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• Check if a type is an instantiation of a template

  - by Pedro Lacerda

  I have structs like struct RGBA (T) {/* ... */} struct BMPFile (DataT) if (is(DataT == RGBA)) {/* ... */} But is(DataT == RGBA) cannot work because DataT is a type and RGBA is a template. Instead I need check if a type is an instantiation of a template in order to declare file like BMPFile!(RGBA!ushort) file; In a comment @FeepingCreature showed struct RGBA(T) { alias void isRGBAStruct; } struct BMPFile (DataT) if (is(DataT.isRGBAStruct)) {} Although to be working I have no tips on alias void isRGBAStruct.

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• An easy way to replace fread()'s with reading from a byte array?

  - by Sam Washburn

  I have a piece of code that needs to be run from a restricted environment that doesn't allow stdio (Flash's Alchemy compiler). The code uses standard fopen/fread functions and I need to convert it to read from a char* array. Any ideas on how to best approach this? Does a wrapper exist or some library that would help? Thanks! EDIT: I should also mention that it's reading in structs. Like this: fread(&myStruct, 1, sizeof(myStruct), f);

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• c struct map to ruby using SWIG

  - by pierr

  Hi, Is there any body can confirm the description here is true? My experience is that I can not use Example::Vector.new at all. C/C++ structs are wrapped as Ruby classes, with accessor methods (i.e. "getters" and "setters") for all of the struct members. For example, this struct declaration: struct Vector { double x, y; }; gets wrapped as a Vector class, with Ruby instance methods x, x=, y and y=. These methods can be used to access structure data from Ruby as follows: $ irb irb(main):001:0> require 'Example' true irb(main):002:0> f = Example::Vector.new #<Example::Vector:0x4020b268> irb(main):003:0> f.x = 10 nil irb(main):004:0> f.x 10.0

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• Nonstatic conversion functions; Casting different types, e.g. DirectX vector to OpenGL vector

  - by Markus

  I am currently working on a game "engine" that needs to move values between a 3D engine, a physics engine and a scripting language. Since I need to apply vectors from the physics engine to 3D objects very often and want to be able to control both the 3D, as well as the physics objects through the scripting system, I need a mechanism to convert a vector of one type (e.g. vector3d<float>) to a vector of the other type (e.g. btVector3). Unfortunately I can make no assumptions on how the classes/structs are laid out, so a simple reinterpret_cast probably won't do. So the question is: Is there some sort of 'static'/non-member casting method to achieve basically this: vector3d<float> operator vector3d<float>(btVector3 vector) { // convert and return } btVector3 operator btVector3(vector3d<float> vector) { // convert and return } Right now this won't compile since casting operators need to be member methods. (error C2801: 'operator foo' must be a non-static member)

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• Sorting in Matlab

  - by smichak

  Hi, I would like to sort elements in a comma-separated list. The elements in the list are structs and I would like the list to be sorted according to one of the fields in the struct. For example, given the following code: L = {struct('obs', [1 2 3 4], 'n', 4), struct('obs', [6 7 5 3], 'n', 2)}; I would want to have a way to sort L by the field 'n'. Matlab's sort function only works on matrices or arrays and on lists of strings (not even lists of numbers). Any ideas on how that may be achieved? Thanks, Micha

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• Is the use of union in this matrix class completely safe?

  - by identitycrisisuk

  Unions aren't something I've used that often and after looking at a few other questions on them here it seems like there is almost always some kind of caveat where they might not work. Eg. structs possibly having unexpected padding or endian differences. Came across this in a math library I'm using though and I wondered if it is a totally safe usage. I assume that multidimensional arrays don't have any extra padding and since the type is the same for both definitions they are guaranteed to take up exactly the same amount of memory? template<typename T> class Matrix44T { ... union { T M[16]; T m[4][4]; } m; }; Are there any downsides to this setup? Would the order of definition make any difference to how this works?

