CODEWITHSUNDEEP
c++memory
Zygmuntix
Zygmuntix

Reputation: 359

What means 'destroying' memory by delete operator in C++?

I would like to know what happens to memory that is destroyed by "delete" operator in C++. Is 'destroying' memory in such way means setting given pieces of memory to 0 or something else?

Upvotes: 2

Views: 321

Answers (3)

Mats Petersson
Mats Petersson

Reputation: 129374

It is destroying (as in calling the relevant destructor for) the object instance passed to delete, and then "frees" the memory so that it can be used for other purposes.

The C++ standard states nothing about what the contents of the memory will be after delete, and it is certainly not guaranteed to be zero or any other value - nor is it guaranteed that it is NOT zero - it may be zerod, it may retain all the values it had before, or some parts of it may be altered and others remain the same.

The goal of C and C++ as languages is to "only do the minimum necessary", so a typical memory free will not overwrite the "old" memory.

You could of course use code in the destructor to set the memory to zero before it is freed.

Since you are not supposed to use memory after it has been deleted, it shouldn't really matter.

Upvotes: 6

LihO
LihO

Reputation: 42083

delete just releases the memory (previously allocated by new) and in case that some object has been stored within this memory, the destructor is also invoked.

delete doesn't change the value of the pointer and neither it modifies the memory that has been released, thus you'll notice that many people are used to assign NULL to this pointer after calling delete just to make sure they will not end up with dereferencing invalid (dangling) pointer, which produces undefined behavior.

Worth to have a look at: Is it good practice to NULL a pointer after deleting it?

Upvotes: 4

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726589

No, it does not mean setting the memory to any particular value+. The memory simply gets back into the heap of values that can be reused. The runtime often use several bytes of the returned chunk to store "bookkeeping" information, but they do not set the entire chunk to a particular value. Once a memory chunk is reused, it is your program that sets its new values.

+ There are memory profiling tools that let you set released memory to some "junk" values to make sure that you get a crash faster.

Upvotes: 3

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