如何在取消引用之前检查迭代器（C ++）？

Question

I am using static analysis tool to find a bugs in my code.It shows the error before dereferencing u have to check your iterator is NULL or not.How can I check this? See the code below:

for(ClientThreadGroupList_t::iterator it = m_vecClientThreadGroup.begin(); it != m_vecClientThreadGroup.end(); it++) 
{
  nCount += (*it)->ConsoleList(pSocket); //error
}

Answer 1

The check for iterator validity is already in your code:

it != m_vecClientThreadGroup.end()

Note that, in general, you can't check whether an iterator is valid or dereferencably; but, when you're iterating over a sequence, you know that the iterators will be dereferencable until you reach the end of the sequence.

Since your sequence apparently contains pointers, you might also need to check whether they are null before dereferencing:

if (*it) {
    nCount += (*it)->ConsoleList(pSocket);
}

Note that this will only check for null pointers; you can't in general check whether or not a non-null pointer actually points to a valid object.

Answer 2

for(ClientThreadGroupList_t::iterator it = m_vecClientThreadGroup.begin(); it != m_vecClientThreadGroup.end(); it++) {
if (*it != 0)
   nCount += (*it)->ConsoleList(pSocket); //error    
}

m_vecClientThreadGroup.begin() returns NULL ptr as i suppose

and are you sure you want to use (*it)->ConsoleList(pSocket) instead of it->ConsoleList(pSocket)?

Answer 3

A pointer can implicitly converted to Boolean type , simply call if(*it) to test nullity of a pointer

for(ClientThreadGroupList_t::iterator it = m_vecClientThreadGroup.begin(); 
    it != m_vecClientThreadGroup.end(); ++it) 
//                                      ^^^^ // better practice
{
    if (*it)  // test if a pointer is null or not
    {
        nCount += (*it)->ConsoleList(pSocket);  
    }
}

Update from comment:

Looks like the issue is multiple threading issue, you need to provide lock mechanism to avoid another thread invalid your current iterators.

Answer 4

You can simply check it with an if condition :

for(ClientThreadGroupList_t::iterator it = m_vecClientThreadGroup.begin(); it != m_vecClientThreadGroup.end(); it++) 
{
    if ( (*it) != NULL ) // or in C++11 != nullptr
        nCount += (*it)->ConsoleList(pSocket); //error
}

It is the call to ConsoleList method on a NULL object who cause the error I suppose. Because, if there is no element in your list , begin() will be the same as end() and you will never enter in the loop.

For brevity you can do :

if ( *it ) // Implicit convertion here...