我应该使用 'malloc(0)' 还是 'free() and then set variable to NULL'？什么更好？

Question

我目前正在写一个函数，禁止用户输入不必要的字符，将数字写入数组，然后从数组转换为int，然后returns。我想创建一个基于动态数组的函数。算法如下：我输入一个数字，数组扩大，擦除数字，数组缩小。当擦除后字符串为空并等待输入数字时，就会出现问题。在这种情况下我应该如何处理？使用malloc(0)或使用free()，然后设置指针等于NULL?

我可以将指针设置为 NULL 并从而清除它吗？可能，答案是否定的我不能，因为我认为记忆单元仍然充满了数据，但我想听到更详细的答案。

我的想法执行不好吗？

在代码的开头，有一个注释，如果数字超过值 2147483647 那么算法会表现得很奇怪，知道为什么会这样吗？

抱歉提出问题，但这真的让我抓狂。我搜索了整个 Internet 并没有找到我的问题的任何答案，即使在 Whosebug 和建议的重复项中也是如此。

long long int number_input(int limit)
{
    // Limit set the number of digits in the number

    // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    // I have no idea why is that happening 
    // 2.147.483.647 = 2147483647
    // 2.147.483.648 = -2147483648
    // -2.147.483.648 = -2147483648
    // -2.147.483.649 = -2147483647

    char c;             // current entered symbol   
    int length = 0;     // length of number (digit capacity, minus included), but index for digits is length - 1
    char* number_array_pointer = NULL;
    long long int number_array_integer = 0;
    int index;      // index for converting string to integer


    if ((number_array_pointer = (char*)realloc(number_array_pointer, sizeof(char) * length)) == NULL) {
        cout << "Fatal: failed to allocate " << sizeof(char) * length << " bytes.\n";
        abort();
    }

    for (;;) {
        c = _getch();
        if (c == 13 && length != 0) {       // Enter breaks down the loop
            if (number_array_pointer[0] == '-' && length < 2) {
                continue;
            }else {
                break;
            }
        }
        else if (c == '\b' && length != 0) {    // Backspace
            cout << "\b \b";
            length--;
            if (length == 0)
            {
                // malloc(0) vs free() then ' = NULL'
                if ((number_array_pointer = (char*)malloc(sizeof(char) * length)) == NULL) {
                    cout << "Fatal: failed to allocate " << sizeof(char) * length << " bytes.\n";
                    abort();
                }
            }       // We cannot realloc 0 bytes
            else if ((number_array_pointer = (char*)realloc(number_array_pointer, sizeof(char) * length)) == NULL) {
                cout << "Fatal: failed to reallocate " << sizeof(char) * length << " bytes.\n";
                abort();
            }
        }
        else if (c == 45 && length == 0) {      // Minus for negative number
            length++;
            if ((number_array_pointer = (char*)realloc(number_array_pointer, sizeof(char) * length)) == NULL) {
                cout << "Fatal: failed to reallocate " << sizeof(char) * length << " bytes.\n";
                abort();
            }
            number_array_pointer[length - 1] = c;
            cout << c;
        }
        else if (c > 47 && c < 58 && length < limit) {      //  Allow to enter only digits
            length++;
            if ((number_array_pointer = (char*)realloc(number_array_pointer, sizeof(char) * length)) == NULL) {
                cout << "Fatal: failed to reallocate " << sizeof(char) * length << " bytes.\n";
                abort();
            }
            number_array_pointer[length - 1] = c;
            cout << c;
        }
        else {
            continue;
        }
    }

    if (number_array_pointer[0] == '-') {
        index = 1;
    }
    else {
        index = 0;
    }

    for (index; index < length; index++)
    {
        number_array_integer *= 10;
        // get the actual digit from ascii code
        number_array_integer += (long long int)number_array_pointer[index] - 48;
    }

    if (number_array_pointer[0] == '-') {
        number_array_integer *= -1;
    }

    free(number_array_pointer);
    return number_array_integer;
}

Answer 1

这是你第二部分的答案，关于如果数字超过值 2147483647

为什么事情会偏离轨道

#include <iostream>
#include <limits>

int main() {
  std::cout<<"The answer is "<<std::numeric_limits<int>::max();
}

Output:

The answer is 2147483647.

这是 32 位整数的最大值（平台上通常的 int 是 32 位）。

关于你的第一部分，如果没有禁忌，请用std::vector！我会让别人回答的。

我应该使用 'malloc(0)' 还是 'free() and then set variable to NULL'？什么更好？

Should i use 'malloc(0)' or 'free() and then set variable to NULL'? What's better?

c++

malloc

user-input

type-conversion

dynamic-memory-allocation