I'm doing some learning exercises in c++ and I ran into an interesting question. Take this sample program.
#include <string>
#include <iostream>
char* pointer1;
void temp() {
char* s1 = new char;
*s1 = 'z';
pointer1 = s1;
std::cout << *pointer1 << std::endl;
for (int i = 0; i < 90000; i++) {
// Waste some processor time.
}
}
int main() {
temp();
std::cout << *pointer1 << std::endl;
delete pointer1;
//delete &pointer1;
std::cout << *pointer1 << std::endl;
return 0;
}
When you run it it prints out 'z' twice then some random garbage. Which is what I expected. If you un-comment 'delete &pointer1' and comment out the first delete and run the program you get an invalid pointer error from the output. I'm assuming this is deleting the address and what's actually stored there is remaining.
My question is when calling 'delete pointer1' does it delete 'char* s1' or just the address to where ever the s1 is stored? When calling 'delete &pointer1' is the address being deleted but s1 still in memory?
The address that pointer1 points to has been allocated by new. Hence, it is correct, even necessary, to call delete on that address.
However, &pointer1 gives you the address where pointer1 itself is stored. And this memory block has not been allocated by new. Hence, it is strictly illegal to call delete on it, which is precisely what the error is telling you.
So, yes, delete pointer1 frees the memory that has been allocated by new previously. But no, delete &pointer1 doesn't do anything but being illegal.
Additionally, accessing memory you called delete on previously is undefined behavior. So, your second *pointer1 is also nothing you want to write in a program that you do not want to crash or, even worse, that gives you unpredictable results.
Related
This question already has answers here:
Deleting C++ pointers to an object
(9 answers)
Why pointer is not NULL after using delete in C++
(2 answers)
Closed 4 years ago.
I've created a dynamic object in C++ using pointer. I set the value for the dynamic object. Then I delete pointer, but when I try to print the value of dynamic object, it is stay the same.
I have tried to print the value of pointer which is the address of the dynamic object, it is stay the same before and after I delete pointer.
#include<iostream>
using namespace std;
struct students
{
string name;
int agee;
};
int main()
{
students *p = NULL;
p = new students;
(*p).name = "Vu Trung Nghia";
(*p).agee = 20;
cout << p->name << " " << p->agee << endl;
delete p;
if(p == NULL)
cout << "It was deleted";
else
cout << (*p).name << " " << (*p).agee << endl;
}
I expect the result is: p == NULL or can't print "Vu Trung Nghia 20"
Actual result: "Vu Trung Nghia 20"
I expect the result is: p == NULL
There is no reason to expect that, since you never assigned p = NULL.
I have tried to print the value of pointer which is the address of the dynamic object, it is stay the same before and after I delete pointer.
This is the expected behaviour. Deleting a pointer has no effect on the value of the pointer.
but when I try to print the value of dynamic object, it is stay the same.
Behaviour of accessing a destroyed object is undefined. You don't ever want your program to have undefined behaviour.
When the behaviour is undefined, anything could happen. "I try to print the value of dynamic object, it is stay the same." is one of the possible behaviours.
So what deleting a pointer really is?
Assuming the pointer points to an object that was allocated with new, then delete destroys the pointed object, and deallocates the memory. Deallocation means that a subsequent new may re-use that memory.
delete frees the pointer from memory, it does NOT change its value or address, the value stays there, the calls which you do after deleteing it invokes Segmentation fault (which can easily be translated to your program having Undefined behvaior)...
This is the reason why people set the pointer to NULL after invoking delete...
delete p;
p = nullptr; // Stop memory leaking...
You will also find that you are no more able to modify the pointer:-
(*p).name = "Hello!"; /* Value is freed/destroyed from memory, cannot be
modified anymore... */
Or just overload your own delete operator:
struct students
{
string name;
int agee;
void students::operator delete ( void* ptr ) {
std::free(ptr);
ptr = nullptr; // Stop memory leaking...
}
};
I just learned pointer and delete pointer in class for C++. I tried this code by my own
# include<iostream>
using namespace std;
int main(){
int num = 10;
int *p = new int;
p = #
cout << *p << endl;
delete p;
cout << num << endl;
return 0;
}
After deleting the pointer p, I cannot print the value of num. But if I delete p at the very end of the program, cout << num << endl; will give me 10. Anyone knows where I did run?
You first leaked a pointer
int *p = new int;
p = # // You just leaked the above int
then illegally deleted something you did not new
delete p; // p points to num, which you did not new
You have already received a couple of good answers that point out the mistake, but I read a deeper misunderstanding of the allocation and deallocation of heap vs stack variables.
