
{
front_p = &newnode;
back_p = &newnode;
}
else
{
back_p->next = &newnode;
back_p = &newnode;
}
current_size ++;
}
int queue::dequeue()
{
//if there is only one node
if (front_p == back_p)
{
front_p = NULL;
back_p = NULL;
}
//if there are two or more
else
front_p = front_p->next;
current_size --;
}
int queue::front()
{
if (front_p != NULL)
return (*front_p).data;
}
bool queue::empty()
{
if (front_p == NULL && back_p == NULL)
return true;
else
return false;
}
int queue::size()
{
return current_size;
}
我的标题文件(queue.h)在这里
class queue
{
public:
queue(); // constructor - constructs a new empty queue.
void enqueue( int item ); // enqueues item.
int dequeue(); // dequeues the front item.
int front(); // returns the front item without dequeuing it.
bool empty(); // true iff the queue contains no items.
int size(); // the current number of items in the queue.
int remove(int item); // removes all occurrances of item
// from the queue, returning the number removed.
private:
class node // node type for the linked list
{
public:
node(int new_data, node * next_node ){
data = new_data ;
next = next_node ;
}
int data ;
node * next ;
};
node* front_p ;
node* back_p ;
int current_size ; // current number of elements in the queue.
};
测试程序(tester.cpp)
#include <iostream>
#include "queue.h"
#include <stdlib.h>
using namespace std;
int main(int argc, char * const argv[])
{
queue q1;
q1.enqueue(5);
cout << "front: " << q1.front() << endl;
cout << "front: " << q1.front() << endl;
cout << "front: " << q1.front() << endl;
q1.enqueue(10);
cout << "front: " << q1.front() << endl;
cout << "front: " << q1.front() << endl;
cout << "size: " << q1.size() << endl;
}
makefile
all: tester
tester: queue.o tester.o
g++ tester.o queue.o -o tester
tester.o: tester.cpp
g++ -c tester.cpp
queue.o: queue.cpp queue.h
g++ -c queue.cpp
clean:
rm -f tester *.o
当我运行测试程序时,我会得到:
front: 5
front: 6299744
front: 6299744
front: 10
front: 6299744
size: 2
您可以看到,在第一个入选之后,正面返回了应有的内容,即队列的前部值。但是之后,它返回了一些奇怪的数字,我不知道它来自哪里!然后,当我再次出入时,它再次打印好。只有在呼叫前两次之后,才开始打印混乱的值。谁能帮我了解发生了什么事?
您的程序陷入不确定的行为,因为您对内存的指针不拥有:
void queue::enqueue(int item)
{
node newnode = node(item, NULL);
if (front_p == NULL) //queue is empty
{
front_p = &newnode;
back_p = &newnode;
}
else
{
back_p->next = &newnode;
back_p = &newnode;
}
current_size ++;
}
在功能末尾,newnode
被破坏,但是front_p
和back_p
仍指向该内存位置。要么动态分配:
node* newnode = new node(item, NULL);
或使用std::shared_ptr<node>
。