一、为什么要用“智能”锁？
用户使用互斥锁时容易出现如下错误：
- 忘记unlock，导致其他线程阻塞
- 多次unlock，导致出现未定义情况
C++互斥锁管理工具能够避免这种情况出现，其实现机理和智能指针一样是RAII。管理套件如下框图所示：
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.box{stroke:none;stroke-width:0;fill:#ECECFF;opacity:0.7}#mermaid-svg-Cy9RNKoySfoMzUwf .edgeLabel text{fill:#333}#mermaid-svg-Cy9RNKoySfoMzUwf .stateLabel text{fill:#000;font-size:10px;font-weight:bold;font-family:'trebuchet ms', verdana, arial;font-family:var(--mermaid-font-family)}#mermaid-svg-Cy9RNKoySfoMzUwf .node circle.state-start{fill:black;stroke:black}#mermaid-svg-Cy9RNKoySfoMzUwf .node circle.state-end{fill:black;stroke:white;stroke-width:1.5}#mermaid-svg-Cy9RNKoySfoMzUwf #statediagram-barbEnd{fill:#9370db}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-cluster rect{fill:#ECECFF;stroke:#9370db;stroke-width:1px}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-cluster rect.outer{rx:5px;ry:5px}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-state .divider{stroke:#9370db}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-state .title-state{rx:5px;ry:5px}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-cluster.statediagram-cluster .inner{fill:white}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-cluster.statediagram-cluster-alt .inner{fill:#e0e0e0}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-cluster .inner{rx:0;ry:0}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-state rect.basic{rx:5px;ry:5px}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-state rect.divider{stroke-dasharray:10,10;fill:#efefef}#mermaid-svg-Cy9RNKoySfoMzUwf .note-edge{stroke-dasharray:5}#mermaid-svg-Cy9RNKoySfoMzUwf .statediagram-note rect{fill:#fff5ad;stroke:#aa3;stroke-width:1px;rx:0;ry:0}:root{--mermaid-font-family: '"trebuchet ms", verdana, arial';--mermaid-font-family: "Comic Sans MS", "Comic Sans", cursive}#mermaid-svg-Cy9RNKoySfoMzUwf .error-icon{fill:#522}#mermaid-svg-Cy9RNKoySfoMzUwf .error-text{fill:#522;stroke:#522}#mermaid-svg-Cy9RNKoySfoMzUwf .edge-thickness-normal{stroke-width:2px}#mermaid-svg-Cy9RNKoySfoMzUwf .edge-thickness-thick{stroke-width:3.5px}#mermaid-svg-Cy9RNKoySfoMzUwf .edge-pattern-solid{stroke-dasharray:0}#mermaid-svg-Cy9RNKoySfoMzUwf .edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-Cy9RNKoySfoMzUwf .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-Cy9RNKoySfoMzUwf .marker{fill:#333}#mermaid-svg-Cy9RNKoySfoMzUwf .marker.cross{stroke:#333}
:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}
#mermaid-svg-Cy9RNKoySfoMzUwf {
color: rgba(0, 0, 0, 0.75);
font: ;
}
管理套件
管理类
lock_guard
unique_lock
scoped_lock
shared_lock
锁定策略类型结构体
defer_lock_t
try_to_lock_t
adopt_lock_t
锁定策略常量
defer_lcok
try_to_lock
adpot_lock
本文只对C++11支持的lock_guard和unique_lock进行介绍，构造函数第二个参数是一个锁定策略结构体，我们不需要自己实例化这个结构体，再传入构造函数，只需要在命名空间中找到这个结构体实例，如std::defer_lock。
struct defer_lock_t { explicit defer_lock_t() = default; };//defer是推迟的意思
struct try_to_lock_t { explicit try_to_lock_t() = default; };
struct adopt_lock_t { explicit adopt_lock_t() = default; };
lock_guard构造函数如下：
explicit lock_guard( mutex_type& m );
lock_guard( mutex_type& m, std::adopt_lock_t t );
lock_guard( const lock_guard& ) = delete;
lock_guard使用的是RAII技术，一旦互斥锁正确地交由lock_guard管理，我们就无需担心锁的释放问题。那么应该怎么交给lock_guard管理？选择正确的构造函数，而不要欺骗编译器。
- 互斥锁未此线程上锁，调用只接受单参数的构造函数，传递一个互斥锁；
- 互斥锁已在此线程上锁，调用接受两个参数的构造函数，传递互斥锁和std::adopt_lock。
两种构造函数对互斥锁的处理不同：
- 对于单参数构造函数，将会调用m.lock()对互斥锁锁定。假如互斥锁已经被锁定或者不是互斥锁类型，结果未定义；
- 对于双参数构造函数，将会假定m已经调用了lock，便不会进行m.lock()操作。假如你欺骗了它，也就是m并没有在构造前加锁，结果未定义。
一个例子：
int k=0;
int main()
{
std::thread th1([](){
for(int i=0;i
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