未定义值的索引处的数组值

Question

在此代码中：

#include <stdio.h>

int main(void) 
{
    int a[2][3] = {{1,2},{4,5,6}};

    printf("%d\n",a[0][2]);
    return 0;
}

输出为 0 - 由于数组未初始化，此输出是否是某些未定义行为的结果？

Answer 1

数组已初始化。您正在定义一个 two-dimensional 数组，该数组由两个数组组成，每个数组包含三个整数。对于这两个中的第一个，你只给出两个值，默认情况下，缺失的一个被初始化为零，所以你的完整数组是 {{1, 2, 0}, {4, 5, 6}。 a[0][2] 会给你那个零。

Answer 2

来自C标准（6.7.9初始化）

19 The initialization shall occur in initializer list order, each initializer provided for a particular subobject overriding any previously listed initializer for the same subobject;151) all subobjects that are not initialized explicitly shall be initialized implicitly the same as objects that have static storage duration.

和

10 If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:

— if it has arithmetic type, it is initialized to (positive or unsigned) zero;

所以实际上这个声明

int a[2][3] = {{1,2},{4,5,6}};

等同于

int a[2][3] = {{1,2, 0},{4,5,6}};

未定义值的索引处的数组值

array value at an index for which value hasn't been defined

c

initialization