c++ - 将数组传递给函数并返回指针

Question

对于我的迷你作业，在一栋 30 层的建筑物中，我必须收集人们在电梯中按的楼层，然后找出每个楼层之间的差异。

所以，我打算设置一个 30 层的数组(我们只被教导数组作为我们唯一的容器)。电梯里的人会点击电梯的按钮，假设是 (5, 10, 14, 19, 29)。

然后我计划将这个数组传递给一个函数，该函数将计算每层楼之间的差异。

到目前为止，这是我的代码，我知道它是错误的，因为它没有编译，我也可能在其他地方错了。

这是错误信息:

main.cpp: In function 'int* calculateDiff(int*, int)':
main.cpp:26:7: warning: address of local variable 'floorsDiffResult' returned [-Wreturn-local-addr]

代码

#include <iostream>
#include <numeric>
#include <algorithm>
using std::cout; 
using std::endl;

int* calculateDiff(int floors[], int floorsSize);

int main() 
{
  int floorsPressed[30] = {5, 10, 14, 19, 29};
  int floorsCounter = 5;

  int* ptr = calculateDiff (floorsPressed, floorsCounter);

  int floorsDiffResult[30];
  for (int i = 0; i < floorsCounter; i++)
  {
    floorsDiffResult[i] = *(ptr + i); //Storing the difference into floorsDiffResult array
    cout << "Difference: " << *(ptr + i) << endl;
  }
}

int* calculateDiff(int floors[], int floorsSize)
{
  int floorsDiffResult[30]; //Create another array to store the difference for other calculations later on such as finding the biggest difference, average of the difference etc.
  std::adjacent_difference(floors, floors + floorsSize, floorsDiffResult);
  std::move(floors + 1, floors + floorsSize, floorsDiffResult); //First element does not give the difference

  return floorsDiffResult;
}

Answer 1

我不知道你在这里尝试做的事情背后的逻辑是否正确，但这里有一个主要问题，你返回的是指向局部变量的指针!

这是未定义的行为，因为它是本地的，并且它的生命周期受限于您的函数范围，之后任何事情都可能发生，甚至是您期望的事情(正确结果)。

所以这里是你可以做的:

int* calculateDiff(int floors[], int* output, int floorsSize);

int main()
{
    int floorsPressed[30] = {5, 10, 14, 19, 29};
    int floorsReturn[30] = {};
    int floorsCounter = 5;

    int* ptr = calculateDiff(floorsPressed, floorsReturn, floorsCounter);

    int floorsDiffResult[30];
    for(int i = 0; i < floorsCounter; i++)
    {
        floorsDiffResult[i] = *(ptr + i); //Storing the difference into floorsDiffResult array
        cout << "Difference: " << *(ptr + i) << endl;
    }
}

int* calculateDiff(int floors[], int* output, int floorsSize)
{
    //int floorsDiffResult[30]; //Create another array to store the difference for other calculations later on such as finding the biggest difference, average of the difference etc.
    std::adjacent_difference(floors, floors + floorsSize, output);
    std::move(floors + 1, floors + floorsSize, output); //First element does not give the difference

    return output;
}

而且你不需要从 calculateDiff 返回一个指针，floorsReturn 将在函数执行后得到你的结果，但我不想改变你的方法很多。