虽然一直强调不要用全局变量。但是对于特殊的应用场合，还是有全局变量的使用（如某些多进程、多线程的共享资源），我们希望在首次运行（载入）时，系统能够帮助我们进行一些必要的初始化。
If a program starts a thread (30.3), the subsequent initialization of a variable is unsequenced with respect to the initialization of a variable defined in a different translation unit. Otherwise, the initialization of a variable is indeterminately sequenced with respect to the initialization of a variable defined in a different translation unit. If a program starts a thread, the subsequent unordered initialization of a variable is unsequenced with respect to every other dynamic initialization. Otherwise, the unordered initialization of a variable is indeterminately sequenced with respect to every other dynamic initialization
对不同的源文件中的全局变量，标准C++对全局变量初始化的顺序并没有要求。对于同一个原文件中，全局变量按照定义先后顺序初始化。
对于堆类型的全局变量的创建和构造，可能在一个构造函数中调用另一个未构造的全局变量，通常会检查另一个指针是否有效，并在无效时构造那个对象。这就出现一个问题：
一个指针在构造之前，被初始化。c/c++运行时，仍然会再次构造那个指针（全局变量为空指针）。
这会引发资源泄露，甚至运行时错误。
解决措施。虽然按需创建是一种很好的思路。但是必须符合c/c++运行机制。解决方式就是不使用堆创建对象，而是使用栈内存对象（c++内部大使用致双检查锁的方式保证构造一次）。我们也可以自己实现双检查锁的思路，但是我们使用的锁的创建过程，本身就是需要双检查锁定的，这是自相矛盾的。
参考C++标准具体介绍：（原文：Storage class specifiers - cppreference.com）
Variables declared at block scope with the specifier?
static?or?thread_local?(since C++11)?have static?or thread?(since C++11)?storage duration but are initialized the first time control passes through their declaration (unless their initialization is?zero-?or?constant-initialization, which can be performed before the block is first entered). On all further calls, the declaration is skipped.If the initialization?throws an exception, the variable is not considered to be initialized, and initialization will be attempted again the next time control passes through the declaration.
If the initialization recursively enters the block in which the variable is being initialized, the behavior is undefined.
If multiple threads attempt to initialize the same static local variable concurrently, the initialization occurs exactly once (similar behavior can be obtained for arbitrary functions with?std::call_once).
Note: usual implementations of this feature use variants of the double-checked locking pattern, which reduces runtime overhead for already-initialized local statics to a single non-atomic boolean comparison.
(since C++11) The destructor for a block-scope static variable?is called at program exit, but only if the initialization took place successfully.
Function-local static objects in all definitions of the same?inline function?(which may be impli
