Question

如何捕获派生（多态）异常对象并将其重新抛出到第二级？在我的情况下，派生对象仅保留到“1级”。

我理解c ++编译器完成的'返回值优化'。在我看来，在下面的代码中，'e'的引用应该不再在'Level 2'有效，但是编译器正在做一些隐式创建本地基类的Exception对象，并且我正在丢失从'Level 0'抛出的原始ExtException。 / p>

我的问题是我如何将'ExtException'对象保留到'Level 2'？

#include <string>
#include <iostream>

using std::endl;

typedef std::string pstring;


class Exception
{
    public:
        Exception ( ) {
            my_id = ++static_count;
            std:: cout << "+Exception obj created.  ID: " << my_id << "  (Total: " << static_count << ")" << endl;
        }

        Exception ( const Exception &e ) {
            my_id = ++static_count;
            std:: cout << "+Exception obj created.  ID: " << my_id << "  (Total: " << static_count << ")" << endl;
        }

        virtual ~Exception() {
            --static_count;
            std:: cout << "-Exception obj deleted.  ID: " << my_id << "  (Total: " << static_count << ")" << endl;
        }

    private:
        static int static_count;
        int        my_id;
};
int Exception:: static_count = 0;



class ExtException : public Exception {
    public:
        ExtException () : Exception()
        { 
            std:: cout << "++ExtException obj created" << endl;
        }

        ExtException ( const ExtException &e ) : Exception()
        { 
            std:: cout << "++ExtException obj created" << endl;
        }

        ~ExtException () {
            std:: cout << "--ExtException obj deleted" << endl;
        }
};





void foo2 () {
    throw ExtException();   // Level 0 throw
}


void foo1 () 
{
    try {
        foo2 ();
    } catch ( Exception &e ) {
        // In my normal understanding of c++ 'e' should no longer be valid here 
        // as it was created on a stack which is no longer exists (i.e Level 0 is scoped out).
        //
        // _BUT_  (actually it is valid. and it is valid because of RVO compiler "Return value optimization"

        std:: cout << "\n--- foo1 catch" << endl;
        throw e;  // Level 1 throw
    }
}


void foo () 
{
    try {
        foo1 ();
    } catch ( Exception &e ) {
        // !!! CAUTION !!!
        // In my opinion 'e' must not be valid here as the original "ExtException" must be destroyed till 
        // this point (which is actually destroyed check output trace).
        // It is destroyed because RVO works for one level only...
        //
        // wait a minute... here the compiler has not left 'e' for FMR hit, instead it has created a new temporary object of 'Exception' class and
        //  'e' is referring to that.
        //
        // Here comes my real question!!!  'e' is no longer referring to an actual  ExtException object but the temporary one, 
        // how could I modify this code to access ExtException object created at 'Level 0'?

        std:: cout << "\n--- foo catch" << endl;
        throw e;  // Level 2 throw
    }
}


int main (void) {

    try {
        foo  ();
    } catch (...) { }
    return 1;
}

以下是上述代码的结果：

+Exception obj created.  ID: 1  (Total: 1)
++ExtException obj created

--- foo1 catch
+Exception obj created.  ID: 2  (Total: 2)
--ExtException obj deleted
-Exception obj deleted.  ID: 1  (Total: 1)

--- foo catch
+Exception obj created.  ID: 2  (Total: 2)
-Exception obj deleted.  ID: 2  (Total: 1)
-Exception obj deleted.  ID: 2  (Total: 0)

Answer 1

首选＆＃34; throw＆＃34;在＆＃34;扔e＆＃34;对于多态异常。

参考：

http://ptgmedia.pearsoncmg.com/images/0321113586/items/sutter_item73.pdf

Answer 2

[C++11: 15.1/3]: throw-expression 初始化一个临时对象，称为异常对象，其类型通过删除任何顶级cv-来确定来自throw操作数的静态类型的限定符，并将类型从“T的数组”或“返回的函数T”调整为“指向T的指针”或“指向函数返回T的指针”。 临时值是一个左值，用于初始化匹配处理程序中指定的变量（15.3）。如果异常对象的类型是不完整类型或指向不完整类型的指针（可能是cv-qualified），则程序格式不正确。除了这些限制和15.3中提到的类型匹配限制外， throw的操作数被完全视为调用中的函数参数（5.2.2）或返回的操作数言。

简而言之，你正在切片，尽管异常处理程序已经接受了引用。

重新投掷应该通过单独编写throw来完成。这正是它的目的：

[C++11: 15.1/8]:没有操作数的 throw-expression 重新抛出当前处理的异常（15.3）。 使用现有临时值重新激活该异常;没有创建新的临时异常对象。 [..]

所以：

throw e;  // no
throw;    // yes

如何捕获派生（多态）异常对象并将其重新抛出到第二级？

2 个答案: