4 ms·

I realize this, but you can't provide multiple global implementations of new. It is difficult to implement multiple allocation strategies in the same applicati

by cmbaus 13y ago

I realize this, but you can't provide multiple global implementations of new. It is difficult to implement multiple allocation strategies in the same application. In another time, I actually did quite a bit of work in this area: http://accu.org/index.php/articles/1308 http://accu.org/index.php/articles/1308

mpyne 13y ago
That's true, but I'm not sure what it would even mean to have multiple implementations of the same function at the same time. Do you mean something like operator overloading where a given type has a different implementation of new that is used for it? Or a way to select different implementations of the global operator new at runtime? It seems to me that it should be possible for the latter to make it switchable, although the code would be inelegant to say the least. AFAIK the former is possible as well, but the problem is that I think the type-specific operator new/delete have to be members of the type which means that you can't add it on later. That's a great article btw.
- cmbaus 13y ago
  I'm going to dig deep here, and say that true flexibility in memory allocation would require multiple global implementations of new, and when an object is deleted it would be destroyed with an implementation of delete the corresponds with the implementation of new. That would be a gigantic change to the language, which I'm sure I don't understand the implications of. After thinking about this some more, I'm going to stand by my original, even if poorly argued, comment that C++ doesn't really live up to the promise of "precise control" when it comes to memory allocation, and this is one area that can have a big effect on performance. C++ has all the overhead of a low level language, with a promise of control that when you really need it is often difficult to access.
  - coliveira 13y ago
    Ok, so let's count... Just from the top of my head, we have global new/delete overloading. At the class level you have new/delete overriding. Then you have allocators that can be used with any STL container. In you templates you can also have custom allocators. Then you have new with placement syntax, which means that you can allocate memory with anything, including something like malloc or gmalloc. Of course, you can also use the stack for automatic memory... You can even use a garbage collector such as Boehm. And you really want to argue that there are no options for memory allocation in C++?... The problem with C++ is exactly the opposite, it has too many options, which makes it difficult to create a global memory management policy, unless you define you own complete framework. Quite the opposite of what you said.
    - comex 13y ago
      I'm not sure whether the previous poster was thinking along these lines, but in some sense C++ makes it harder to control allocations compared to C due to a combination of increased abstraction and a general assumption that objects are fixed size. For example, if a C struct contains an array whose size is known at the time of allocation, there's a good chance the elements will be located after the end of the struct rather than being in a separate allocation. Of course, you could do this in C++, but neither language nor library provides sugar, so it ends up looking unnecessary compared to just using a std::vector. I won't opine on whether it actually is unnecessary.
      - mpyne 13y ago
        Well at a language level C++ shares more or less the same behavior as C. If you input the same "plain old data" struct into C++, with an array whose size is known at compile-time, it would be placed where C places it. Now you're correct that the standard library provided with C++ does not quite give you this same level of control. If you want node elements placed somewhere specific you either have to pick the right container, write a custom allocator (including rebind support so that any required container-internal elements also can use your allocator), etc. But this is a complaint about a C++-only feature that C doesn't even provide. On the very few times that C does provide an standard library function that will automatically allocate storage from the heap, it is even less customizable than the C++ equivalent. Normally your only option is something allocated with malloc(3), you're told to use free(3) when appropriate, and if you need something different you run into trying to replace malloc/free and friends and then somehow telling your custom malloc when to use custom behavior and when to use stdlib behavior. But you could do this in C++ code too. Now you can certainly easily write your own library with better support for custom allocation/deallocation. Things like the Apache Portable Runtime, for instance, or glib. But once you've bought into the idea of simply ignoring the language standard library anyways, you've bought into something else that you could do in C++ if you really felt like it were useful. The problem with C++ is that the standard library makes it almost straightforward to do what you want and still be able to use standard containers without having to write your own, but we have to remember that we're talking about a problem that essentially can't even be spoken about in C. So it's not so much that C "doesn't have a problem", as that this problem couldn't even be expressed in C.
pcwalton 13y ago
You can just provide an allocate() static method and destroy() method on your object that use placement new and call the destructor manually. Or don't and just use placement new/manual destructor calling yourself. Are you trying to argue that C++ programs can't use custom allocators because of language restrictions? That's demonstrably false; Gecko and WebKit, just to name a couple of C++ codebases I'm relatively familiar with, use custom allocators everywhere.
- cmbaus 13y ago
  Fair enough. My point was poorly made, and truthfully it has been along time since I've done significant C++ work. But it is difficult for the users of libraries like the STL to mix and match their allocation strategies, so for all practical purposes, most applications end up using the default implementation, or a single override of global new.
  - coliveira 13y ago
    Actually if you may create a policy for a particular type by implementing operator new at the class level. I think you're not well informed, because C++ is the language that gives you the most flexibility to design memory management schemes.