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• struct with template variables in c++

  - by monkeyking

  I'm playing around with template, so I'm not trying to reinvent the std::vector, I'm trying to get a grasp of templateting in c++. Can I do the following template <typename T> typedef struct{ size_t x; T *ary; }array; What I'm trying to do is a basic templated version of typedef struct{ size_t x; int *ary; }iArray; It looks like its working if I use a class instead of struct, so is it not possible with typedef structs? thanks

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• memset on array of structures in C++

  - by garry

  I have another memset question. It appears as if the code I am editing may have some issues (or it's not done the same way in different files) A::LRM las[9]; //A and LRM are both structures with BOOLS and INTS memset(&las, 0, sizeof(las)); typedef Sec SecArray[16]; SecArray rad_array; memset(rad_array, 0, sizeof(SecArray)); The second example appears to be correct because rad_array is the same as the first position in the array. Then the sizeof(SecArray)) would make sense. The first one doesn't seem correct to me. All structs are just BOOLs and INTS nothing dynamic in them. My understanding from my other post about memset was that it followed this format. memset("pointer to object", "what to set it to", "size of object") Can anyone tell me what exactly is going on here if I am incorrect with my theory.

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• Identifying the parts of this typedef struct in C?

  - by Tommy

  Please help me identify the parts of this typdef struct and what each part does and how it can be used: typedef struct my_struct { int a; int b; int c; } struct_int, *p_s; struct_int struct_array[5]; my_struct is the...? struct_int is the...? *p_s is the...and can be used to point to what? struct_array is the...? Also, when creating the array of structs, why do we use struct_int instead of my_struct ? Thank You!

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• Lua-Objective-C bridge on the iphone

  - by John Smith

  I have partially ported the LuaObjCBridge to the iPhone. Most things work but there are still some issues I have to deal with. There are sections where #defines are defined with-respect-to intel or ppc. Is the ARM chip closer to intel or ppc? Here is the most relevant section where most of the defines are: #if defined(__ppc__)||defined(__PPC__)||defined(__powerpc__) #define LUA_OBJC_METHODCALL_INT_IS_SHORTEST_INTEGRAL_TYPE #define LUA_OBJC_METHODCALL_PASS_FLOATS_IN_MARG_HEADER #define LUA_OBJC_POWER_ALIGNMENT #elif defined(__i386__)||defined(__arm__) #warning LuaObjCBridge is not fully tested for use on Intel chips. #define LUA_OBJC_METHODCALL_RETURN_STRUCTS_DIRECTLY // Use this or the code was crashing for me for structs LUA_OBJC_METHODCALL_RETURN_STRUCTS_DIRECTLY_LIMIT #define LUA_OBJC_METHODCALL_USE_OBJC_MSGSENDV_FPRET #define LUA_OBJC_METHODCALL_RETURN_STRUCTS_DIRECTLY_LIMIT 8 #define LUA_OBJC_INTEL_ALIGNMENT #endif For now I added arm with i386, but I could be wrong

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• In game programming are global variables bad?

  - by Joe.F

  I know my gut reaction to global variables is "badd!" but in the two game development courses I've taken at my college globals were used extensively, and now in the DirectX 9 game programming tutorial I am using (www.directxtutorial.com) I'm being told globals are okay in game programming ...? The site also recommends using only structs if you can when doing game programming to help keep things simple. I'm really confused on this issue, and all the research I've been trying to do is very confusing. I realize there are issues when using global variables (threading issues, they make code harder to maintain, the state of them is hard to track etc) but also there is a cost associated with not using globals, I'd have to pass a loooot of information around very often which can be confusing and I imagine time-costing, although I guess pointers would speed the process up (this is my first time writing a game in C++.) Anyway, I realize there is probably no "right" or "wrong" answer here since both ways work, but I want my code to be as proper as I can so any input would be good, thank you very much!