I realised this has become a pretty long post, so maybe if people think it is useful I should put it as a community Wiki somewhere. Hopefully it clarifies some of your confusion though.
Stack
The stack is a limited and fixed size storage. Local variables will be created here if you don't specify otherwise, and they will be automatically cleaned up when they are no longer needed. That means you don't have to explicitly allocate them - they will start existing the moment you declare them. Also you don't have to deallocate them - they will die when they fall out of scope, loosely speaking: when you reach the end brace of the block they are defined in.
int main() {
int a; // variable a is born here
a = 3;
a++;
} // a goes out of scope and is destroyed here
Pointers
A pointer is just a variable, but instead of an int which holds a whole number or a bool which holds a true/false value or a double which holds a floating point, a pointer holds a memory address. You can request the address of a stack variable using the address operator &:
{
int a = 3, b = 4;
int* p = &a; // p's value is the address of b, e.g. 0x89f2ec42
p = &b; // p now holds the address of b, e.g. 0x137f3ed0.
p++; // p now points one address space further, e.g. 0x137f3ed4
cout << p; // Prints 0x137f3ed4
} // Variables a, b and p go out of scope and die
Note that you should not assume that a and b are "next to" each other in memory, or that if p has a "used" address as its value then you can also read and write to the address at p + 1.
As you probably know, you can access the value at the address by using the pointer indirection operator, e.g.
int* p = &a; // Assume similar as above
*p = 8;
cout << a; // prints 8
cout << &a << p; // prints the address of a twice.
Note that even though I am using a pointer to point at another variable, I don't need to clean up anything: p is just another name for a, in a sense, and since both p and what it points to are cleaned up automatically there is nothing for me to do here.
Heap
The heap memory is a different kind of memory, which is in theory unlimited in size. You can create variables here, but you need to tell C++ explicitly that you want to do so. The way to do this is by calling the new operator, e.g. new int will create an integer on the heap and return the address. The only way you can do something sensible with the allocated memory, is save the address this gives you. The way you do this, is store it in a pointer:
int* heapPtr = new int;
and now you can use the pointer to access the memory:
*heapPtr = 3;
cout << heapPtr; // Will print the address of the allocated integer
cout << *heapPtr; // Will print the value at the address, i.e. 3
The thing is that variables created on the heap will keep on living, until you say you don't need them anymore. You do that by calling delete on the address you want to delete. E.g. if new gave you 0x12345678 that memory will be yours until you call delete 0x12345678. So before you exit your scope, you need to call
delete heapPtr;
and you will tell your system that the address 0x12345678 is available again for the next code that comes along and needs space on the heap.
Leaking memory
Now there is a danger here, and that is, that you may lose the handle. For example, consider the following:
void f() {
int* p = new int;
}
int main() {
f();
cout << "Uh oh...";
}
The function f creates a new integer on the heap. However, the pointer p in which you store the address is a local variable which is destroyed as soon as f exits. Once you are back in the main function, you suddenly have no idea anymore where the integer you allocated was living, so you have no way to call delete on it anymore. This means that - at least for the duration of your program - you will have memory that according to your operating system is occupied, so you cannot use it for anything else. If you do this too often, you may run out of memory even though you can't access any of it.
This is one of the errors you are making:
int* p = new int;
allocates a new integer on the heap and stores the address in p, but in the next line
p = #
you overwrite that with another address. At this point you lose track of the integer on the heap and you have created a memory leak.
Freeing memory
Aside from freeing memory not often enough (i.e. not instead of once), the other error you can make is freeing it too often. Or, to be more precise, you can make the error of accessing memory after you have told your OS you don't need it anymore. For example, consider the following:
int main() {
int* p = new int;
*p = 10;
delete p; // OK!
*p = 3; // Errr...
}
That last line is very wrong! You have just returned the memory allocated when you called delete, but the address is still stored in p. After you call delete, your OS is allowed to re-allocate the memory at any time - for example, immediately after another thread could call new double and get the same address. At that point, if you write *p = 3 you are therefore writing to memory that is no longer yours which may lead to disaster, if you happen to overwrite the location in memory where the nuke's launch codes are stored, or nothing may happen at all because the memory is never used for anything else before your program ends.
Always release your own memory, and nothing but your own memory
We have concluded the following: memory allocated on the stack is not yours to claim, and not yours to release. Memory allocated on the heap is yours to claim, but you must also release it once and only once.
The following examples are incorrect:
{
int a = 3;
int* p = &a;
delete a;
} // Uh oh... cannot clean up a because it is not ours anymore!
