动态内存分配不匹配

Question

现在我坚持使用内存分配几个小时。基本上，我必须添加一个新的图形元素的矢量图形，其初始化VectorGraphic是必要的。

第一个问题是用InitVectorGraphic方法内的存储器分配被固定？ （我认为）现在我坚持的第二个问题是，即使内存分配自InitVectorGraphic方法，pElements在AddGraphicElement方法中没有内存。 （即，即使是在一个方法（InitVectorGraphic法）初始化之后，改变不会在其他方式体现）

这里是我的全码：

#define _CRT_SECURE_NO_WARNINGS 
#include<stdio.h> 
#include<conio.h> 
#include<stdlib.h> 

enum{ 
    RUNNING = 1 
}; 

struct Point   
{ 
    int x, y; 
}; 

struct Line   
{ 
    Point start; 
    Point end; 
}; 

struct GraphicElement  
{ 
    enum{ 
     SIZE = 256 
    }; 
    unsigned int numLines; 
    Line* pLines; 
    char name[SIZE]; 
}; 

typedef struct   
{ 
    unsigned int numGraphicElements; 
    GraphicElement* pElements; 
}VectorGraphic; 

void InitVectorGraphic(VectorGraphic* image){ 
    image = (VectorGraphic*)malloc(sizeof(VectorGraphic)); 
    (*image).pElements = (GraphicElement*)malloc(sizeof(GraphicElement)* 256); 
//Problem part 1 
    }; 

void AddGraphicElement(VectorGraphic* image){ 
    printf("\nADDING A Graphic Element"); //Problem part 2 
    int index = (*image).numGraphicElements; 

    printf("\nPlease enter the name of the new GraphicElement(<256 characters): "); 
    scanf("%s", &(*image).pElements[index].name); 
    printf("How many lines are there in the new GraphicElement? "); 
    scanf("%d", &(*image).pElements[index].numLines); 
    (*image).pElements[index].pLines = (Line*)malloc(sizeof(Line)* (*image).pElements[index].numLines); 
    for (int i = 0; i < (*image).pElements[index].numLines; i++){ 
     Line line; 
     printf("Please enter the x coord of the start point of line index %d: ", i); 
     scanf("%d", &line.start.x); 
     printf("Please enter the y coord of the start point of line index %d: ", i); 
     scanf("%d", &line.start.y); 
     printf("Please enter the x coord of the end point of line index %d: ", i); 
     scanf("%d", &line.end.x); 
     printf("Please enter the y coord of the end point of line index %d: ", i); 
     scanf("%d", &line.end.y); 

     (*image).pElements[index].pLines[i] = line; 
    } 

    (*image).numGraphicElements = (*image).numGraphicElements + 1; 
    printf("Added"); 
    //add graphicElement to Image 

}; 
void ReportVectorGraphic(VectorGraphic* image){ 
    printf("\nVectorGraphic Report"); 
    for (int i = 0; i < (*image).numGraphicElements; i++){ 
     printf("\nReporting Graphic Element #%d", i); 
     printf("\nGraphic Element name: %s", (*image).pElements[i].name); 
     for (int j = 0; j < (*image).pElements[i].numLines; j++){ 
      printf("\nLine #%d start x: %d", j, (*image).pElements[i].pLines[j].start.x); 
      printf("\nLine #%d start y: %d", j, (*image).pElements[i].pLines[j].start.y); 
      printf("\nLine #%d end x: %d", j, (*image).pElements[i].pLines[j].end.x); 
      printf("\nLine #%d end y: %d", j, (*image).pElements[i].pLines[j].end.y); 
     }  
    } 

}; 

void CleanUpVectorGraphic(VectorGraphic* image){ 
    free(image); 
}; 

VectorGraphic Image; 


int main()   
{ 
    char response; 
    InitVectorGraphic(&Image); 
    while (RUNNING)    
    { 
     printf("\nPlease select an option:\n"); 
     printf("1. Add a Graphic Element\n"); 
     printf("2. List the Graphic Elements\n"); 
     printf("q. Quit\n"); printf("CHOICE: "); 
     fflush(stdin); 
     scanf("%c", &response); 
     switch (response)      
     { 
     case '1':AddGraphicElement(&Image); 
      break; 
     case '2':ReportVectorGraphic(&Image); 
      break; 
     case 'q':CleanUpVectorGraphic(&Image); 
      return 0; 
     default:printf("Please enter a valid option\n"); 
     } 
     printf("\n"); 
    } 
} 

什么解决的办法？

Answer 1

这是非常标准的指针混淆。你需要以下变化：

VectorGraphic* Image; //make it a pointer 
void InitVectorGraphic(VectorGraphic** image) //expect pointer-to-pointer 
{ 
    *image = (VectorGraphic*)malloc(sizeof(VectorGraphic)); //assign allocated buffer to pointer 
    (*image)->pElements = (GraphicElement*)malloc(sizeof(GraphicElement)* 256); 
} 

InitVectorGraphic(&Image); //call remains the same 

在你的旧代码，您正在泄漏内存，你能想到的形象当作如下局部定义的变量：

执行

InitVectorGraphic(VectorGraphic* image) 
{ 
    image = malloc(); //Here you are allocating memory to a local variable not to the address Image 
    //no free()'ing of image variable 
} 

步骤，按您的旧代码是简单地做两个任务，第二个任务覆盖原来的一个：

VectorGraphic *image = &Image; 
image = malloc();