c++异常处理(转载)
2013-12-16 19:56
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例1:自定义一个继承自excepton的异常类myException
C++标准中,定义在<stdexcept>中的任何异常类都派生自exception Class,本例也只是简单地由exception继承,在try段抛出一个异常并捕捉。代码如下:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.0
decript:define a exception class named myException
derived from base class exception
which is declared in <exception>
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
myException():exception("ERROR! Don't divide a number by integer zero.\n")
{
}
};
//entry of the application
int main()
{
int x=100,y=0;
try
{
if(y==0) throw myException();
else cout<<x/y;
}
catch(myException& me)
{
cout<<me.what();
}
system("pause");
return 0;
}
结果如下:
ERROR! Don't divide a number by integer zero.
请按任意键继续. . .
显然,异常被捕捉到了。此处需要说明的是,VC对异常处理类exception进行了扩展,本例之所以能够使用exception("ERROR!....")的初始化方法正出于这样的原因,C++标准是不允许这样做的。
与此同时,VC又没有遵循标准,有力地支持terminate和unexpected,它只保留了语法,却在编译运行时不提供支持。为了结合terminate和unexpected更加深入了解C++的异常处理,下面的例子采用Dev cpp IDE实现。
例2:依照C++标准实现自定义异常类myException并将throw语句封装到函数check()中
涉及到的更改正如标题所述,(1)重写基类的what()函数,返回错误信息;(2)将throw myException()封装到check()函数中;(3)允许check()函数抛出myException类型的异常。代码如下:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.1
decript:define a exception class named myException
according to C++ standard,
derived from base class exception
which is declared in <exception>
!also,encapusulate throw into a function
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()//#1
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) throw(myException)//#2
{
if(y==0) throw myException();
}
//entry of the application
int main()
{
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(myException& me)
{
cout<<me.what();
}
system("pause");
return 0;
}
结果与例1完全相同。需说明的是,紧跟check()后的throw列表表明允许该函数抛出的异常类型。这里不得不产生疑问,如果抛出了一个不被允许的异常类型将怎样?
例3:抛出unexpected异常
check函数体之后的throw列表,规定了允许抛出的异常类型,一旦违背,就将触发unexpected。可以把unexpected看作系统自动调用的CALLBACK函数,不同的是,也可以手工触发它的执行。本例的情况属于前者。代码如下:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.3
decript:define an unexpected excption handler,
set it by using set_unexpected,
modify the throw list of function check
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) throw()//#1 only int-type exception is permitted
{
if(y==0) throw myException();
}
void myUnexpected()
{
cout<<"Unexpected exception caught!\n";
system("pause");
exit(-1);
}
//entry of the application
int main()
{
unexpected_handler oldHandler=set_unexpected(myUnexpected);
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(myException& me)
{
cout<<me.what();
}
system("pause");
return 0;
}
结果如下:
Unexpected exception caught!
请按任意键继续. . .
check函数的throw列表为空,即不允许抛出任何类型的异常,然而实际上当异常发生时,系统不能等闲视之,它将调用unexpected处理方法。所以,限定一个函数throw列表为空是值得程序员警醒的事,需要特别留意。如果将#1处的代码修改为throw(int)等也能得到相同的结果。所谓unexpected异常,说白了就是函数体允许抛出异常类型范围之外的异常。如果check函数后面根本没有throw,则表示函数任何类型的异常都被允许。
例4:抛出函数体允许的异常,但没被捕捉到的情况
思考这样一个问题,如果函数check的throw列表中有异常类型myException,而且在y==0时,它的确抛出myException类型的异常,但是没有被catch到,这时会发生什么?
在正式回答这个问题之前,先讨论“没被catch到”的意思。比如,修改例3的代码如下:(##为修改之处)
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.4.1
decript:
how to understand "exception not caucht"?
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) //any type of exception is permitted
{
if(y==0) throw myException();
}
void myUnexpected()
{
cout<<"Unexpected exception caught!\n";
system("pause");
exit(-1);
}
//entry of the application
int main()
{
unexpected_handler oldHandler=set_unexpected(myUnexpected);
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(int &e) //##1 no catch sentence matches the throw type
{
cout<<e<<endl;
}
/* ##2 if add this part, any type which's not handler before will
be caught
catch(...)
