4 ms·

> FWIW my assumption always is that it's "a borrow". But for how long? Forget about multithreading for a moment, but can my function store the pointer for late

by klmr 9y ago

> FWIW my assumption always is that it's "a borrow". But for how long? Forget about multithreading for a moment, but can my function store the pointer for later use? This is incredibly common in OOP, and dangling pointers/references are one of the (if not the) top sources of bugs in C and C++. And unlike other sources of bugs, there’s no good fundamental remedy against it.

jstimpfle 9y ago
> But for how long? As I wrote, "lifetime of the function". Maybe that was poorly worded - of course I mean "until the function call returns". (I think "borrow" is universally understood in this way). That means, again, don't assume you are allowed to keep the pointer argument in an associated datastructure. In general we don't know where the data is coming from, how it was allocated, how or when it will be freed, etc. It's just that: data. And as I wrote in the other comment, don't do OOP. It's wrong. I think it has had a good twenty years of hype to prove any merits, and failed to do so. Yes, OOP without a garbage collector cannot be done reliably because the pointers will kill you. But that's not an indication that pointer management is impossible. It's an indication that we shouldn't do OOP. Even in a GCed language it's still a pain to maintain all these redundant links, and to write algorithms that collect the data that was littered in a thousand places and is accessible only through a hard to understand object interface. There are much simpler ways to organize a program. Pointer management can be pretty easy when the project structure is right, and we focus on the data instead an overly bureaucratic object graph of funky objects that do almost nothing in the name of "reusability".
- klmr 9y ago
  It’s not that simple. First off, OOP is actually useful if not abused. Even in modern C++ it definitely has its place. And secondly, you run into the same problem outside the limited scope of OOP. Every large, well-designed library provides its own solution to this problem of “holding onto” a value and extending its lifetime, if necessary. Obviously I agree with you that a bare pointer shouldn’t ever be (ab)used for this purpose but as mentioned that’s easier said than done, since the alternatives aren’t trivial. Lifetime extension really is difficult.
  - jstimpfle 9y ago
    My general approach is to divide a project into modules which hold their data in global tables (typically a handful of dynamically allocated arrays). The data's lifetime is managed by that module and that module only. Most of the data never needs to be exposed to other modules, and most of the functionality related to that data is encapsulated in the module. Other modules use integer handles to access data. They might convert the integer handle to a pointer and iterate over the data with the help of the module, but these pointers are not stored. The module's interface has very clear and simple rules which commands from its API invalidate those pointers (because they might trigger reallocation). But of course only few commands do invalidate pointers, and they are typically not interleaved with the other commands. And typically getting pointers is not even needed, because the relevant functionality is implemented in the module that owns the data. There are generally no pointers inside the data (perhaps a few const char pointers to static or quasi-static data) and most intra-module links are realized using integer indexes, just like it's done in relational databases. So in a way that is actually "object-oriented", but not in the usual sense of creating a million instances of some class that are linked in a complicated way. Every module is a kind of object. So the number of these objects is typically static. All the linking between these objects is simply done by: the linker :-)

by klmr 9y ago

jstimpfle 9y ago

> But for how long? As I wrote, "lifetime of the function". Maybe that was poorly worded - of course I mean "until the function call returns". (I think "borrow" is universally understood in this way). That means, again, don't assume you are allowed to keep the pointer argument in an associated datastructure. In general we don't know where the data is coming from, how it was allocated, how or when it will be freed, etc. It's just that: data. And as I wrote in the other comment, don't do OOP. It's wrong. I think it has had a good twenty years of hype to prove any merits, and failed to do so. Yes, OOP without a garbage collector cannot be done reliably because the pointers will kill you. But that's not an indication that pointer management is impossible. It's an indication that we shouldn't do OOP. Even in a GCed language it's still a pain to maintain all these redundant links, and to write algorithms that collect the data that was littered in a thousand places and is accessible only through a hard to understand object interface. There are much simpler ways to organize a program. Pointer management can be pretty easy when the project structure is right, and we focus on the data instead an overly bureaucratic object graph of funky objects that do almost nothing in the name of "reusability".

klmr 9y ago
It’s not that simple. First off, OOP is actually useful if not abused. Even in modern C++ it definitely has its place. And secondly, you run into the same problem outside the limited scope of OOP. Every large, well-designed library provides its own solution to this problem of “holding onto” a value and extending its lifetime, if necessary. Obviously I agree with you that a bare pointer shouldn’t ever be (ab)used for this purpose but as mentioned that’s easier said than done, since the alternatives aren’t trivial. Lifetime extension really is difficult.
- jstimpfle 9y ago
  My general approach is to divide a project into modules which hold their data in global tables (typically a handful of dynamically allocated arrays). The data's lifetime is managed by that module and that module only. Most of the data never needs to be exposed to other modules, and most of the functionality related to that data is encapsulated in the module. Other modules use integer handles to access data. They might convert the integer handle to a pointer and iterate over the data with the help of the module, but these pointers are not stored. The module's interface has very clear and simple rules which commands from its API invalidate those pointers (because they might trigger reallocation). But of course only few commands do invalidate pointers, and they are typically not interleaved with the other commands. And typically getting pointers is not even needed, because the relevant functionality is implemented in the module that owns the data. There are generally no pointers inside the data (perhaps a few const char pointers to static or quasi-static data) and most intra-module links are realized using integer indexes, just like it's done in relational databases. So in a way that is actually "object-oriented", but not in the usual sense of creating a million instances of some class that are linked in a complicated way. Every module is a kind of object. So the number of these objects is typically static. All the linking between these objects is simply done by: the linker :-)