error: binding reference of type ‘sylar::RWMutex&’ to ‘const RWMutexType’ {aka ‘const sylar::RWMutex’} discards qualifiers

C++编译的时候，遇到了这个错误。

翻译这个错误就是，将一个 引用类型，绑定到了一个 常量类型上面。这个是不允许的。

 

In file included from /home/henry/workspace/henry-sylar/tests/test_config.cpp:1:
/home/henry/workspace/henry-sylar/tests/../sylar2023/config.h: In instantiation of ‘const T sylar::ConfigVar<T, FromStr, ToStr>::getValue() const [with T = int; FromStr = sylar::LexicalCast<std::basic_string<char>, int>; ToStr = sylar::LexicalCast<int, std::basic_string<char> >]’:
/home/henry/workspace/henry-sylar/tests/test_config.cpp:65:82:   required from here
/home/henry/workspace/henry-sylar/tests/../sylar2023/config.h:279:40: error: binding reference of type ‘sylar::RWMutex&’ to ‘const RWMutexType’ {aka ‘const sylar::RWMutex’} discards qualifiers
  279 |             RWMutexType::ReadLock lock(m_mutex);
      |                                        ^~~~~~~
In file included from /home/henry/workspace/henry-sylar/tests/../sylar2023/log.h:16,
                 from /home/henry/workspace/henry-sylar/tests/../sylar2023/config.h:8,
                 from /home/henry/workspace/henry-sylar/tests/test_config.cpp:1:
/home/henry/workspace/henry-sylar/tests/../sylar2023/thread.h:70:26: note:   initializing argument 1 of ‘sylar::ReadScopedLockImp<T>::ReadScopedLockImp(T&) [with T = sylar::RWMutex]’
   70 |     ReadScopedLockImp(T& mutex)

 

getValue是 常函数。里面的变量都应该是常量。

而我们这里的 m_mutex，是外部的一个引用，不是常量。

因此这里的函数报错了。

 RWMutexType m_mutex;

const T getValue() const {
        RWMutexType::ReadLock lock(m_mutex);
        return m_val;
    }

 

解决方案1：将getValue函数的const属性去掉。

解决方案2：声明 m_mutex 的时候，添加mutable属性。getValue还是const函数。

       mutable RWMutexType m_mutex;