{
cout<<"Unkown exception caught!\n";
}
*/
system("pause");
return 0;
}
编译运行,程序将会出错,因为check函数抛出的myException异常没有被处理。在缺省情况下,一旦出现抛出异常没被处理的问题,系统将自动调用abort()函数,终止程序允许,在控制台将会看到这样的提示:
This application has requested the Runtime to terminate it in an unusual way.Please contact the application's support team for more information.
不过可以增加##2部分的代码,catch(...)表示捕捉任何类型的异常。
注意:check函数不被允许的异常类型并不会进入到catch语句的判断中来,因此catch(...)对unexpected exception没有作用。
仍然考虑没有##2部分的情况。正如前面所述,系统将自动调用abort()函数终止程序。实际上,它触发的是terminate,类似于unexpected,仍然可以自定义terminate的处理方法。甚至terminate语法上跟unexpected都十分近似。修改代码为:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.4.2
decript:
how to understand "exception not caucht"?
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) //any type of exception is permitted
{
if(y==0) throw myException();
}
void myUnexpected()
{
cout<<"Unexpected exception caught!\n";
system("pause");
exit(-1);
}
void myTerminate() //##1 set it be the terminate handler
{
cout<<"Unhandler exception!\n";
system("pause");
exit(-1);
}
//entry of the application
int main()
{
unexpected_handler oldHandler=set_unexpected(myUnexpected);
terminate_handler preHandler=set_terminate(myTerminate);
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(int &e) //no catch sentence matches the throw type
{
cout<<e<<endl;
}
system("pause");
return 0;
}
结果如下:
Unhandler exception!
请按任意键继续. . .
结论:C++为异常处理提供了友好的支持。
用户可以自定义异常类型,异常类型并不受到限制,可以是内建数据类型如int,double等,也可以是自定义的类,也可以从C++某个异常类继承下来。例1采用了派生自exception的方法。
除此之外,在定义函数时,可以显式指定函数体抛出的异常类型。隐式情况下,缺省允许函数抛出任何类型的异常。有可以增加throw语句,对异常类型加以限制。特别的是,throw()表示不允许函数抛出任何类型的异常。如果违反了throw列表规定的异常类型,系统将调用unexpected hanlder进行处理,可以自定义unexpected异常处理方法。例2和例3对它们进行了说明。
如果对于函数体throw列表合法的异常被抛出,但是却没有被程序捕捉处理,系统将调用terminate handler进行处理。缺省情况下,只是简单调用abort()函数终止程序,同样可以自定义terminate处理方法。例4对它进行了说明
C++标准中,定义在<stdexcept>中的任何异常类都派生自exception Class,本例也只是简单地由exception继承,在try段抛出一个异常并捕捉。代码如下:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.0
decript:define a exception class named myException
derived from base class exception
which is declared in <exception>
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
myException():exception("ERROR! Don't divide a number by integer zero.\n")
{
}
};
//entry of the application
int main()
{
int x=100,y=0;
try
{
if(y==0) throw myException();
else cout<<x/y;
}
catch(myException& me)
{
cout<<me.what();
}
system("pause");
return 0;
}
/*++ test.cpp version:1.0 decript:define a exception class named myException derived from base class exception which is declared in <exception> created:2011-08-14 author: btwsmile --*/ #include<exception> #include<iostream> using namespace std; //customized exception class 'myException' class myException:public exception { public: myException():exception("ERROR! Don't divide a number by integer zero.\n") { } }; //entry of the application int main() { int x=100,y=0; try { if(y==0) throw myException(); else cout<<x/y; } catch(myException& me) { cout<<me.what(); } system("pause"); return 0; }
结果如下:
ERROR! Don't divide a number by integer zero.
请按任意键继续. . .
