引发异常时, 编译器总是创建一个临时拷贝...

http://blog.csdn.net/g5dsk/article/details/5851614

引发异常时, 编译器总是创建一个临时拷贝, 即使异常规范和 catch 块中指定的是引用.

备注: 指定引用的目的并不是避免创建拷贝以提高效率, 而是基类引用可以执行派生类对象.

 // Code

 #include <iostream>


 class MyException
 {
 public:
     MyException()
     {
         ++nCnt;
         nObjId = nCnt;
         std::cout << "<+" << nObjId << "> MyException default constructor has been invoked. " << std::endl;
     }

     MyException(const MyException& /* rhs */) // Comment 'rhs' to avoid warning C4100.
     {
         ++nCnt;
         nObjId = nCnt;
         std::cout << "<+" << nObjId << "> MyException copy constructor has been invoked. " << std::endl;
     }

     virtual ~MyException()
     {
         std::cout << "<-" << nObjId << "> MyException destructor has been invoked. " << std::endl;
         --nCnt;
         nObjId = -100000; // Set object id invalid.
     }

     int ObjId()
     {
         return nObjId;
     }
 private:
     static int nCnt;
     int nObjId;
 };

 /* static */ int MyException::nCnt = 0; // Cannot use 'static' here!


 int main()
 {
     try
     {
         MyException me;
         throw me;
     }
     catch (MyException& me)
     {
         std::cout << "<" << me.ObjId() << "> The exception has been caught. " << std::endl;
         std::cout << "____A____" << std::endl;
     }

     std::cout << "____B____" << std::endl;

     std::cout << std::endl;

     try
     {
         MyException me;
         throw me;
     }
     catch (MyException me)
     {
         std::cout << "<" << me.ObjId() << "> The exception has been caught. " << std::endl;
         std::cout << "____A____" << std::endl;
     }

     std::cout << "____B____" << std::endl;

     std::cout << std::endl;

     try
     {
         MyException me;
         std::cout << "In try-block, address of me is 0x" << &me << std::endl;
         throw &me;
     }
     catch (MyException* pme)
     {
         std::cout << "In catch-block, pme points to 0x" << pme << std::endl;
         std::cout << "<" << pme->ObjId() << "> The exception has been caught. " << std::endl;
         std::cout << "____A____" << std::endl;
     }

     std::cout << "____B____" << std::endl;

     std::cout << std::endl;

     try
     {
         MyException* pme = new MyException;
         std::cout << "In try-block, pme points to 0x" << pme << std::endl;
         throw pme;
     }
     catch (MyException* pme)
     {
         std::cout << "In catch-block, pme points to 0x" << pme << std::endl;
         std::cout << "<" << pme->ObjId() << "> The exception has been caught. " << std::endl;
         std::cout << "____A____" << std::endl;
     }

     std::cout << "____B____" << std::endl;

     std::cout << std::endl;

     try
     {
         MyException* pme = new MyException;
         std::cout << "In try-block, pme points to 0x" << pme << std::endl;
         throw pme;
     }
     catch (MyException* pme)
     {
         std::cout << "In catch-block, pme points to 0x" << pme << std::endl;
         std::cout << "<" << pme->ObjId() << "> The exception has been caught. " << std::endl;
         delete pme;
         pme = NULL;
         std::cout << "____A____" << std::endl;
     }

     std::cout << "____B____" << std::endl;

     return 0;
 }

 // Output

 <+1> MyException default constructor has been invoked.
 <+2> MyException copy constructor has been invoked.
 <-1> MyException destructor has been invoked.
 <2> The exception has been caught.
 ____A____
 <-2> MyException destructor has been invoked.
 ____B____

 <+1> MyException default constructor has been invoked.
 <+2> MyException copy constructor has been invoked.
 <+3> MyException copy constructor has been invoked.
 <-1> MyException destructor has been invoked.
 <3> The exception has been caught.
 ____A____
 <-3> MyException destructor has been invoked.
 <-2> MyException destructor has been invoked.
 ____B____

 <+1> MyException default constructor has been invoked.
 In try-block, address of me is 0x0038FBA0
 <-1> MyException destructor has been invoked.
 In catch-block, pme points to 0x0038FBA0
 <-100000> The exception has been caught.
 ____A____
 ____B____

 <+1> MyException default constructor has been invoked.
 In try-block, pme points to 0x003A1EA0
 In catch-block, pme points to 0x003A1EA0
 <1> The exception has been caught.
 ____A____
 ____B____

 <+2> MyException default constructor has been invoked.
 In try-block, pme points to 0x003A1ED8
 In catch-block, pme points to 0x003A1ED8
 <2> The exception has been caught.
 <-2> MyException destructor has been invoked.
 ____A____
 ____B____