mi12.htm

一个非常适合初学者入门的有关c++的文档

catch (Widget w) ...                // catch exception by value


catch (Widget& w) ...               // catch exception by
                                    // reference


catch (const Widget& w) ...         // catch exception by
                                    // reference-to-const


Right away we notice another difference between parameter passing and exception propagation. A thrown object (which, as explained above, is always a temporary) may be caught by simple reference; it need not be caught by reference-to-const. Passing a temporary object to a non-const reference parameter is not allowed for function calls (see Item 19), but it is for exceptions.
Let us overlook this difference, however, and return to our examination of copying exception objects. We know that when we pass a function argument by value, we make a copy of the passed object (see Item E22), and we store that copy in a function parameter. The same thing happens when we pass an exception by value. Thus, when we declare a catch clause like this, 
catch (Widget w) ...                // catch by value


we expect to pay for the creation of two copies of the thrown object, one to create the temporary that all exceptions generate, the second to copy that temporary into w. Similarly, when we catch an exception by reference, 
catch (Widget& w) ...               // catch by reference

catch (const Widget& w) ...         // also catch by reference


we still expect to pay for the creation of a copy of the exception: the copy that is the temporary. In contrast, when we pass function parameters by reference, no copying takes place. When throwing an exception, then, we expect to construct (and later destruct) one more copy of the thrown object than if we passed the same object to a function.
We have not yet discussed throwing exceptions by pointer, but throw by pointer is equivalent to pass by pointer. Either way, a copy of the pointer is passed. About all you need to remember is not to throw a pointer to a local object, because that local object will be destroyed when the exception leaves the local object's scope. The catch clause would then be initialized with a pointer to an object that had already been destroyed. This is the behavior the mandatory copying rule is designed to avoid.
The way in which objects are moved from call or throw sites to parameters or catch clauses is one way in which argument passing differs from exception propagation. A second difference lies in what constitutes a type match between caller or thrower and callee or catcher. Consider the sqrt function from the standard math library: 
double sqrt(double);                 // from <cmath> or <math.h>


We can determine the square root of an integer like this: int i;

double sqrtOfi = sqrt(i);


There is nothing surprising here. The language allows implicit conversion from int to double, so in the call to sqrt, i is silently converted to a double, and the result of sqrt corresponds to that double. (See Item 5 for a fuller discussion of implicit type conversions.) In general, such conversions are not applied when matching exceptions to catch clauses. In this code, void f(int value)
{
  try {
    if (someFunction()) {      // if someFunction() returns
      throw value;             // true, throw an int
    ...
    }
  }
  catch (double d) {           // handle exceptions of
    ...                        // type double here
  }

  ...

}


the int exception thrown inside the try block will never be caught by the catch clause that takes a double. That clause catches only exceptions that are exactly of type double; no type conversions are applied. As a result, if the int exception is to be caught, it will have to be by some other (dynamically enclosing) catch clause taking an int or an int& (possibly modified by const or volatile).
Two kinds of conversions are applied when matching exceptions to catch clauses. The first is inheritance-based conversions. A catch clause for base class exceptions is allowed to handle exceptions of derived class types, too. For example, consider the diagnostics portion of the hierarchy of exceptions defined by the standard C++ library (see Item E49):
A catch clause for runtime_errors can catch exceptions of type range_error and overflow_error, too, and a catch clause accepting an object of the root class exception can catch any kind of exception derived from this hierarchy.
This inheritance-based exception-conversion rule applies to values, references, and pointers in the usual fashion: 
catch (runtime_error) ...               // can catch errors of type
catch (runtime_error&) ...              // runtime_error,
catch (const runtime_error&) ...        // range_error, or
                                        // overflow_error


catch (runtime_error*) ...              // can catch errors of type
catch (const runtime_error*) ...        // runtime_error*,
                                        // range_error*, or
                                        // overflow_error*


The second type of allowed conversion is from a typed to an untyped pointer, so a catch clause taking a const void* pointer will catch an exception of any pointer type: 
catch (const void*) ...                 // catches any exception
                                        // that's a pointer


The final difference between passing a parameter and propagating an exception is that catch clauses are always tried in the order of their appearance. Hence, it is possible for an exception of a derived class type to be handled by a catch clause for one of its base class types even when a catch clause for the derived class is associated with the same try block! For example, try {
  ...
}
catch (logic_error& ex) {              // this block will catch
  ...                                  // all logic_error
}                                      // exceptions, even those
                                       // of derived types

catch (invalid_argument& ex) {         // this block can never be
  ...                                  // executed, because all
}                                      // invalid_argument
                                       // exceptions will be caught
                                       // by the clause above


Contrast this behavior with what happens when you call a virtual function. When you call a virtual function, the function invoked is the one in the class closest to the dynamic type of the object invoking the function. You might say that virtual functions employ a "best fit" algorithm, while exception handling follows a "first fit" strategy. Compilers may warn you if a catch clause for a derived class comes after one for a base class (some issue an error, because such code used to be illegal in C++), but your best course of action is preemptive: never put a catch clause for a base class before a catch clause for a derived class. The code above, for example, should be reordered like this: try {
  ...
}
catch (invalid_argument& ex) {          // handle invalid_argument
  ...                                   // exceptions here
}
catch (logic_error& ex) {               // handle all other
  ...                                   // logic_errors here
}


There are thus three primary ways in which passing an object to a function or using that object to invoke a virtual function differs from throwing the object as an exception. First, exception objects are always copied; when caught by value, they are copied twice. Objects passed to function parameters need not be copied at all. Second, objects thrown as exceptions are subject to fewer forms of type conversion than are objects passed to functions. Finally, catch clauses are examined in the order in which they appear in the source code, and the first one that can succeed is selected for execution. When an object is used to invoke a virtual function, the function selected is the one that provides the best match for the type of the object, even if it's not the first one listed in the source code. Back to Item 11: Prevent exceptions from leaving destructors
Continue to Item 13: Catch exceptions by reference