显然,异常被捕捉到了。此处需要说明的是,VC对异常处理类exception进行了扩展,本例之所以能够使用exception("ERROR!....")的初始化方法正出于这样的原因,C++标准是不允许这样做的。
与此同时,VC又没有遵循标准,有力地支持terminate和unexpected,它只保留了语法,却在编译运行时不提供支持。为了结合terminate和unexpected更加深入了解C++的异常处理,下面的例子采用Dev cpp IDE实现。
例2:依照C++标准实现自定义异常类myException并将throw语句封装到函数check()中
涉及到的更改正如标题所述,(1)重写基类的what()函数,返回错误信息;(2)将throw myException()封装到check()函数中;(3)允许check()函数抛出myException类型的异常。代码如下:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.1
decript:define a exception class named myException
according to C++ standard,
derived from base class exception
which is declared in <exception>
!also,encapusulate throw into a function
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()//#1
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) throw(myException)//#2
{
if(y==0) throw myException();
}
//entry of the application
int main()
{
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(myException& me)
{
cout<<me.what();
}
system("pause");
return 0;
}
/*++ test.cpp version:1.1 decript:define a exception class named myException according to C++ standard, derived from base class exception which is declared in <exception> !also,encapusulate throw into a function created:2011-08-14 author: btwsmile --*/ #include<exception> #include<iostream> using namespace std; //customized exception class 'myException' class myException:public exception { public: const char* what()const throw()//#1 { return "ERROR! Don't divide a number by integer zero.\n"; } }; void check(int y) throw(myException)//#2 { if(y==0) throw myException(); } //entry of the application int main() { int x=100,y=0; try { check(y); cout<<x/y; } catch(myException& me) { cout<<me.what(); } system("pause"); return 0; }
结果与例1完全相同。需说明的是,紧跟check()后的throw列表表明允许该函数抛出的异常类型。这里不得不产生疑问,如果抛出了一个不被允许的异常类型将怎样?
例3:抛出unexpected异常
check函数体之后的throw列表,规定了允许抛出的异常类型,一旦违背,就将触发unexpected。可以把unexpected看作系统自动调用的CALLBACK函数,不同的是,也可以手工触发它的执行。本例的情况属于前者。代码如下:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.3
decript:define an unexpected excption handler,
set it by using set_unexpected,
modify the throw list of function check
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) throw()//#1 only int-type exception is permitted
{
if(y==0) throw myException();
}
void myUnexpected()
{
cout<<"Unexpected exception caught!\n";
system("pause");
exit(-1);
}
//entry of the application
int main()
{
unexpected_handler oldHandler=set_unexpected(myUnexpected);
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(myException& me)
{
cout<<me.what();
}
system("pause");
return 0;
}
/*++ test.cpp version:1.3 decript:define an unexpected excption handler, set it by using set_unexpected, modify the throw list of function check created:2011-08-14 author: btwsmile --*/ #include<exception> #include<iostream> using namespace std; //customized exception class 'myException' class myException:public exception { public: const char* what()const throw() { return "ERROR! Don't divide a number by integer zero.\n"; } }; void check(int y) throw()//#1 only int-type exception is permitted { if(y==0) throw myException(); } void myUnexpected() { cout<<"Unexpected exception caught!\n"; system("pause"); exit(-1); } //entry of the application int main() { unexpected_handler oldHandler=set_unexpected(myUnexpected); int x=100,y=0; try { check(y); cout<<x/y; } catch(myException& me) { cout<<me.what(); } system("pause"); return 0; }
结果如下:
Unexpected exception caught!
请按任意键继续. . .
check函数的throw列表为空,即不允许抛出任何类型的异常,然而实际上当异常发生时,系统不能等闲视之,它将调用unexpected处理方法。所以,限定一个函数throw列表为空是值得程序员警醒的事,需要特别留意。如果将#1处的代码修改为throw(int)等也能得到相同的结果。所谓unexpected异常,说白了就是函数体允许抛出异常类型范围之外的异常。如果check函数后面根本没有throw,则表示函数任何类型的异常都被允许。
例4:抛出函数体允许的异常,但没被捕捉到的情况
思考这样一个问题,如果函数check的throw列表中有异常类型myException,而且在y==0时,它的确抛出myException类型的异常,但是没有被catch到,这时会发生什么?
在正式回答这个问题之前,先讨论“没被catch到”的意思。比如,修改例3的代码如下:(##为修改之处)
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.4.1
decript:
how to understand "exception not caucht"?
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) //any type of exception is permitted
{
if(y==0) throw myException();
}
void myUnexpected()
{
cout<<"Unexpected exception caught!\n";
system("pause");
exit(-1);
}
//entry of the application
int main()
{
unexpected_handler oldHandler=set_unexpected(myUnexpected);
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(int &e) //##1 no catch sentence matches the throw type
{
cout<<e<<endl;
}
/* ##2 if add this part, any type which's not handler before will
be caught
catch(...)
