我正在研究一个使用指针创建动态数组的程序，然后重新分配它。
问题是程序运行成功但输出错误。任何帮助，将不胜感激。
int main()
{
int n;
cout<<"Please enter the size of the array: ";
cin>>n;
int *arr1;
arr1 = new int[n];
int *arr1_1;
arr1_1=arr1;
cout<<"Enter "<<n<<" elements in the array \n";
int x=0;
while(x<n)
{
cin>>arr1[0];
arr1++;
x++;
}
x=0;
arr1=arr1_1;
cout<<"Here's what you've inserted in the array: \n";
while(x<n)
{
cout<<arr1[0]<<endl;;
arr1++;
x++;
}
arr1=arr1_1;
cout<<"\n\n";
cout<<"Do you want to extend the size of the array? \nAnswer with 'Y' or 'N'. "<<endl;
char ans;
cin>>ans;
int n1;
if(ans=='Y' || ans=='y'){
cout<<"According to you, What should be the new size of array? ";
cin>>n1;
cout<<"You can enter new elements in your existing array now.\n";
}
else{
cout<<"Ok, then! Have a nice day.";
}
int *arr2, *arr2_1;
arr2=(int *) realloc (arr1, n1*sizeof(int));
arr2_1=arr2;
x=0;
while(x<n)
{
arr2[0]=arr1[0];
arr1++;
arr2++;
x++;
}
x=n;
while(x<n1){
cin>>arr2[n+1];
arr2++;
x++;
}
arr2=arr2_1;
x=0;
while(x<n1){
cout<<arr2[0]<<endl;
arr2++;
x++;
}
return 0;
}
答案 0 :(得分：3)
有两个主要问题：第一个问题是您使用两个不同的系统进行内存分配。 new和realloc都会返回指针，但new和realloc不属于同一系统，不应混用。你要么使用C ++，要么使用C，不要混用。
第二个问题是当你第二次读入数据时，你没有重置arr2指向从上面的复制循环中分配数据的开始，所以你写的是界限。
此外，如果您决定采用C方式并使用malloc，realloc和free，您应该知道realloc正是如此，它会重新分配现有的内存，包括复制数据，因此您不必进行手动复制。此外，为什么首先使用relloc（和复制），当你做的第一件事就是覆盖所有数据？
作为旁注，我认为你了解指针及其工作原理是好的，但是将来当你想使用“动态数组”时，你应该用std::vector来做。
答案 1 :(得分：0)
我修改了您的代码并对其进行了评论。我希望它可以帮到你：
#include <iostream>
using namespace std;
int main()
{
//int n;//CALL YOUR VARIABLES WITH SUGGESTIVE NAMES
int array_size;
do
{
cout << "Please enter the size of the array: ";
cin >> array_size;
}
while(array_size <= 0);//you don't want a pointer which points to 0 elements or less
int* arr1 = nullptr;//INITIALIZE YOUR POINTERS TO NULL or nullptr (C++11)
arr1 = new int[array_size];
int* arr1_1 = nullptr;
arr1_1 = arr1;
//arr1_1 is pointing to the same memory location of arr1
//So once you delete arr1 or arr1_1 pointer, you don't have anymore to free the same memory location
//If you do it, you will probably have an error.
cout << "\nEnter "<< array_size <<" elements in the array:\n";
//int x=0;
//WHY ARE YOU USING A WHILE LOOP? THIS IS REALLY THE CASE WHERE YOU SHOULD USE A FOR LOOP
//CAUSE YOU KNOW EXACTLY THE NUMBER OF TIMES THE LOOP IS GOING TO ITERATE.
