为什么要在C ++中避免使用单例

Question

People use singleton everywhere. Read some threads recently from stackoverflow that singleton should be avoided in C++, but not clear why is that.

Some might worry about memory leak with undeleted pointers, things such as exceptions will skip the memory recycle codes. But will the auto_ptr solve this problem?

Answer 1

In general, as mentioned in another answer, you should avoid mutable global data. It introduces a difficulty in tracking code side effects.

However your question is specifically about C++. You could, for instance, have global immutable data that is worth sharing in a singleton. In C++, specifically, it's nearly impossible to safely initialize a singleton in a multithreaded environment.

Multithreaded Environment

You can use the "construct on first use" idiom to make sure the singleton is properly initialized exactly by the time it is needed: http://www.parashift.com/c++-faq-lite/static-init-order.html .

However, what happens if you have 2 (or more) threads which all try to access the singleton for the first time, at exactly the same time ? This scenario is not as far fetched as it seems, if the shared immutable data is data required by your calculateSomeData thread, and you initialize several of these threads at the same time.

Reading the discussion linked above in the C++ FAQ Lite, you can see that it's a complex question in the first place. Adding threads makes it much harder.

On Linux, with gcc, the compiler solves this problem for you - statics are initialized within a mutex and the code is made safe for you. This is an enhancement, the standard requires no such behavior.

In MSVC the compiler does not provide this utility for you and you get a crash instead. You may think "that's ok, I'll just put a mutex around my first use initialization!" However, the mutex itself suffers from exactly the same problem, itself needing to be static.

The only way to make sure your singleton is safe for threaded use is to initialize it very early in the program before any threads are started. This can be accomplished with a trick that causes the singleton to be initialized before main is called.

Singletons That Rely on Other Singletons

This problem can be mostly solved with the construct on first use idiom, but if you have the problem of initializing them before any threads are initialized, you can potentially introduce new problems.

Cross-platform Compatibility

If you plan on using your code on more than one platform, and compile shared libraries, expect some issues. Because there is no C++ ABI interface specified, each compiler and platform handles global statics differently. For instance, unless the symbols are explicitly exported in MSVC, each DLL will have its own instance of the singleton. On Linux, the singletons will be implicitly shared between shared libraries.

Answer 2

避免使用可变全局变量，无论它们是否是单例，因为它们引入了无约束的通信线：您不知道代码的哪个部分影响其他部分，或何时发生。