# V757. It is possible that an incorrect variable is compared with null after type conversion using 'dynamic_cast'.

The analyzer has detected a potential error that may lead to memory access by a null pointer.

The situation that the analyzer detected deals with the following algorithm. A pointer to the base class is first cast to a pointer to the derived class by using the 'dynamic_cast' operator. Then the same pointer is checked for a 'nullptr' value, though it is the pointer received as a result of casting that this check should have been applied to.

Here's an example. In this code, the 'baseObj' pointer will not address an instance of the 'Derived' class, in which case, when calling the 'Func' function, the null pointer will be dereferenced. The analyzer will output a warning pointing out two lines. The first line is the spot where the pointer is checked for 'nullptr'; the second is where the pointer to the base class is cast to a pointer to the derived class.

    Base *baseObj;
    ....
    Derived *derivedObj = dynamic_cast<Derived *>(baseObj);
    if (baseObj != nullptr)
    {
        derivedObj->Func();
    }

In this example before using it is most likely the pointer received as a result of casting that the programmer intended to check for 'nullptr' before using it. This is the fixed version of the code:

    Base *baseObj;
    ....
    Derived *derivedObj = dynamic_cast<Derived *>(baseObj);
    if (derivedObj != nullptr)
    {
        derivedObj->Func();
    }

This diagnostic is classified as:

CWE-690 CERT-EXP12-C

You can look at examples of errors detected by the V757 diagnostic.