C++ 中的变量初始化

My understanding is that an int variable will be initialized to 0 automatically; however, it is not. The code below prints a random value.

int main () 
{   
    int a[10];
    int i;
    cout << i << endl;
    for(int i = 0; i < 10; i++)
        cout << a[i] << " ";
    return 0;
}

• 哪些规则(如果有)适用于初始化?
• 具体来说，在什么情况下变量会自动初始化?

推荐答案

if会自动初始化

• 它是一个类/结构实例，其中默认构造函数初始化所有原始类型；比如 MyClass 实例；
• 您使用数组初始值设定项语法，例如int a[10] = {}(全为零)或 int a[10] = {1,2};(除前两项外全为零:a[0] == 1 和 a[1] == 2)
• 同样适用于非聚合类/结构，例如我的类实例 = {};(有关这方面的更多信息，请参阅 这里)
• 这是一个全局/外部变量
• 变量是定义的static(无论是在函数内部还是在全局/命名空间范围内)-感谢 Jerry
• it's a class/struct instance in which the default constructor initializes all primitive types; like MyClass instance;
• you use array initializer syntax, e.g. int a[10] = {} (all zeroed) or int a[10] = {1,2}; (all zeroed except the first two items: a[0] == 1 and a[1] == 2)
• same applies to non-aggregate classes/structs, e.g. MyClass instance = {}; (more information on this can be found here)
• it's a global/extern variable
• the variable is defined static (no matter if inside a function or in global/namespace scope) - thanks Jerry

永远不要相信一个普通类型的变量(int、long、...)会自动初始化！它可能发生在像 C# 这样的语言中，但不会发生在 C &C++.

Never trust on a variable of a plain type (int, long, ...) being automatically initialized! It might happen in languages like C#, but not in C & C++.