The Secret Life of C++: Objects
We talked about structs in C. Lets take a look at C++ structs and classes.Just Fields
First, a basic struct with no constructor:- _Znwj
- Translates into the new operator over the type
unsigned int - _ZdlPv
- delete a pointer to a void.
Simple Methods
Next, a basic class this time (really no difference), still no constructor, this time we call some simple get/set methods:- _ZN8onefield8setFieldEi
- method onefield::setField(int)
- _ZNK8onefield8getFieldEv
- Konst method onefield::getField(void)
Notice that the this pointer gets passed as the first
argument to both functions, as you might expect.
Constructors
Lets give our simple object a constructor and a destructor, and see how that goes:- _ZN8onefieldC1Ei
- onefield Constructor (C1 means it a is complete object constructor) that takes an int parameter.
- _ZN8onefieldD1Ev
- onefield complete object desctructor, no parameters.
Temporary Objects
When you call a function with an object, and you don't pass a pointer/reference, the function gets a copy of the object. Similarly when you return an object, there is a temporary one created for the return.
Temporary Objects as Arguments
When passing a temporary object as an argument, the caller must make space on the stack, call the constructor, pass a reference/pointer to the object, and then call the destructor when the life of the object is over. It could also be allocated on the heap.Lets look at the first case, using our onefield object again:
- _ZN8onefieldC1ERKS_
- onefield constructor that takes a
Reference to a Konst onefield.
S_is shorthand. In this case for onefield. - _Z5print8onefield_Z5print8onefield
- This is our print(onefield) function.
Temporary Objects as Return Values
When returning an object from a function, if the object is non-trivial, than the caller allocates temporary space and passes a pointer to the space as a first argument to the function. The function calls the constructor. The caller is responsible for calling the destructor when the object is no longer needed.Examples, using a twofield object:
Virtual Methods - Call by Pointer/Reference
How do virtual methods work? With a vtable. What a vtable? Lets look at an example: basic-vtable.cc, basic-vtable.s, basic-vtable.listing.- _ZTV8onefield
- A pointer to the VTable of the onefield object.
- _ZTI8onefield
- A pointer to the typeinfo structure for onefield.
- _ZTVN10__cxxabiv117__class_type_infoE+8
- Pointer to the VTable for the implementation object of class_type_info.
Virtual Methods - Overriding
Lets do an example where we override a virtual method: subclass-vtable.cc, subclass-vtable.s, subclass-vtable.listing.- _ZTV17onefield_subclass
- This is the vtable for onefield_subclass.
Virtual Methods - Call by "Value"
When you call a virtual method on a local or temporary variable, the compiler really does know the type of the variable, and so it doesn't have to go through the vtable. This is a useful optimization. It can be seen in the basic-vtable example above.It is also why you have to make sure to pass a subclass into a method expecting a superclass by reference, not by copyconstructing an instance of the superclass. Becuase the later technique will cause the superclass functions to get called, rather than the subclass ones, even if they are virtual.
Pure Virtual Methods
Pure virtual methods are just funny entries in the vtable. Specifically pointers to __cxa_pure_virtual. An example: vtable-null.cc, vtable-null.s, vtable-null.listing.Virtual Destructors
You want a virtual destructor becuase otherwise someone might delete your object when they have a pointer to a superclass, and your destructor won't get run. Simple as that.Subclasses
To see subclasses that add data values, we can look back at our subclass-vtable example: subclass-vtable.cc, subclass-vtable.s, subclass-vtable.listing.