## Pointers Are Complicated
What is the problem with "pointers are just integers"? Let us consider the following example:<br>
## Pointers Are Complicated
What is the problem with "pointers are just integers"? Let us consider the following example:<br>
-(I am using C++ code here mostly because writing unsafe code is easier in C++, and unsafe code is where these problems really show up.)
+(I am using C++ code here mostly because writing unsafe code is easier in C++, and unsafe code is where these problems really show up. C has all the same issues, as does unsafe Rust.)
However, multiplying that "integer" by 2 leads to an error, because it is entirely unclear what it means to multiply such an abstract pointer by 2.
This is the most lazy thing to do, and we do it because it is not clear what else to do -- in our abstract machine, there is no single coherent "address space" that all allocations live in, that we could use to map every pointer to a distinct integer.
However, multiplying that "integer" by 2 leads to an error, because it is entirely unclear what it means to multiply such an abstract pointer by 2.
This is the most lazy thing to do, and we do it because it is not clear what else to do -- in our abstract machine, there is no single coherent "address space" that all allocations live in, that we could use to map every pointer to a distinct integer.
We could now start to enrich this model with extra data like a base address for each allocation, and somehow use that when casting an integer back to a pointer... but that's where it gets really complicated, and anyway discussing such a model is not the point of this post.
The point it to discuss the *need* for such a model.
If you are interested, I suggest you read [this paper](http://www.cis.upenn.edu/%7Estevez/papers/KHM+15.pdf) that explores the above idea of adding a base address.
We could now start to enrich this model with extra data like a base address for each allocation, and somehow use that when casting an integer back to a pointer... but that's where it gets really complicated, and anyway discussing such a model is not the point of this post.
The point it to discuss the *need* for such a model.
If you are interested, I suggest you read [this paper](http://www.cis.upenn.edu/%7Estevez/papers/KHM+15.pdf) that explores the above idea of adding a base address.
## Conclusion
We have seen that in languages like C++ and Rust (unlike on real hardware), pointers can be different even when they point to the same address, and that a byte is more than just a number in `0..256`.
## Conclusion
We have seen that in languages like C++ and Rust (unlike on real hardware), pointers can be different even when they point to the same address, and that a byte is more than just a number in `0..256`.
With this, I think we are ready to look at a first draft of my "2018 memory model" (working title ;) -- in the next post. :)
Thanks to @rkruppe and @nagisa for help in finding arguments for why `Uninit` is needed.
With this, I think we are ready to look at a first draft of my "2018 memory model" (working title ;) -- in the next post. :)
Thanks to @rkruppe and @nagisa for help in finding arguments for why `Uninit` is needed.