C++ error: pointer addition must be within an array, how should I modify it?

C is a powerful high-level programming language, but sometimes you will inevitably encounter various errors during use. Among them, the error message that pointer addition must be within an array often appears, confusing many beginners. This article will explain the causes of this error and how to correct it.

First of all, you need to understand the concept of pointer addition. Pointer addition means adding an integer to a pointer in order to access the data at the pointed memory address. For example, if the pointer p points to the first element of an array of integers, then p 1 will point to the second element in the array.

However, pointer addition must be used within an array. This is because the array is stored contiguously in memory, and each of its elements can be accessed through pointer addition. If the pointer addition exceeds the bounds of the array, unknown behavior will occur, causing the program to crash or other errors.

The following is an example of a common error:

int arr[5] = {1,2,3,4,5};
int *p = &arr[2];
p = p + 6;  //这里会报错：指针加法必须在数组内。

In this example, we create an array containing 5 integers and point the pointer p to the third element. Then, we add 6 to p and try to access the 9th element of the array. However, since the array only has 5 elements and the pointer exceeds the boundaries of the array, an error will be reported: pointer addition must be within the array.

So, how to solve this problem? The simplest way is to check if the pointer addition exceeds the bounds of the array. In this example, it can be modified like this:

int arr[5] = {1,2,3,4,5};
int *p = &arr[2];
if (p + 6 < &arr[5]) {
    p = p + 6;
} else {
    //处理越界的情况
}

By judging whether the result of pointer addition exceeds the boundary of the array, we can avoid the error that pointer addition must be within the array.

In addition to manually checking whether pointer addition is legal, you can also use algorithm functions in the STL library, such as std::advance. This function can move the iterator a specified distance, but will automatically check for out-of-bounds situations to ensure that pointer addition is within the array and avoid errors. Modify the code as follows:

#include <algorithm>

int arr[5] = {1,2,3,4,5};
int *p = &arr[2];
std::advance(p, 6);

Using the std::advance function, we can directly move the pointer p by 6 positions without manually checking whether pointer addition is legal. In this way, you can avoid the error that pointer addition must be within an array.

In short, pointer addition must be within an array. This is a basic rule of C language. When writing code, be sure to use pointer addition with caution to avoid exceeding the boundaries of the array and causing program errors. If you want to use pointer addition, be sure to make sure it is within an array and check accordingly, or use an algorithm function in the STL library. This improves the reliability and stability of the program.

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