Allocating memory

[adam2016]

hey guys,I am reading an article which I will post below,

https://www.codeproject.com/Articles/4795/C-Standard-Allocator-An-Introduction-and-Implement

it is explaining how std::allocators work.


They go onto discuss how the new and delete keyword are actually implemented

 A* a = ::operator new(sizeof(A)); 
a->A::A();
if ( a != 0 ) {  // a check is necessary for delete
    a->~A();
    ::operator delete(a);
}

I understand the first line we are calling operator new to allocate memory with malloc as far as I know

but the next line is what I don't understand a->is calling A's constructor,I have never seen this syntax before and never knew it was possible to create an object like this?

thanks

[tpb]

It's pseudo syntax. For instance, you don't pass sizeof(A) to new in real C++.

Maybe it should have been written less like C++. In English:

operator new:
  Set an A pointer to newly allocated memory the size of A
  Call A's constructor on the pointer.
  Return the pointer.

operator delete:
  If the given pointer is null, do nothing.
  Otherwise, Call the destructor on the pointer. And free the allocated memory. 

[adam2016]

Why can't you pass sizeof(A) to new in real C++?

int main()
{

    // allocate memory with operator new
    void* mem = ::operator new(sizeof(int));
    // create object/type (int) in the memory we allocated
    int* integer = new(mem) int;
    *integer = 7;

    cout << *integer << endl;

}

the following code will compile and execute fine,we are allocating memory enough for the size of an int


but as you said

A* a = ::operator new(sizeof(A)); 
a->A::A();
if ( a != 0 ) {  // a check is necessary for delete
    a->~A();
    ::operator delete(a);
}

is not valid C++ code,I don't know why the author didn't say it was pseudo code,yeah very poor choice on his part,quite confusing

thanks

[tpb]

adam2016 wrote:

Why can't you pass sizeof(A) to new in real C++?

I didn't actually say you "can't", I said you "don't", but should have added "usually" in the sense that if you want an object you just use the type unless you're doing something "fancy".