{
cout<<"Unkown exception caught!\n";
}
*/
system("pause");
return 0;
}
/*++ test.cpp version:1.4.1 decript: how to understand "exception not caucht"? created:2011-08-14 author: btwsmile --*/ #include<exception> #include<iostream> using namespace std; //customized exception class 'myException' class myException:public exception { public: const char* what()const throw() { return "ERROR! Don't divide a number by integer zero.\n"; } }; void check(int y) //any type of exception is permitted { if(y==0) throw myException(); } void myUnexpected() { cout<<"Unexpected exception caught!\n"; system("pause"); exit(-1); } //entry of the application int main() { unexpected_handler oldHandler=set_unexpected(myUnexpected); int x=100,y=0; try { check(y); cout<<x/y; } catch(int &e) //##1 no catch sentence matches the throw type { cout<<e<<endl; } /* ##2 if add this part, any type which's not handler before will be caught catch(...) { cout<<"Unkown exception caught!\n"; } */ system("pause"); return 0; }
编译运行,程序将会出错,因为check函数抛出的myException异常没有被处理。在缺省情况下,一旦出现抛出异常没被处理的问题,系统将自动调用abort()函数,终止程序允许,在控制台将会看到这样的提示:
This application has requested the Runtime to terminate it in an unusual way.Please contact the application's support team for more information.
不过可以增加##2部分的代码,catch(...)表示捕捉任何类型的异常。
注意:check函数不被允许的异常类型并不会进入到catch语句的判断中来,因此catch(...)对unexpected exception没有作用。
仍然考虑没有##2部分的情况。正如前面所述,系统将自动调用abort()函数终止程序。实际上,它触发的是terminate,类似于unexpected,仍然可以自定义terminate的处理方法。甚至terminate语法上跟unexpected都十分近似。修改代码为:
[cpp] view plaincopyprint?
/*++ test.cpp
version:1.4.2
decript:
how to understand "exception not caucht"?
created:2011-08-14
author: btwsmile
--*/
#include<exception>
#include<iostream>
using namespace std;
//customized exception class 'myException'
class myException:public exception
{
public:
const char* what()const throw()
{
return "ERROR! Don't divide a number by integer zero.\n";
}
};
void check(int y) //any type of exception is permitted
{
if(y==0) throw myException();
}
void myUnexpected()
{
cout<<"Unexpected exception caught!\n";
system("pause");
exit(-1);
}
void myTerminate() //##1 set it be the terminate handler
{
cout<<"Unhandler exception!\n";
system("pause");
exit(-1);
}
//entry of the application
int main()
{
unexpected_handler oldHandler=set_unexpected(myUnexpected);
terminate_handler preHandler=set_terminate(myTerminate);
int x=100,y=0;
try
{
check(y);
cout<<x/y;
}
catch(int &e) //no catch sentence matches the throw type
{
cout<<e<<endl;
}
system("pause");
return 0;
}
/*++ test.cpp version:1.4.2 decript: how to understand "exception not caucht"? created:2011-08-14 author: btwsmile --*/ #include<exception> #include<iostream> using namespace std; //customized exception class 'myException' class myException:public exception { public: const char* what()const throw() { return "ERROR! Don't divide a number by integer zero.\n"; } }; void check(int y) //any type of exception is permitted { if(y==0) throw myException(); } void myUnexpected() { cout<<"Unexpected exception caught!\n"; system("pause"); exit(-1); } void myTerminate() //##1 set it be the terminate handler { cout<<"Unhandler exception!\n"; system("pause"); exit(-1); } //entry of the application int main() { unexpected_handler oldHandler=set_unexpected(myUnexpected); terminate_handler preHandler=set_terminate(myTerminate); int x=100,y=0; try { check(y); cout<<x/y; } catch(int &e) //no catch sentence matches the throw type { cout<<e<<endl; } system("pause"); return 0; }
结果如下:
Unhandler exception!
请按任意键继续. . .
结论:C++为异常处理提供了友好的支持。
用户可以自定义异常类型,异常类型并不受到限制,可以是内建数据类型如int,double等,也可以是自定义的类,也可以从C++某个异常类继承下来。例1采用了派生自exception的方法。
除此之外,在定义函数时,可以显式指定函数体抛出的异常类型。隐式情况下,缺省允许函数抛出任何类型的异常。有可以增加throw语句,对异常类型加以限制。特别的是,throw()表示不允许函数抛出任何类型的异常。如果违反了throw列表规定的异常类型,系统将调用unexpected hanlder进行处理,可以自定义unexpected异常处理方法。例2和例3对它们进行了说明。
如果对于函数体throw列表合法的异常被抛出,但是却没有被程序捕捉处理,系统将调用terminate handler进行处理。缺省情况下,只是简单调用abort()函数终止程序,同样可以自定义terminate处理方法。例4对它进行了说明
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