C++ Interview Questions: Part 2

Part 2

Q: Can I overload the destructor for my class?
A: No. You can have only one destructor for a class Fred. It's always called Fred::~Fred(). It never takes any parameters, and it never returns anything. You can't pass parameters to the destructor anyway, since you never explicitly call a destructor (well, almost never).

Q: Should I explicitly call a destructor on a local variable?
A: No! The destructor will get called again at the close } of the block in which the local was created. This is a guarantee of the language; it happens automagically; there's no way to stop it from happening. But you can get really bad results from calling a destructor on the same object a second time! Bang! You're dead!

Q: What if I want a local to "die" before the close } of the scope in which it was created? Can I call a destructor on a local if I really want to?
A: No! [For context, please read the previous FAQ]. Suppose the (desirable) side effect of destructing a local File object is to close the File. Now suppose you have an object f of a class File and you want File f to be closed before the end of the scope (i.e., the }) of the scope of object f:

void someCode()
{
File f;
...insert code that should execute when f is still open...
We want the side-effect of f's destructor here!
...insert code that should execute after f is closed...
}
There is a simple solution to this problem. But in the mean time, remember: Do not explicitly call the destructor!

Q: OK, OK already; I won't explicitly call the destructor of a local; but how do I handle the above situation?
A: Simply wrap the extent of the lifetime of the local in an artificial block {...}:
void someCode()
{
{
File f;
...insert code that should execute when f is still open...
} f's destructor will automagically be called here!
...insert code here that should execute after f is closed...}

Q: What if I can't wrap the local in an artificial block?
A: Most of the time, you can limit the lifetime of a local by wrapping the local in an artificial block ({...}). But if for some reason you can't do that, add a member function that has a similar effect as the destructor. But do not call the destructor itself!
For example, in the case of class File, you might add a close() method. Typically the destructor will simply call this close() method. Note that the close() method will need to mark the File object so a subsequent call won't re-close an already-closed File. E.g., it might set the fileHandle_ data member to some nonsensical value such as -1, and it might check at the beginning to see if the fileHandle_ is already equal to -1:

class File {
public:
void close();
~File();
...
private:
int fileHandle_; // fileHandle_ >= 0 if/only-if it's open
};

File::~File()
{
close();
}

void File::close()
{
if (fileHandle_ >= 0) {
...insert code to call the OS to close the file...
fileHandle_ = -1;
}
}

Note that the other File methods may also need to check if the fileHandle_ is -1 (i.e., check if the File is closed).
Note also that any constructors that don't actually open a file should set fileHandle_ to -1.


Q: But can I explicitly call a destructor if I've allocated my object with new?
A: Probably not. Unless you used placement new, you should simply delete the object rather than explicitly calling the destructor. For example, suppose you allocated the object via a typical new expression: Fred* p = new Fred(); Then the destructor Fred::~Fred() will automagically get called when you delete it via: delete p; // Automagically calls p->~Fred() You should not explicitly call the destructor, since doing so won't release the memory that was allocated for the Fred object itself. Remember: delete p does two things: it calls the destructor and it deallocates the memory.

Q: What is "placement new" and why would I use it?
A: There are many uses of placement new. The simplest use is to place an object at a particular location in memory. This is done by supplying the place as a pointer parameter to the new part of a new expression:

#include // Must #include this to use "placement new"
#include "Fred.h" // Declaration of class Fred
void someCode()
{
char memory[sizeof(Fred)]; // Line #1
void* place = memory; // Line #2
Fred* f = new(place) Fred(); // Line #3 (see "DANGER" below)
// The pointers f and place will be equal
...
}

Line #1 creates an array of sizeof(Fred) bytes of memory, which is big enough to hold a Fredobject.

Line #2 creates a pointer place that points to the first byte of this memory (experienced C
programmers will note that this step was unnecessary; it's there only to make the code moreobvious).

Line #3 essentially just calls the constructor Fred::Fred(). The this pointer in the Fred
constructor will be equal to place. The returned pointer f will therefore be equal to place.
ADVICE: Don't use this "placement new" syntax unless you have to. Use it only when you really care that an object is placed at a particular location in memory. For example, when your hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that memory location.

DANGER: You are taking sole responsibility that the pointer you pass to the "placement new" operator points to a region of memory that is big enough and is properly aligned for the object type that you're creating. Neither the compiler nor the run-time system make any attempt to check whether you did this right. If your Fred class needs to be aligned on a 4 byte boundary but you supplied a location that isn't properly aligned, you can have a serious disaster on your hands (if you don't know what "alignment" means, please don't use the placement new syntax). You have been warned. You are also solely responsible for destructing the placed object. This is done by explicitly calling the destructor:

void someCode()
{
char memory[sizeof(Fred)];
void* p = memory;
Fred* f = new(p) Fred();
...
f->~Fred(); // Explicitly call the destructor for the placed object
}
This is about the only time you ever explicitly call a destructor.

Note: there is a much cleaner but more sophisticated way of handling the destruction / deletion situation.