C++ 静态成员在初始化后重新初始化

Question

我正在尝试实现“HasUtility”模板的设计class，允许创建模板化静态成员（实用程序）并维护所有实用程序的列表（基础classes）

代码如下：

StaticUtilityBase.h:

#include <vector>
#include <iostream>

class StaticUtilityBase {
public:
    static std::vector<StaticUtilityBase*> all;
    StaticUtilityBase() {
        all.push_back(this);
        printAllState();
    }
    static void printAllState() {
        std::cout << "All size is " << all.size() << ", address is " << &all << std::endl;
    }
};

template <typename T> class StaticUtility : public StaticUtilityBase {};

template <typename T> class HasUtility  {
public:
    static StaticUtility<T> utility;
};

template <typename T> StaticUtility<T> HasUtility<T>::utility;

TemplateUtility.cpp:

#include "StaticUtilityBase.h"
std::vector<StaticUtilityBase*> StaticUtilityBase::all;

和主要的：

#include <iostream>
#include "StaticUtilityBase.h"
class A {};
class B {};
int main() {
    std::cout << "Initializations done" << std::endl;
    StaticUtilityBase::printAllState();
    HasUtility<A>::utility; //Tell to the compiler that the static member is used so the constructor
    HasUtility<B>::utility; // of utility is used before main execution, during initializations
}

执行时得到以下结果：

All size is 1, address is 0x562942baa1f0
All size is 2, address is 0x562942baa1f0
Initializations done
All size is 0, address is 0x562942baa1f0

所有尺寸最后应该还是2。看起来“all”在“utility”成员初始化之后被重新初始化了！地址相同，所以我认为它是同一个对象。也许它是从一个空的复制过来的？？欢迎任何帮助！

我使用 gcc 和 C++17

Answer 1

您正在从另一个静态对象（在本例中 template <typename T> StaticUtility<T> HasUtility<T>::utility;）的构造函数中访问一个静态对象（在本例中 StaticUtilityBase::all）。

由于在跨翻译单元的不同静态对象的初始化之间没有顺序保证，因此您尝试执行的操作将行不通。

有关详细信息，请参阅 https://en.cppreference.com/w/cpp/language/siof。

验证这一点的简单方法是向 StaticUtilityBase 添加第二个静态对象，它在构建时打印出来： static int printer = []()->int{std::cout << "Initing StaticUtilityBase\n"; return 0;}()

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