{
int* p = new int;
delete p;
*p = 3; // Uh oh, cannot touch this memory anymore!
delete p; // Uh oh, cannot touch this memory anymore!
}
Why does it print 10?
Well, to be honest, you were just "lucky" there. Actually, the way your operating system manages memory, is generally pretty lazy. When you tell it "I would like some memory" it doesn't zero it for you. That is why it is a bad idea to write
int main() {
int a;
a = a + 3;
cout << a;
}
You get allocated a variable a somewhere in the memory, but the value of a will be whatever was in that memory location. It might be zero, or some random number that depends on how the bits fell when you booted your computer. That is why you should always initialize the variable:
int a = 0;
Similarly, when you say "I don't need this memory" anymore, the OS doesn't zero it. That would be slow and unnecessary: all it needs to do is mark the memory as "free to be re-allocated". So if you give it back and access it immediately afterwards, the probability that it has not been re-allocated yet is pretty large. Therefore
int* p = new int;
*p = 10;
delete p;
cout << *p;
is not guaranteed to print 10. The address p is pointing to may have been (partially) taken (and initialized!) by someone else immediately after the delete. But if it hasn't, the memory will still contain the value 10 there so even though it isn't yours anymore, C++ will still allow you to access it. Basically, when you are using pointers, you are telling it "trust me, I'm a programmer - you don't need to do all kinds of slow checks to make sure I'm staying where I'm supposed to be, instead I'll be careful about that myself!"
using namespace std;
int main(){
int num = 10; // a) an int is created on stack
int *p = new int; // b) another int is allocated on heap
p = # // c) address of int from stack is assigned to p and the pointer
// allocated in b) is leaked: as nothing points to it anymore,
// it can't be deleted
cout << *p << endl;
delete p; // d) deleting a pointer that was not dynamically allocated
// and is pointing to stack.
cout << num << endl;
return 0;
}
here if I use delete or delete[] the output is still 70. Can I know why?
#include<iostream>
using namespace std;
int main()
{
int* c = new int[100];
for(int i=0; i<98; i++)
{
c[i] = i;
}
cout<<c[70]<<endl;
delete[] c;
or
delete c;
cout<<c[70]<<endl; //outputs 70 even after delete[] or delete
return 0;
}
Accessing deleted memory is undefined behavior. Deleting with the wrong delete is also UB. Any further discussion is pointless in the sense that you cannot reliably expect any outcome.
In many cases, UB will just do the "correct" thing, but you need to be aware that this is completely "by chance" and could change with another compiler, another version of the same compiler, the weather... To get correct code, you need to avoid all cases of UB, even those that seemingly work.
Using new will just allocate some memory to your program and return a pointer pointing at the said memory address, reserving as much memory as needed for the datatype. When you use delete later, it "frees" the memory, but doesn't delete it's content. If you had an int with the value 70 stored at that address, it will still contain 70, until another application wants some memory, gets said address and puts another value in there.
If you use new to allocate memory for an array, you will reserve following blocks of memory until there are enough blocks for your specified array length.
Let's say you do the following:
int main() {
int* array = new int[10]; // array now points to the first block of the allocated memory
delete array; // since array points to the first block of the array, it will only free that block, but nothing else, causing a memory leak
delete[] array; // will free all memory allocated by the previous new
// Note that you should never free allocated memory twice, like in this code sample. Using delete on already freed memory is undefined behaviour!
]
Always use delete for single variables and delete[] for arrays.
A demonstration of your problem:
int main() {
int* c = new int[10]; // We allocate memory for an array of 10 ints
c[0] = 1; // We set the value of the first int inside the array to 1
delete[] c;
/*
* We free the previously allocated memory.
* Note that this does not delete the CONTENT of the memory!
* c does still point towards the first block of the array!
*/
std::cout << c[0];
/*
* Firstly, this is undefined behaviour (Accessing deallocated memory).
* However, this will output 1,
* unless some other process allocated the memory at the address
* and filled it with another value already. (Very unlikely)
*/
return 0;
}
If you want to delete / overwrite the content of the deleted memory, you can use std::memset.
Example:
#include <cstring>
int main() {
std::size_t length = 10;
int* c = new int[length];
c[0] = 1;
std::cout << c[0] << std::endl; // Will output 1
std::memset( c, 0, length ); // Fill the memory with 0 bytes
delete[] c; // Now we free the array's memory
std::cout << c[0] << std::endl; // Will output 0
}
As others pointed its undefined behaviour and anything can happen.
These can be easily caught with the help of tools like valgrind.
