Swaping addresses of pointers

[pearl temple]

I'm creating Conway's Game of life in c++ and I have 2 vectors inside a Class:

#include<vector>
using namespace std;
class Board
{
private:
    std::vector<std::vector<bool>>* CurrentFrame;
    std::vector<std::vector<bool>>* NextFrame;
    
public:
    Board();
    ~Board();
};

Board::Board()
{
}

Board::~Board()
{
}

I was wondering if there is any way to change what they point to without creating a reference to them?
ei:

std::vector<std::vector<bool>> CurrentFrame;
Auto* PtrToFrame = &CurrentFrame;

[obtuse barn]

When your question is answered use !solved to mark the question as resolved.

Remember to ask specific questions, provide necessary details, and reduce your question to its simplest form. For more information use !howto ask.

[static spear]

Yes

But what exactly are you referring to, what exactly do you want to swap

[pearl temple]

what the pointers are pointing to

[static spear]

CurrentFrame = something else will change what the pointer points to

[pearl temple]

like if int a was pointing to Ox00000000 and b was pointing to 0x0000000F, I want b to point to Ox00000000

I don't want it to memory copy the entire array

[static spear]

CurrentFrame = something else won't copy the entire vector

[queen wind]

int* a = &x;
int* b = &y;
std::swap(a, b);

[static spear]

it only sets what vector is being pointed to

[pearl temple]

?

[static spear]

std::vector<std::vector<bool>>* CurrentFrame;

[pearl temple]

oh

[queen wind]

who owns those vectors btw

if not the board

[static spear]

If CurrentFrame is not a pointer, you can swap the pointers to the underlying buffer with .swap

[pearl temple]

It is a data value of the board

so ```cpp
std::swap(CurrentFrame,NextFrame)

[queen wind]

no idea what that means

[queen wind]

that swaps the pointers.

i mean, it's basically just

auto temp = p1;
p1 = p2;
p2 = temp;

[pearl temple]

I don't want it to copy all of the values of the vector

[queen wind]

it doesn't

it swaps pointers

those pointers point to the vectors

i'm not sure you really want pointers to vectors anyway

where do those pointers even come from

do they point to some local variables?

[pearl temple]

yes

I just forget to write new()

[queen wind]

don't use new

[pearl temple]

how do I allocate then?

[queen wind]

also then your "yes" answer doesn't make sense.

[queen wind]

who said you need to allocate

because you don't

[pearl temple]

Ok....

[pearl temple]

So are you saying this will work if I replace it with CurrentFrame and NextFrame?

X and Y I mean

A and B could be Anything, but swaping a and b would switchit?

[queen wind]

in theory yes. but i don't think that's what you want anyway

[pearl temple]

yeah

[queen wind]

so why didn't you make CurrentFrame and NextFrame std::vector<std::vector<bool>>

instead of std::vector<std::vector<bool>>*

[pearl temple]

And how would I swap it

[queen wind]

std::swap(current_frame, next_frame);

yes, even if they are vectors

std::swap is just that smart

i would still like you to answer my question.

[pearl temple]

And that was?

[queen wind]

@pearl temple this one.

[pearl temple]

I really haven't ever swaped addresses and like nothing on stack exchange helps

[queen wind]

but that still doesn't answer my question

why did you use pointers

why did you declare those data members as pointers

[pearl temple]

So I could swap addresses

Object 1 -------------------> 0x00000000
        \ /
         X
        / \
Object 2 -------------------> 0x0000000F

Rough diagram

[queen wind]

okay so, i think you're thinking about memory wrong

[pearl temple]

I probably am

[queen wind]

when you swap pointers, the objects at those addresses do not change at all

when you do

int* a = &x;
int* b = &y;
swap(a, b);

then x is still x and y is still y, it's only as if you declared them as

int* a = &y;
int* b = &x;

[pearl temple]

Then How come when I pass it through a swap funvtion, the address doesn't change?

[queen wind]

that's the effect of swapping pointers.

[queen wind]

what is "it" here?

the pointers?

