How to determine if an object is an instance of certain derived C++ class from a pointer to a base class in GDB?

Question

I'm debugging a C++ program with GDB.

I have a pointer to an object of certain class. The pointer is declared to be of some super class which is extended by several sub-classes.

There is no fields in the object to specify the precise class type of this object but some virtual functions (e.g. bool is_xxx()) are defined to tell the class type at runtime.

Is there some way to tell the precise class type of an object in GDB without calling these virtual functions. Calling such functions in GDB may generate confusing result when the program is multi-threaded.

Answer 1

Use ptype. If you use it by itself, you get the declared type of the pointer:

(gdb) ptype ptr type = class SuperClass {   // various members } * 

To get the actual type of the object pointed to, set the "print object" variable:

(gdb) set print object on (gdb) ptype ptr type = /* real type = DerivedClass * */ class SuperClass {   // various members } * 

Answer 2

On my system ptype or whatis also only show the obvious.

(gdb) whatis pObject type = QObject * 

But printing the first entry of the vtable helped me:

(gdb) p /a (*(void ***)pObject)[0] $4 = 0xb4b4cdf4 <QMessageBox::metaObject() const> 

Here the pObject pointed to a QMessageBox which is derived from QObject. This only works if vtable-entry points to a method that is overridden by the derived class.

See also: Print C++ vtables using GDB

Edit: Printing only the pointer to the vtable works more reliable (though the output uses the mangled name and is not so readable):

(gdb) p /a (*(void ***)pObject) $5 = 0xb4af33a0 <_ZTV11QMessageBox+8>