What is the purpose of a function try block? [dupl

问题:

Possible Duplicate:
When is a function try block useful?
Difference between try-catch syntax for function

This code throws an int exception while constructing the Dog object inside class UseResources. The int exception is caught by a normal try-catch block and the code outputs :

Cat()  
Dog()  
~Cat()  
Inside handler

#include <iostream>
using namespace std;

class Cat
{
    public:
    Cat() { cout << "Cat()" << endl; }
    ~Cat() { cout << "~Cat()" << endl; }
};

class Dog
{
    public:
    Dog() { cout << "Dog()" << endl; throw 1; }
    ~Dog() { cout << "~Dog()" << endl; }
};

class UseResources
{
    class Cat cat;
    class Dog dog;

    public:
    UseResources() : cat(), dog() { cout << "UseResources()" << endl; }
    ~UseResources() { cout << "~UseResources()" << endl; }
};

int main()
{
    try
    {
        UseResources ur;
    }
    catch( int )
    {
        cout << "Inside handler" << endl;
    }
}

Now, if we replace the definition of the UseResources() constructor, with one that uses a function try block, as below,

UseResources() try : cat(), dog() { cout << "UseResources()" << endl; } catch(int) {}

the output is the same

Cat()  
Dog()  
~Cat()  
Inside handler 

i.e., with exactly the same final result.

What is then, the purpose of a function try block ?

回答1:

Imagine if UseResources was defined like this:

class UseResources
{
    class Cat *cat;
    class Dog dog;

    public:
    UseResources() : cat(new Cat), dog() { cout << "UseResources()" << endl; }
    ~UseResources() { delete cat; cat = NULL; cout << "~UseResources()" << endl; }
};

If Dog::Dog() throws, then cat will leak memory. Becase UseResources's constructor never completed, the object was never fully constructed. And therefore it does not have its destructor called.

To prevent this leak, you must use a function-level try/catch block:

UseResources() try : cat(new Cat), dog() { cout << "UseResources()" << endl; } catch(...)
{
  delete cat;
  throw;
}

To answer your question more fully, the purpose of a function-level try/catch block in constructors is specifically to do this kind of cleanup. Function-level try/catch blocks cannot swallow exceptions (regular ones can). If they catch something, they will throw it again when they reach the end of the catch block, unless you rethrow it explicitly with throw. You can transform one type of exception into another, but you can't just swallow it and keep going like it didn't happen.

This is another reason why values and smart pointers should be used instead of naked pointers, even as class members. Because, as in your case, if you just have member values instead of pointers, you don't have to do this. It's the use of a naked pointer (or other form of resource not managed in a RAII object) that forces this kind of thing.

Note that this is pretty much the only legitimate use of function try/catch blocks.

---

More reasons not to use function try blocks. The above code is subtly broken. Consider this:

class Cat
{
  public:
  Cat() {throw "oops";}
};

So, what happens in UseResources's constructor? Well, the expression new Cat will throw, obviously. But that means that cat never got initialized. Which means that delete cat will yield undefined behavior.

You might try to correct this by using a complex lambda instead of just new Cat:

UseResources() try
  : cat([]() -> Cat* try{ return new Cat;}catch(...) {return nullptr;} }())
  , dog()
{ cout << "UseResources()" << endl; }
catch(...)
{
  delete cat;
  throw;
}

That theoretically fixes the problem, but it breaks an assumed invariant of UseResources. Namely, that UseResources::cat will at all times be a valid pointer. If that is indeed an invariant of UseResources, then this code will fail because it permits the construction of UseResources in spite of the exception.

Basically, there is no way to make this code safe unless new Cat is noexcept (either explicitly or implicitly).

By contrast, this always works:

class UseResources
{
    unique_ptr<Cat> cat;
    Dog dog;

    public:
    UseResources() : cat(new Cat), dog() { cout << "UseResources()" << endl; }
    ~UseResources() { cout << "~UseResources()" << endl; }
};

In short, look on a function-level try-block as a serious code smell.

回答2:

Ordinary function try blocks have relatively little purpose. They're almost identical to a try block inside the body:

int f1() try {
  // body
} catch (Exc const & e) {
  return -1;
}

int f2() {
  try {
    // body
  } catch (Exc const & e) {
    return -1;
  }
}

The only difference is that the function-try-block lives in the slightly larger function-scope, while the second construction lives in the function-body-scope -- the former scope only sees the function arguments, the latter also local variables (but this doesn't affect the two versions of try blocks).

The only interesting application comes in a constructor-try-block:

Foo() try : a(1,2), b(), c(true) { /* ... */ } catch(...) { }

This is the only way that exceptions from one of the initializers can be caught. You cannot handle the exception, since the entire object construction must still fail (hence you must exit the catch block with an exception, whether you want or not). However, it is the only way to handle exceptions from the initializer list specifically.

Is this useful? Probably not. There's essentially no difference between a constructor try block and the following, more typical "initialize-to-null-and-assign" pattern, which is itself terrible:

Foo() : p1(NULL), p2(NULL), p3(NULL) {
  p1 = new Bar;
  try {
    p2 = new Zip;
    try {
      p3 = new Gulp;
    } catch (...) {
      delete p2;
      throw;
    }
  } catch(...) {
    delete p1;
    throw;
  }
}

As you can see, you have an unmaintainable, unscalable mess. A constructor-try-block would be even worse because you couldn't even tell how many of the pointers have already been assigned. So really it is only useful if you have precisely two leakable allocations. Update: Thanks to reading this question I was alerted to the fact that actually you cannot use the catch block to clean up resources at all, since referring to member objects is undefined behaviour. So [end update]

In short: It is useless.