C++ 零初始化

Question

我无法理解 class 中的某个成员何时以及为何根据 http://en.cppreference.com/w/cpp/language/zero_initialization 被零初始化。

考虑以下测试程序：

#include <iostream>
#include <stdio.h>

class MyTest {
private:
    const static unsigned int dimension = 8;
    void (* myFunctions [dimension])();

public: 
    MyTest() {}

    void print() { 
        for(unsigned int i=0; i < MyTest::dimension; i++) {
            printf("myFunctions[%d] = %p\n", i, this->myFunctions[i]);
        }   
    }
};


int main() {
    //We declare and initialize an object on the stack 
    MyTest testObj = {};
    testObj.print();

    return 0;
}

我正在声明一个 class 以具有签名 "void functionname()" 的 8 个函数指针数组。当我将 main 中的 class 的对象声明并初始化为 MyTest testObj = {}; 或 MyTest testObj; 时，我希望它被零初始化，即所有指针都是空指针。

然而，在我的 Windows 10 机器和 g++ -m32 -o test -std=c++14 test.cpp && test 机器上用 g++ 5.3.0 编译给出输出：

myFunctions[0] = 76dd6b7d
myFunctions[1] = 00401950
myFunctions[2] = 0061ff94
myFunctions[3] = 004019ab
myFunctions[4] = 00401950
myFunctions[5] = 00000000
myFunctions[6] = 003cf000
myFunctions[7] = 00400080

这看起来像是堆栈中未初始化的值..

如果我将对象的声明移到 main 之外（作为全局变量），它会再次打印全零。

如果我没有正确理解 cppreference，那是因为我有一个具有静态存储持续时间的变量，因此被零初始化。它通过零初始化我的 class（即 myFunctions）数组的所有非静态数据成员来初始化我的 class 类型。一个数组是通过对它的每个元素进行零初始化来初始化的，在我的函数指针的情况下，它是一个空指针。

当我用 MyTest testObj = {}; 声明对象时，为什么它不对堆栈进行零初始化？

Answer 1

以下

MyTest testObj = {};

不是 MyTest 的零初始化，而是简单地调用其默认构造函数。 cppreference 页面解释了原因（强调我的）：

As part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors, including value initialization of elements of aggregates for which no initializers are provided.

MyTest是一个class类型，a有一个构造函数。

用

定义构造函数

MyTest() = default;

将改为零初始化对象。

下面的相关标准引述（强调我的）。

来自[dcl.init#8]：

To value-initialize an object of type T means:

if T is a (possibly cv-qualified) class type with either no default constructor ([class.ctor]) or a default constructor that is user-provided or deleted, then the object is default-initialized;

if T is a (possibly cv-qualified) class type without a user-provided or deleted default constructor, then the object is zero-initialized and the semantic constraints for default-initialization are checked, and if T has a non-trivial default constructor, the object is default-initialized;

...

来自[dcl.init.list]：

List-initialization of an object or reference of type T is defined as follows:

...

Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

C++ 零初始化

C++ Zero-Initialization

c++

c++14

zero-initialization