C++ exception handling, error

Question

I am using a library, RapidXML, but my problem is more general. The library parses xml like item->first_node("CRAP")->first_node("CRAP") Now, if I put this in an if statement it will crash. If I put this: item->first_node("CRAP") it won't.

I am a beginner in C++ and I don't know much about exceptions but:

try
{
    if(item->first_node("CRAP")->first_node("CRAP"))
    {

    }
    cout << "OK";
} catch (...)
{
    cout << "CRASH";
}

The above crashes. How to check if my node exists without crashes (and without looping all the items one by one)?

Always post full error messages (i.e. copy and paste the entire error message from start to finish) instead of using vague terms like "it crashes". — JBentley, Dec 20 '13 at 03:33
@chris: I believe this is it, but how to check if a null pointer pointing to a null pointer is null? — Blazer, Dec 20 '13 at 03:34
You cannot have a null pointer pointing to a null pointer (that makes no sense). But to check if a pointer is null, see [here](http://stackoverflow.com/questions/6185821/so-how-do-we-check-if-a-pointer-is-null-pointer). — JBentley, Dec 20 '13 at 03:36
What does this have to do with C++ exceptions? Does the library specify that `first_node` ever throws anything? — Potatoswatter, Dec 20 '13 at 03:42
It throws this: http://rapidxml.sourceforge.net/manual.html#classrapidxml_1_1parse__error — Blazer, Dec 20 '13 at 03:51

score 2 · Answer 1 · answered Dec 20 '13 at 03:40

2

You simply need to take it one step at a time:

if (item != 0) // check if item is null
{
    rapidxml::xml_node<char>* node = item->first_node("CRAP"); // Try to grab first child node
    if (node != 0)
    {
        // okay got a valid node, grab next one
        rapidxml::xml_node<char>* next = node->first_node("CRAP");
        if (next != 0)
        {
           // Okay
        }
    }
}

When you try it in one step, i.e. item->first_node("CRAP")->first_node("CRAP"), you never check that the first call to first_node returned a null pointer (assuming item is a valid pointer also).

answered Dec 20 '13 at 03:40

Jesse Good

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Assuming `item` is not NULL to begin with. – Remy Lebeau Dec 20 '13 at 03:41
1

`auto` implies C++11, so you should compare against `nullptr` instead of `0`. – Remy Lebeau Dec 20 '13 at 03:43

score 2 · Answer 2 · answered Dec 20 '13 at 03:40

Sounds like either item is NULL or item->first_node("CRAP") is returning NULL. Try this, see what output you get:

try
{
    node *n; // <-- use whatever type first_node() actually returns

    if (!item)
        cout << "item is NULL";
    else
    {
        n = item->first_node("CRAP");
        if (!n)
            cout << "first node is NULL";
        else
        {
            n = n->first_node("CRAP");
            if (!n)
                cout << "second node is NULL";
            else
                cout << "OK";
        }
    }
}
catch (...)
{
    cout << "CRASH";
}

Ernest Friedman-Hill · Answer 3 · 2013-12-20T03:51:21.687

2

Always test whether an expression is NULL before using it as part of a longer expression. Never write things like

if(item->first_node("CRAP")->first_node("CRAP"))

if first_node("CRAP") can return NULL. Instead, write something like

if(item->first_node("CRAP") && item->first_node("CRAP")->first_node("CRAP"))

This works because the '&&' (logical and) operator uses lazy evaluation: it won't bother to evaluate its second operand if the first one evaluates to false.

edited Dec 20 '13 at 03:51

answered Dec 20 '13 at 03:41

Ernest Friedman-Hill

80,601
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Heh. Hard to type good answers on an iPhone, don't know why I even try. – Ernest Friedman-Hill Dec 20 '13 at 03:44
@ErnestFriedman-Hill The backtick is available by holding down the quote key on the punctuation keyboard. – Potatoswatter Dec 20 '13 at 03:45
@RemyLebeau -- clarity is more important than performance, except when it's not. Oftentimes a single conditional expression is preferable to a couple of nested blocks, unless you're in a tight loop. – Ernest Friedman-Hill Dec 20 '13 at 03:46
1

It might be worth mentioning that the reason this works is because of [lazy evaluation](http://stackoverflow.com/questions/4613551/good-practice-in-c-lazy-evaluation) (for those who might not be aware of it). – JBentley Dec 20 '13 at 03:47
Yes but if I have 20 items (ITEMS1->ITEM2 etc), how to check it without making my code x100? – Blazer Dec 20 '13 at 03:50
@Blazer That's a great question, and the answer really is that an API that returns `NULL` to mean "not found" is difficult to use this way. A nicer API could return a special "null object" that returned "false" or other null object values from all its methods. You can read about this idea here: http://en.wikipedia.org/wiki/Null_Object_pattern . An API designed this way is much more convenient to use with chained method calls. – Ernest Friedman-Hill Dec 20 '13 at 03:55
@Blazer: Well, you can write a simple wrapper that observes Null Object Pattern as suggested. – Siyuan Ren Dec 20 '13 at 04:56

C++ exception handling, error

3 Answers3