C++ - Memory leaks: where is the pointer (meta) information stored?

Question

This is a basic question of which I can't find any answer.

Given the next code, a memory leak will occur:

   int main(){
          A* a = new A();
          // 1
     } 
     //2

Lets say that a got the value 1000. That is, the address 1000 on the heap is now taken by an A object. On 1, a == 1000 and on 2 a is out of scope. But some information is missing.

In real life, the address 1000 is the address of a byte in the memory. This byte does not have the information that it stores a valuable information.

My questions:

1. who keeps this information?

2. how is this information is kept?

3. which component "knows" from where to where the pointer a points to? How can the computer know that a points to sizeof(A) bytes?

Thanks!

Answer 1

1. This information is kept in your program, in the variable a
2. The compiler knows this while compiling. Run-time only the allocator knows that "at this particular address sizeof(A) bytes are reserved" - and you can't use that info, you're simply expected to treat these bytes as if they contained an A

Answer 2

The language standard doesn't say.

All we know is that if we do delete a, the memory is released again.

There are several options, like allocating everything that is sizeof(a) from a certain memory pool with addresses 1000 to 1000+x. Or someone (the language runtime or the OS) can keep at table somewhere. Or something else.