C ++函数返回指针到结构变得很奇怪 [英] C++ Function Return Pointer to Struct Goes Weird

问题描述

我正在编写一个任务，创建一个程序，可以使用链表从文本文件中读取多项式。其中，当我试图从read_poly函数返回链表poly_pointer的开始指针很奇怪。

预期输出应为-12

但是我得到的是-10

如果我在read_poly中返回前添加一行代码

  cout< curr_ptr-> coef; 
  

输出将突然变为2-12
任何人都可以提供一些解释为什么和如何解决这个问题？

Polynomial.h

  #ifndef _POLYNOMIAL_H_ 
 #define _POLYNOMIAL_H_ 
 
 using namespace std; 
 
 typedef struct poly_node * poly_pointer; 
 
 typedef struct poly_node {
 int coef; 
 int expon; 
 poly_pointer link; 
}; 
 
 poly_pointer addNode（int coef，int expon）; 
 
 #endif 
  

Polynomial.cpp

  #include< iostream> 
 #include< fstream> 
 #include< string> 
 #includePolynomial.h
 using namespace std; 
 
 poly_pointer addNode（int coef，int expon）
 {
 poly_node a; 
 poly_pointer ptr =& a; 
 a.coef = coef; 
 a.expon = expon; 
 return ptr; 
} 
 
 poly_pointer read_poly（const char * fileName）
 {
 poly_pointer start_ptr，curr_ptr; 
 start_ptr = curr_ptr = addNode（-1，6）; 
 curr_ptr = curr_ptr-> link = addNode（2，3）; 
 return start_ptr; 
} 
  

main.cpp

  #include< iostream> 
 #includePolynomial.h
 using namespace std; 
 
 int main（void）
 {
 poly_pointer a，b，d，e，f; 
 a = read_poly（input1.txt）; 
 cout<< a-> coef; 
 cout<< a-> link-> coef; 
 
 cout<< \\\
-eop-; 
 cin.get（）; 
 return 0; 
} 
  

解决方案

addNode本地分配的poly_node。

以下是内存的稀疏分配，但会工作。

  poly_pointer a = new poly_pointer（）; 
 a-> coef = coef; 
 a-> expon = expon; 
 return a; 
  

I am working on an assignment which to create a program that can read a polynomial from a text file using linked list. Which when i tried to return the starting pointer of the linked list "poly_pointer" from the read_poly function things being weird.

The expected output should be -12

But what i got is -10

And if i add one single line of code right before return in read_poly

cout << curr_ptr->coef;

the output would suddenly turns to 2-12 May anyone provide some explanation on why and how to fix this problem?

Polynomial.h

#ifndef _POLYNOMIAL_H_
#define _POLYNOMIAL_H_

using namespace std;

typedef struct poly_node *poly_pointer;

typedef struct poly_node {
  int coef;
  int expon;
  poly_pointer link;
};

poly_pointer addNode(int coef, int expon);

#endif

Polynomial.cpp

#include <iostream>
#include <fstream>
#include <string>
#include "Polynomial.h"
using namespace std;

poly_pointer addNode(int coef, int expon)
{
    poly_node a;
    poly_pointer ptr = &a;
    a.coef = coef;
    a.expon = expon;
    return ptr;
}

poly_pointer read_poly(const char* fileName)
{
    poly_pointer start_ptr, curr_ptr;
    start_ptr = curr_ptr = addNode(-1, 6);
    curr_ptr = curr_ptr->link = addNode(2, 3);
    return start_ptr;
}

main.cpp

#include <iostream>
#include "Polynomial.h"
using namespace std;

int main(void)
{
    poly_pointer a, b, d, e, f;
    a = read_poly("input1.txt");
    cout << a->coef;
    cout << a->link->coef;

    cout << "\n-eop-";
    cin.get();
    return 0;
}

解决方案

addNode is returning a pointer to a locally allocated poly_node.

The following is sloppy allocation of memory, but will work.

  poly_pointer a = new poly_pointer();
  a->coef = coef;
  a->expon = expon;
  return a;

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