问题描述
我有一个关于 boost::shared_ptr 的问题.
I have a question about boost::shared_ptr<T>.
有很多线程.
using namespace boost;
class CResource
{
// xxxxxx
}
class CResourceBase
{
public:
void SetResource(shared_ptr<CResource> res)
{
m_Res = res;
}
shared_ptr<CResource> GetResource()
{
return m_Res;
}
private:
shared_ptr<CResource> m_Res;
}
CResourceBase base;
//----------------------------------------------
// Thread_A:
while (true)
{
//...
shared_ptr<CResource> nowResource = base.GetResource();
nowResource.doSomeThing();
//...
}
// Thread_B:
shared_ptr<CResource> nowResource;
base.SetResource(nowResource);
//...
第一季度
如果Thread_A不关心nowResource是最新的，这部分代码会不会有问题?
Q1
If Thread_A do not care the nowResource is the newest, will this part of code have problem?
我的意思是当Thread_B没有完全SetResource()时，Thread_A通过GetResource()得到一个错误的智能点代码>?
I mean when Thread_B do not SetResource() completely, Thread_A get a wrong smart point by GetResource()?
线程安全是什么意思?
如果不关心资源是不是最新的，shared_ptr 在 nowResource 被释放时会导致程序崩溃还是会破坏 shared_ptr?
If I do not care about whether the resource is newest, will the shared_ptr<CResource> nowResource crash the program when the nowResource is released or will the problem destroy the shared_ptr<CResource>?
推荐答案
来自 boost 文档:
shared_ptr 对象提供相同的内置线程安全级别类型.shared_ptr 实例可以是读取"(仅使用 const 访问操作)同时由多个线程.不同 shared_ptr实例可以写入"(使用可变操作访问例如 operator= 或 reset)同时由多个线程(即使这些实例是副本，并共享相同的引用计数在下面.)
shared_ptrobjects offer the same level of thread safety as built-in types. Ashared_ptrinstance can be "read" (accessed using only const operations) simultaneously by multiple threads. Differentshared_ptrinstances can be "written to" (accessed using mutable operations such asoperator=or reset) simultaneously by multiple threads (even when these instances are copies, and share the same reference count underneath.)
任何其他同时访问都会导致未定义的行为.
Any other simultaneous accesses result in undefined behavior.
所以你的使用是不安全的，因为它使用了m_res的同时读写.boost 文档中的示例 3 也说明了这一点.
So your usage is not safe, since it uses simultaneous read and write of m_res. Example 3 in the boost documentation also illustrates this.
您应该使用单独的 mutex 保护对 SetResource/GetResource 中的 m_res 的访问.
You should use a separate mutex that guards the access to m_res in SetResource/GetResource.
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