I'm studying c++ and I'm reading about pointers. I'm curious about the following scenarios:
Scenario 1:
If I'm not mistaken, if the user types -1, there will be a memory leak:
#include <iostream>
using namespace std;
int main(){
int *p = new int;
cout << "Please enter a number: ";
cin >> *p;
if (*p == -1){
cout << "Exiting...";
return 0;
}
cout << "You entered: " << *p << endl;
delete p;
return 0;
}
Scenario 2:
But what happens in the following code? From what I've read and correct me if I'm wrong, when declaring a pointer like in the second scenario the pointer gets cleared out once you are out of scope. So if the user doesn't enter -1, the *p will be auto-cleared?
#include <iostream>
using namespace std;
int main(){
int x;
int *p = &x;
cout << "Please enter a number: ";
cin >> *p;
if (*p == -1){
cout << "Exiting...";
return 0;
}
cout << "You entered: " << *p << endl;
return 0;
}
What happens if I enter -1 in the second scenario?
Do not focus on the fact that you are using pointers that much. Memory leaks are usually about memory that the pointer points to, not about the pointer itself.
In this code:
int x;
int *p = &x;
there is no memory leak since there is no memory that would require explicit deallocation (no memory that has been allocated dynamically). int x is a variable with automatic storage duration that will be cleaned up automatically when the execution goes out of scope and int *p = &x; is just a pointer that holds the address of the memory where x resides.
But you are right that in code like:
Resource* r = new Resource();
if (something) {
return -1;
}
delete r;
there is a memory leak since there is a return path (exit path) that doesn't free the allocated memory. Note that the same would happen if the exception would be thrown instead of return being called... ensuring that all resources are freed properly is one of the main reasons why you should learn more about smart pointers, the RAII idiom and try to prefer objects with automatic storage duration over dynamically allocated ones.
In the second scenario, everything's fine.
Indeed, you haven't allocated memory (while you did in the first scenario). In the first case, the pointer "holds" the memory you allocated through new. In the second case, it points to a local variable with automatic-storage duration (i.e. it will be removed when going out of scope).
Simple rule: If you used new you must use delete (unless you're using a smart pointer). In the first scenario, if you type -1, you end up with one new and zero delete, which yields a memory leak. In the second case, you haven't allocated anything, the pointer points to memory that is already managed.
In the second scenario you do not allocate memory so you should not bother about some memory leak. You have to use delete or delete[] if you explicitly allocate memory with new or new for arrays.
In the second scenario p points to local variable x. It is the compiler that allocated memory for x in stack. So it is the compiler that properly will free this memory after x will be out of its scope.
In the first main, you dynamically allocating the int and manually delete it when exiting from main. However, if int is equal to -1, you do not delete it and return so that's why you have a memory leak.
In second example, the int is allocated on the stack and you're taking the address of it. When main returns, the int is deallocated automatically. If you tried to call delete on it in the main, you would get a crash.
This code causes a Segmentation Fault:
int main(){
char *p;
char a[50] = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
p = (char *)malloc(50*sizeof(char));
if(!p){
cout << "Allocation Failure";
cout << "\n";
}
else{
cout << "Allocation Success";
cout << "\n";
p = a;
cout << p;
cout << "\n";
free(p);
}
return 0;
}
The output after executing this program is:
Allocation Success
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Segmentation fault
I am not able to find the bug. What may be reason?
This:
p = a;
copies the pointer, not the contents of the pointed-to memory. p now points at the first element of the array a. So when you do free(p), you're trying to free a non-dynamic array, which doesn't make any sense.1
You should investigate strncpy() to copy strings.
1. And it also causes a memory leak.
You're calling free on a block of memory that wasn't allocated using one of the malloc methods.
When you do: p = a you're assigning to p the memory used by the stack array a. That memory wasn't allocated using malloc and hence it can't be freed using free.
Furthermore with that re-assignment, you'll lose track of the block that you originally allocated with malloc and assigned to p, causing a memory leak.
char a[50] allocates an array of fifty characters on the stack. a therefore points at an address on the stack.
p = a sets p to point at the same address as a. Therefore after p = a, p points at an address on the stack. Remember, p contains an address.
Afterwards, free(p) tries to free a region of memory on the stack, which is illegal. You can only free memory you got from malloc().
you actually meant memcpy(p,a,50); not p=a, remember C does not have a string data type.
p = a; // p points a
Executing this line, p should be pointing to const string (char *).
You cannot free() anything that is not obtained by calling malloc or calloc. Since in this example you try to free() a const string, you get an error.