[pearl temple]

yes

[queen wind]

the pointers just contain addresses. swapping them has no effect on the objects they are pointing to.

[pearl temple]

?

[queen wind]

?

[pearl temple]

but how are the addresses still the same? What Changed?

[queen wind]

the only thing that changed is what the pointers point to

..... ok wait

English is an unhelpful language when explaining this stuff

[queen wind]

tell me in your words, what is a pointer

[pearl temple]

A pointer is a variable that contains an address of another part of ram

[queen wind]

yes.

good.

okay

maybe i can show you something

;compile

#include <iostream>
#include <iomanip>
#include <utility>
using namespace std;

int main() {
    int x = 1;
    int y = 3;
    int* p1 = &x;
    int* p2 = &y;
    cout << boolalpha;
    cout << (p1 == &x) << endl;
    cout << (p2 == &y) << endl;
    cout << x << ' ' << y << endl;
    std::swap(p1, p2);
    cout << (p1 == &y) << endl;
    cout << (p2 == &x) << endl;
    cout << x << ' ' << y << endl;
}

[compact hollow]

Program Output

true
true
1 3
true
true
1 3

darkblueflow 💠#5176 | 37ms | c++ | x86-64 gcc 12.2 | godbolt.org

[pearl temple]

;compile

#include <iostream>
#include <iomanip>
#include <utility>
using namespace std;

int main() {
    int x = 1;
    int y = 3;
    int* p1 = &x;
    int* p2 = &y;
    cout << boolalpha;
    cout << p1 << endl;
    cout << p2 << endl;
    std::swap(p1, p2);
    cout << p1 << endl;
    cout << p2 << endl;
}```

[compact hollow]

Critical error:

You must provide a valid language or compiler!

;compile c++
```
int main() {}
```

Requested by: ThePlatupusEveryoneLoves#0865

[pearl temple]

;compile cpp

#include <iostream>
#include <iomanip>
#include <utility>
using namespace std;

int main() {
    int x = 1;
    int y = 3;
    int* p1 = &x;
    int* p2 = &y;
    cout << boolalpha;
    cout << p1 << endl;
    cout << p2 << endl;
    std::swap(p1, p2);
    cout << p1 << endl;
    cout << p2 << endl;
}```

[compact hollow]

Program Output

0x7ffd12c6198c
0x7ffd12c61988
0x7ffd12c61988
0x7ffd12c6198c

ThePlatupusEveryoneLoves#0865 | 67ms | c++ | x86-64 gcc 12.2 | godbolt.org

[pearl temple]

ohh

but does this work with countainers?

[queen wind]

btw you can just edit your message instead of typing another one

[pearl temple]

ohh

[queen wind]

after the edit, the compiler bot will just recompile

[pearl temple]

you can?

[queen wind]

yes, reduces spam

[pearl temple]

neat

[queen wind]

so please do that

[pearl temple]

will do

[queen wind]

so, std::swap works this way

[pearl temple]

ok

[queen wind]

if the type of the objects that we wanna swap has a special swap() method, it will use that

otherwise it will just create a temporary and move the two objects between them

[pearl temple]

so vector::swap()?

[queen wind]

yeah

[pearl temple]

ok

[queen wind]

idk what it exactly does, but it doesn't matter

it will probably just swap the internal pointers

doesn't matter.

the point is:

pointers are also just objects that contain addresses.

so when you swap pointers, you never affect the objects that they point to. you just change which object they point to.

p1 points to what p2 just pointed to.
and p2 points to what p1 just pointed to.

[pearl temple]

Thanks!

[queen wind]

got it?

[pearl temple]

yup

[queen wind]

so when you wanna swap the vectors, you swap them directly

like std::swap(v1, v2);

or whatever they are called

[pearl temple]

do I use the one in the vector class?

[queen wind]

you can call it directly, or you can use std::swap

doesn't matter.

[pearl temple]

ok then

[queen wind]

just don't use new, that would be a completely avoidable source of potential mistakes