Skip to content

Static local variables

对于static local variable，cppreference是在Storage class specifiers#Static local variables章节进行描述的。

cppreference Storage class specifiers#Static local variables

Dnitialization

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.

NOTE: 上面这段话关于initialization的描述是不易理解的？它的意思是：对于static local variable，它的initialization的发生时间如下：

  • zero- or constant-initialization can be performed before the block is first entered
  • others are initialized the first time control passes through their declaration

Initialization of static local variable concurrently (since C++11)

NOTE: 因为C++11支持multiple thread，所以标准需要对此进行特别的规定。

这种情况的典型就是：线程执行函数中声明了一个static local variable。

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:

static local object，具备如下特性:

1、construct on first use、lazy initialization

2、call once

3、guaranteed-destruction(后"End"章节进行了介绍)

Meyers singleton就是利用了static local object的上述两个特性来实现的

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.

NOTE:

1、double-checked locking pattern在工程parallel-computing的Synchronization\Lock章节描述。

2、在C++\Pattern\Singleton\Cpp-and-the-Perils-of-Double-Checked-Locking中解释了为何double-checked locking pattern无法实现singleton。

End

The destructor for a block-scope static variable is called at program exit, but only if the initialization took place successfully.

NOTE:

1、利用这个特性可以实现exit guard，关于此参见:

a、stackexchange Modern C++ Singleton Template ，其中的static MemGuard g; // clean up on program end正是这个用法

Inline

Function-local static objects in all definitions of the same inline function (which may be implicitly inline) all refer to the same object defined in one translation unit.

NOTE: 能够保证唯一性

cppreference Initialization#Static local variables

For initialization of locals (that is, block scope) static and thread-local variables, see static local variables.

Initializer is not allowed in a block-scope declaration of a variable with external or internal linkage. Such a declaration must appear with extern and cannot be a definition.

NOTE:

对于static local variable with external linkage，如果带上initializer，则它就是definition了，显然这是违反了ODR的。

一般，我们使用的static local variable是no linkage的。

Application

Singleton

参见C++\Pattern\Singleton。