CODEWITHSUNDEEP
c++operator-overloading
shreyasva
shreyasva

Reputation: 13446

Overloading new, delete in C++

i came across this line is stroustrup An operator function must either be a member or take at least one argument of a user-defined type (functions redefining the new and delete operators need not).

Dont operator new and operator delete take an user defined type as one of their arguments? what does it mean, am i missing something here

Upvotes: 4

Views: 1149

Answers (4)

fredoverflow
fredoverflow

Reputation: 263098

a + b is just syntactic sugar for a.operator+(b) or operator+(a, b).

On the other hand, new Foo(x, y, z) is NOT just syntactic sugar for operator new(Foo, x, y, z) or something like that. It is way more complicated:

void* address = operator new(sizeof(Foo)); // here is the behavior you can replace
try {
    new(address) Foo(x, y, z);
} catch (...) {
    operator delete(address);
}

As you can see, the function operator new merely allocates memory, which is only half of what the operator new actually does. In my opinion, it would have made far more sense to have named that thing allocate_memory or something like that. It definitely IS NOT an operator like operator+.

Upvotes: 2

Potatoswatter
Potatoswatter

Reputation: 137780

operator new and operator delete are looked up based on the type of the operand in the new or delete expression, and any extra parenthetical arguments to new. Thus they may be overloaded (and operator new is subject to overload resolution), but by a different mechanism than the other operators. (C++03 §13.5/5)

As they work with raw memory, they never deal with pointers to the client class type. operator new always takes a size_t argument (and possibly other arguments, none of which need be of user-defined type) and returns void *. operator delete always takes a void * argument and optionally a size_t, no matter how it was looked up.

I can think of two reasons for this:

  • They deal with raw memory, not containing a constructed object. It is always an error for operator new or operator delete to attempt to access an object within the returned block of memory. (Memory not used to construct objects, however, is fair game.)
  • A class member operator new or operator delete may be multiply- and/or virtually-inherited, such that the void* in question cannot be made to point to the not-yet-constructed or already-destroyed subobject, by any voodoo.

Upvotes: 0

AnT stands with Russia
AnT stands with Russia

Reputation: 320391

The quote from Stroustrup apparently applies to operator overloading. C++ language supports operator overloading for user-defined types only. This means that the overloading function (operator <something>) has to be either a member of user-defined type or be a standalone function with at least one argument of user-defined type. This is exactly what is meant by the quote in question.

Yet standalone (non-member) operator new and operator delete functions are not required to take a value of user defined type as one of their arguments. This might be seen as something that contradicts your quote.

However, in reality there's no contradiction. These operators are not really overloaded. When you provide your own versions of standalone operator new/operator delete, you are actually replacing the library-provided ones. This is the official term from the language specification: replacement, not overloading. Which is why the above quote does not really apply to operator new and operator delete.

Upvotes: 6

Ken Bloom
Ken Bloom

Reputation: 58770

You can overload the normal global new operator, for all classes to add functionality (e.g. logging or leak detection) if you so desire, but there's no way to call the old definition of the new operator, so you'll probably be stuck calling malloc() from inside your redefined operator new to actually get the memory that you need.

Upvotes: 3

Related Questions