unique_lock是一类独占锁，当其获取锁的所有权后，其他所有的unique_lock和shared_lock均无法获取到锁。

当unique_lock无法获取锁时，其会阻塞，直到其他对象释放锁，才会继续执行。

以下是代码验证（windows环境下执行）：

#define  _CRT_SECURE_NO_WARNINGS
#include <iostream>     
#include <mutex>
#include <windows.h>
using namespace std;

class A {
    
    
public:
    void funcOne() {
    
    
        std::unique_lock<std::mutex> um(m_mutex);
        for (int i = 0; i < 5; ++i) {
    
    
            cout << "funcOne owns m_mutex" << std::endl;
            Sleep(1000);
        }
    }

    void funcTwo() {
    
    
        cout << "funcTwo not owns m_mutex" << std::endl;

        //  由于funcOne先执行，获取到m_mutex的所有权，因此funcTwo阻塞在此处，直到锁被释放
        std::unique_lock<std::mutex> um(m_mutex); 
        for(int i = 0; i < 5; ++i) {
    
    
            cout << "=====>>> funcTwo owns m_mutex" << std::endl;
            Sleep(1000);
        }
    }
private:
    std::mutex m_mutex;
};

int main() {
    
    
   
    A a;

    // 执行funcOne
    std::thread myObjOne(&A::funcOne, &a);

    // 确保funcOne先于funcTwo执行
    Sleep(100); 

    // 执行funcTwo
    std::thread myObjTwo(&A::funcTwo, &a);

    myObjOne.join();
    myObjTwo.join();

    return 0;
}

以下是执行结果:

funcOne owns m_mutex
funcTwo not owns m_mutex
funcOne owns m_mutex
funcOne owns m_mutex
funcOne owns m_mutex
funcOne owns m_mutex
=====>>> funcTwo owns m_mutex
=====>>> funcTwo owns m_mutex
=====>>> funcTwo owns m_mutex
=====>>> funcTwo owns m_mutex
=====>>> funcTwo owns m_mutex

由此可见，当unique_lock无法获取到锁时，它是会阻塞的。

谢谢阅读