#include <iostream>
#include <memory>

class Test
{

};


void dowork(std::unique_ptr<Test> p){
    std::cout << "Hello, dowork!" << std::endl;
}

int main() {
    std::cout << "Hello, World!" << std::endl;
    std::unique_ptr<Test> p(new Test());
    
    dowork(p);
    return 0;
}

Report errors ：main.cpp: In function ‘int main()’:
main.cpp:18:13: error: use of deleted function ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = Test; _Dp = std::default_delete<Test>]’
   18 |     dowork(p);
      |             ^
In file included from /usr/include/c++/9/memory:80,
                 from main.cpp:2:
/usr/include/c++/9/bits/unique_ptr.h:414:7: note: declared here
  414 |       unique_ptr(const unique_ptr&) = delete;
      |       ^~~~~~~~~~
main.cpp:10:35: note:   initializing argument 1 of ‘void dowork(std::unique_ptr<Test>)’
   10 | void dowork(std::unique_ptr<Test> p){

problem ：unique_ptr The memory area pointed to can only consist of one unique_ptr Object to specify . hold p As an incoming parameter , You will find two unique_ptr At the same time, it points to a memory area , The actual parameter p And formal parameters .

1 If this is the case main Function std::unique_ptr<Test> p Ownership passes to function dowork

modify ：

#include <iostream>
#include <memory>

class Test
{

};


void dowork(std::unique_ptr<Test> p){
    std::cout << "Hello, dowork!" << std::endl;
}

int main() {
    std::cout << "Hello, World!" << std::endl;
    std::unique_ptr<Test> p(new Test());
    
    dowork(std::move(p));
    std::cout << "p in main" <<  p.get() <<  std::endl;   
    return 0;
}

2 dowork Is only for use p, Then change the parameter type of the function to the form of reference , In this way, the parameter will not be copied in the process of parameter transfer ,main Medium p and dowork Medium p Represents the same object .

#include <iostream>
#include <memory>

class Test
{

};


void dowork(const std::unique_ptr<Test> &p){
    std::cout << "Hello, dowork! => " <<  p.get() << std::endl;
}

int main() {
    std::cout << "Hello, World!" << std::endl;
    std::unique_ptr<Test> p(new Test());
    
    dowork(std::move(p));
    std::cout << "p in main =>" <<  p.get() <<  std::endl;   
    return 0;
}

3 Use std::unique_ptr Don't use raw pointers after managing them

#include <iostream>
#include <memory>

class Test
{
public:
    Test(){
    	std::cout << "Hello, Test!" << std::endl;    
    }
    ~Test(){
    	std::cout << "Hello, ~Test!" << std::endl;        
    }	
    
    int test(){
     	return num;  
    }
private:    
    int num = 10;
};


void dowork(std::unique_ptr<Test> p){
    std::cout << "Hello, dowork! => " <<  p.get() << std::endl;
}

int main() {
    std::cout << "Hello, World!" << std::endl;
    Test* p = new Test();
    std::unique_ptr<Test>  ptr(p);
    std::cout << "p in main =>" <<  p    << "  p.test:"   << p->test() <<  std::endl;    
    ptr.reset();
    std::cout << "p in main =>" <<  p    << "  p.test:"   << p->test() <<  std::endl;  
    
    while(1); 
    return 0;
}

stay  ptr call reset after ,p The reference count is 0, Deconstruction , stay ubuntu9.4.0

  Next p You can still call , But it's junk value