by cmbaus 13y ago

mpyne 13y ago

That's true, but I'm not sure what it would even mean to have multiple implementations of the same function at the same time. Do you mean something like operator overloading where a given type has a different implementation of new that is used for it? Or a way to select different implementations of the global operator new at runtime? It seems to me that it should be possible for the latter to make it switchable, although the code would be inelegant to say the least. AFAIK the former is possible as well, but the problem is that I think the type-specific operator new/delete have to be members of the type which means that you can't add it on later. That's a great article btw.

cmbaus 13y ago
I'm going to dig deep here, and say that true flexibility in memory allocation would require multiple global implementations of new, and when an object is deleted it would be destroyed with an implementation of delete the corresponds with the implementation of new. That would be a gigantic change to the language, which I'm sure I don't understand the implications of. After thinking about this some more, I'm going to stand by my original, even if poorly argued, comment that C++ doesn't really live up to the promise of "precise control" when it comes to memory allocation, and this is one area that can have a big effect on performance. C++ has all the overhead of a low level language, with a promise of control that when you really need it is often difficult to access.
- coliveira 13y ago
  Ok, so let's count... Just from the top of my head, we have global new/delete overloading. At the class level you have new/delete overriding. Then you have allocators that can be used with any STL container. In you templates you can also have custom allocators. Then you have new with placement syntax, which means that you can allocate memory with anything, including something like malloc or gmalloc. Of course, you can also use the stack for automatic memory... You can even use a garbage collector such as Boehm. And you really want to argue that there are no options for memory allocation in C++?... The problem with C++ is exactly the opposite, it has too many options, which makes it difficult to create a global memory management policy, unless you define you own complete framework. Quite the opposite of what you said.
  - comex 13y ago
    I'm not sure whether the previous poster was thinking along these lines, but in some sense C++ makes it harder to control allocations compared to C due to a combination of increased abstraction and a general assumption that objects are fixed size. For example, if a C struct contains an array whose size is known at the time of allocation, there's a good chance the elements will be located after the end of the struct rather than being in a separate allocation. Of course, you could do this in C++, but neither language nor library provides sugar, so it ends up looking unnecessary compared to just using a std::vector. I won't opine on whether it actually is unnecessary.
    - mpyne 13y ago
      Well at a language level C++ shares more or less the same behavior as C. If you input the same "plain old data" struct into C++, with an array whose size is known at compile-time, it would be placed where C places it. Now you're correct that the standard library provided with C++ does not quite give you this same level of control. If you want node elements placed somewhere specific you either have to pick the right container, write a custom allocator (including rebind support so that any required container-internal elements also can use your allocator), etc. But this is a complaint about a C++-only feature that C doesn't even provide. On the very few times that C does provide an standard library function that will automatically allocate storage from the heap, it is even less customizable than the C++ equivalent. Normally your only option is something allocated with malloc(3), you're told to use free(3) when appropriate, and if you need something different you run into trying to replace malloc/free and friends and then somehow telling your custom malloc when to use custom behavior and when to use stdlib behavior. But you could do this in C++ code too. Now you can certainly easily write your own library with better support for custom allocation/deallocation. Things like the Apache Portable Runtime, for instance, or glib. But once you've bought into the idea of simply ignoring the language standard library anyways, you've bought into something else that you could do in C++ if you really felt like it were useful. The problem with C++ is that the standard library makes it almost straightforward to do what you want and still be able to use standard containers without having to write your own, but we have to remember that we're talking about a problem that essentially can't even be spoken about in C. So it's not so much that C "doesn't have a problem", as that this problem couldn't even be expressed in C.

pcwalton 13y ago

You can just provide an allocate() static method and destroy() method on your object that use placement new and call the destructor manually. Or don't and just use placement new/manual destructor calling yourself. Are you trying to argue that C++ programs can't use custom allocators because of language restrictions? That's demonstrably false; Gecko and WebKit, just to name a couple of C++ codebases I'm relatively familiar with, use custom allocators everywhere.

cmbaus 13y ago
Fair enough. My point was poorly made, and truthfully it has been along time since I've done significant C++ work. But it is difficult for the users of libraries like the STL to mix and match their allocation strategies, so for all practical purposes, most applications end up using the default implementation, or a single override of global new.
- coliveira 13y ago
  Actually if you may create a policy for a particular type by implementing operator new at the class level. I think you're not well informed, because C++ is the language that gives you the most flexibility to design memory management schemes.