//while(x<n)
//{
//cin>>arr1[0];//WHY arr1[0]? YOU ARE GOING TO PUT AND OVERRIDE ALL THE ELEMENTS IN THE FIRST POSITION
//arr1++;
//x++;
//}
for(int i=0; i<array_size; ++i)
{
cout << "Enter your "<< i + 1 << ". number: ";
cin >> arr1[i];//YOU HAVE TO PUT THE ELEMENT AT THE 'i' POSITION
}
//x=0;//YOU DON'T NEED ANYMORE THIS VARIABLE BECAUSE YOU SHOULD USE A FOR LOOP
arr1=arr1_1;//WHAT EXACTLY IS THIS? THEY ARE ALREADY POINTING TO THE SAME MEMORY LOCATION
//SO THIS ASSIGNMENT IS SUPERFLOUS
cout << "\nHere's what you have inserted in the array: \n";
//STILL: USE A FOR LOOP ;)
//while(x<n)
//{
// cout<<arr1[0]<<endl;;
// arr1++;
//x++;
//}
for(int i=0; i<array_size; ++i)//JUST 1 STATEMENT, YOU DON'T NEED PARENTHESIS
cout << "Element number "<< i + 1 << " is: "<< arr1_1[i] << '\n';
//arr1=arr1_1;
cout<<"\n\n";
cout<<"Do you want to extend the size of the array? \nAnswer with 'Y' or 'N'. "<<endl;
char ans;
cin >> ans;
//int n1;//SUGGESTIVE NAMES HELP YOU TO READ THE CODE WHEN YOU WILL NEED TO DO IT
int new_size;
bool isSizeChanged = false;
if(ans=='Y' || ans=='y')
{
isSizeChanged = true;//YOU WILL SEE AT THE END,
//WHEN YOU WANT TO FREE THE POINTERS, THAT THIS VARIABLE IS USEFUL
cout<<"According to you, what should be the new size of array? ";
cin >> new_size;
if(new_size > 0)//YOU DON'T WANT TO MAKE arr1_1 INITIALIZE WITH 0 ELEMENTS
{
cout<<"You can enter new elements in your existing array now.\n";
//YOU SHOULD PUT HERE THE CODE WHICH REALLOCATES MEMORY
//Both arr1 and arr1_1 are pointing to the same memory location,
//but you can make 1 of them to point to another location!
arr1_1 = new int[new_size];
//COPYING THE ELEMENTS FROM arr1 TO arr1_1,
//WHICH NOW IS POINTING TO A DIFFERENT MEMORY LOCATION
if(new_size >= array_size)//NOTE THAT IF THIS CONDITION IS TRUE, YOU WILL HAVE SOME LOCATIONS POINTED WITH RANDOM VALUES!!!
{
for(int i=0; i<array_size; ++i)
arr1_1[i] = arr1[i];
}
else//new_size is less than array_size
{
for(int i=0; i<new_size; ++i)//NOTE THAT NOW I AM USING new_size
arr1_1[i] = arr1[i];
}
cout << "\nYour new array contains:\n";
for(int i=0; i<new_size; ++i)
cout << "Element number "<< i +1 << ". is: "<<arr1_1[i] << '\n';
}
else
cout << "Sorry, we cannot change the size of the pointer to 0 or less.\n";
}
else
cout<<"Ok, then! Have a nice day.";
//WHY ARE YOU DECLARING A NEW POINTERS?
//YOU CAN USE 1 OF THE PREVIOUS POINTERS
//int *arr2, *arr2_1;
//YOU SHOULDN'T USE THE C STYLE TO REALLOCATE A POINTER, IT'S HARDER TO DO IT
//arr2=(int *) realloc (arr1, n1*sizeof(int));
//arr2_1=arr2;//OK, THIS WOULD HAVE WORKED :)
//x=0;
//I WOULD SUGGEST YOU TO USE A FOR LOOP ALSO IN THIS SITUATION
//while(x<n)
//{
//arr2[0]=arr1[0];//YOU ARE COPYING THE FIRST ELEMENT OF arr1 TO THE FIRST ELEMENT OF arr2, ALWAYS!!
//arr1++;
//arr2++;
//x++;
//}
/*
x=n;
while(x<n1)
{
cin>>arr2[n+1];
arr2++;
x++;
}
arr2=arr2_1;
x=0;
while(x<n1)
{
cout<<arr2[0]<<endl;
arr2++;
x++;
}*/
//FREEING THE MEMORY POINTED MY THE 2 POINTERS
if(isSizeChanged)
{
delete [] arr1;
delete [] arr1_1;
}
else
delete [] arr1;//YOU JUST NEED TO DELETE OR arr1 or arr1_1
//BECAUSE THEY ARE STILL POINTING TO THE SAME MEMORY LOCATION
return 0;
}