如何在 C++ 中延迟静态对象（记录器）的销毁？

Question

I got a class which holds the application's logger as a unique_ptr. The logger can be set through a static function. Also, one can obviously log messages. I left out any thread synchronization (mutexes) to make things easier.

class LoggerWrapper {
 public:
  static void SetLogger(std::unique_ptr<logger::ILogger> new_logger);

  static void Log(const std::string& message);

 private:
  static std::unique_ptr<logger::ILogger> logger_;
};

void LoggerWrapper::Log(const std::string& message) {
  if (!logger_) {
    // cannot log
  } else {
    logger_->OnLogEvent(message);
  }
}

void LoggerWrapper::SetLogger(std::unique_ptr<logger::ILogger> new_logger) {
  logger_ = std::move(new_logger);
}

My problem is: The unique_ptr gets destructed before some of the other classes inside the application. Eg is the DTOR of Class Foo wants to log something, the unique_ptr might have already been destroyed (which is the case at the moment). This causes the ILogger implementation to be destroyed, resulting in no log output being possible.

Does anyone have an idea on how to easily fix this? I somehow need to "delay" the destruction of the static unique_ptr. I also tried changing it to a shared_ptr, but that only caused SIGABRTs with "pure virtual method called" errors.

Thanks in advance!

EDIT: Created a minimal working example which contradicts my experience. In this case, the static logger outlives the Foo class.

EDIT2: My application uses exit . That seems to change the order of destruction.

EDIT3: exit does not destroy local objects.

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#include <iostream>
#include <memory>
#include <string>

using namespace std;

class ILogger {
  public:
  ILogger() {
      std::cout << "ILogger CTOR" << std::endl;
  }
  ~ILogger() {
      std::cout << "ILogger DTOR" << std::endl;
  }
  
  virtual void OnLogEvent(const std::string& log_message) {
        std::cout << "OnLogEvent: " << log_message << std::endl;
  }
};

class LoggerWrapper {
 public:
  static void SetLogger(std::unique_ptr<ILogger> new_logger) {
  logger_ = std::move(new_logger);
}

  static void Log(const std::string& message) {
  if (!logger_) {
    // cannot log
  } else {
    logger_->OnLogEvent(message);
  }
};

 private:
  static std::unique_ptr<ILogger> logger_;
};

class Foo {
  public:
   Foo(const std::string& name) : name_{name} {
       LoggerWrapper::Log(name_ + ": CTOR");
   }
   ~Foo() {
       LoggerWrapper::Log(name_ + ": DTOR");
   }
  private:
   std::string name_;
};

// declaring logger_ first causes it to be deleted AFTER foo
std::unique_ptr<ILogger> LoggerWrapper::logger_;
std::unique_ptr<Foo> foo;


int main()
{
    LoggerWrapper::SetLogger(std::make_unique<ILogger>());
    foo = std::make_unique<Foo>("Global FOO");
    
    // local variables do NOT get destroyed when calling exit!
    auto foo_local = Foo("Local FOO");

    exit(1);
}

Answer 1

This is trivial.

First you don't use global static objects ( you should not be using global state like that). You use function static objects so you can control the order of creation/destruction.

So change this:

std::unique_ptr<ILogger> LoggerWrapper::logger_;
std::unique_ptr<Foo> foo;

Into:

 class GlobalLogger
 {
     public:
         ILogger& getLogger() {
             static ILogger  logger;  // if you must use unique_ptr you can do that here
             return logger;           // But much simpler to use a normal object.
         }
  };
  class GlobalFoo
  {
      public:
          Foo& getFoo() {
              // If there is a chance that foo is going to 
              // use global logger in its destructor
              // then it should simply call `GlobalLogger::getLogger()`
              // in the constructor of Foo. You then 
              // guarantee the order of creation and thus destruction.
              // Alternatively, you can call it here in thus
              // function just before the declaration of foo.
              static Foo foo;
              return foo;
          }
  };

  // Where you were using `logger_` use `GlobalLogger::getLogger()`
  // Where you were using `foo`     use `GlobalFoo::getFoo()`

If we use your original code as the starting point we can do this:

#include <iostream>
#include <memory>
#include <string>

// Please don't do this.
// This is the number one worst practice.
// https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice
using namespace std;

class ILogger {
    public:
        ILogger() {
            std::cout << "ILogger CTOR" << std::endl;
        }
        ~ILogger() {
            std::cout << "ILogger DTOR" << std::endl;
        }

        virtual void OnLogEvent(const std::string& log_message) {
            std::cout << "OnLogEvent: " << log_message << std::endl;
        }
};

class LoggerWrapper
{
    // Here store the logger
    // as a static member of this private function.
    // The the SetLogger() Log() functions get this reference.
    static std::unique_ptr<ILogger>& getLogReference() {
        static std::unique_ptr<ILogger> logger;
        return logger;
    }

    public:
        static void SetLogger(std::unique_ptr<ILogger> new_logger) {
            // Save the new reference.
            getLogReference() = std::move(new_logger);
        }

        // Use the logger if it has been set.
        static void Log(const std::string& message) {
            std::unique_ptr<ILogger>& logger_ = getLogReference();
            if (!logger_) {
                // cannot log
            } else {
                logger_->OnLogEvent(message);
            }
        };
};

class Foo {
    public:
        Foo(const std::string& name) : name_{name} {
            // This calls Log()
            // Which calls getLogReference()
            // Which forces the creation of the function static
            // variable logger so it is created before this
            // object is fully initialized (if it has not already
            // been created).
            // 
            // This means this object was created after the logger
            LoggerWrapper::Log(name_ + ": CTOR");
        }
        ~Foo() {
            // Because the Log() function was called in the
            // constructor we know the loger was fully constructed first
            // thus this object will be destroyed first
            // so the logger object is guaranteed to be
            // available in this objects destructor
            // so it is safe to use.
            LoggerWrapper::Log(name_ + ": DTOR");
        }
    private:
        std::string name_;
};


std::unique_ptr<Foo>& globalFoo() {
    // foo may destroy an object created later
    // that has a destructor that calls LoggerWrapper::Log()
    // So we need to call the Log function here before foo
    // is created.
    LoggerWrapper::Log("Initializing Global foo");
    // Note: Unless somebody else has explicitly called SetLogger()
    // the above line is unlikely to log anything as the logger
    // will be null at this point.

    static std::unique_ptr<Foo> foo;
    return foo;
}

int main()
{
    LoggerWrapper::SetLogger(std::make_unique<ILogger>());
    globalFoo() = std::make_unique<Foo>("Global FOO");

    // local variables do NOT get destroyed when calling exit!
    auto foo_local = Foo("Local FOO");

    exit(1);
}

Answer 2

The order in which static (global) objects are created and destroyed is undefined. This leave you a few options.

1. Don't use global objects, just raw use pointers which you can destroy yourself in the correct order.
2. Use a pointer to your logger that you never destroy. This technically is a memory leak but the kernel will cleanup when your application exits.
3. Use a shared_ptr for your logger. Each object that uses the logger gets a shared_ptr and the logger will be cleaned up after the last object is destroyed.