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• C# Struct No Parameterless Constructor? See what I need to accomplish

  - by Changeling

  I am using a struct to pass to an unmanaged DLL as so - [StructLayout(LayoutKind.Sequential)] public struct valTable { public byte type; public byte map; public byte spare1; public byte spare2; public int par; public int min; public byte[] name; public valTable() { name = new byte[24]; } } The code above will not compile because VS 2005 will complain that "Structs cannot contain explicit parameterless constructors". In order to pass this struct to my DLL, I have to pass an array of struct's like so valTable[] val = new valTable[281]; What I would like to do is when I say new, the constructor is called and it creates an array of bytes like I am trying to demonstrate because the DLL is looking for that byte array of size 24 in each dimension. How can I accomplish this?

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• Lua operations, that works in mutitheaded environment

  - by SBKarr

  My application uses Lua in multithreaded environment with global mutex. It implemented like this: Thread locks mutex, Call lua_newthread Perform some initialization on coroutine Run lua_resume on coroutine Unlocks mutex lua_lock/unlock is not implemented, GC is stopped, when lua works with coroutine. My question is, can I perform steps 2 and 3 without locking, if initialisation process does not requires any global Lua structs? Can i perform all this process without locking at all, if coroutine does not requires globals too? In what case I generally can use Lua functions without locking?

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• Using std::bad_alloc for C pointers

  - by otibom

  I'm using a library written in C in a C++ project. I'd like to use C++ exceptions to handle C errors. In particular, it would be nice to have an exception thrown if an allocation fails. I can do this in constructors of classes which hold C-style pointers to C structs : if (c_object == NULL) throw std::bad_alloc(); But if the class is responsible for several C objects they are no ways of free-ing all already allocated pointers since the destructor isn't called. I have a feeling I could use smart-pointers, but I don't have much experience with them. What's more, I have to have access to the original C pointers to use the C api properly. Is there an elegant solution to this ?

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• C++: Delete a struct?

  - by Rosarch

  I have a struct that contains pointers: struct foo { char* f; int* d; wchar* m; } I have a vector of shared pointers to these structs: vector<shared_ptr<foo>> vec; vec is allocated on the stack. When it passes out of scope at the end of the method, its destructor will be called. (Right?) That will in turn call the destructor of each element in the vector. (Right?) Does calling delete foo delete just the pointers such as foo.f, or does it actually free the memory from the heap?

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• Resolve circular typedef dependency?

  - by Nick

  What is the best way to resolve the following circular dependency in typedef-ing these structs? Note the C language tag - I'm looking for a solution in standard gcc C. typedef struct { char* name; int age; int lefthanded; People* friends; } Person; typedef struct { int count; int max; Person* data; } People;

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• C# how to sort a list without implementing IComparable manually?

  - by JL

  I have a fairly complex scenario and I need to ensure items I have in a list are sorted. Firstly the items in the list are based on a struct that contains a sub struct. For example: public struct topLevelItem { public custStruct subLevelItem; } public struct custStruct { public string DeliveryTime; } Now I have a list comprised of topLevelItems for example: var items = new List<topLevelItem>(); I need a way to sort on the DeliveryTime ASC. What also adds to the complexity is that the DeliveryTime field is a string. Since these structs are part of a reusable API, I can't modify that field to a DateTime, neither can I implement IComparable in the topLevelItem class. Any ideas how this can be done? Thank you

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• Allocate from buffer in C

  - by Grimless

  I am building a simple particle system and want to use a single array buffer of structs to manage my particles. That said, I can't find a C function that allows me to malloc() and free() from an arbitrary buffer. Here is some pseudocode to show my intent: Particle* particles = (Particle*) malloc( sizeof(Particle) * numParticles ); Particle* firstParticle = <buffer_alloc>( particles ); initialize_particle( firstParticle ); // ... Some more stuff if (firstParticle->life < 0) <buffer_free>( firstParticle ); // @ program's end free(particles); Where <buffer_alloc> and <buffer_free> are functions that allocate and free memory chunks from arbitrary pointers (possibly with additional metadata such as buffer length, etc.). Do such functions exist and/or is there a better way to do this? Thank you!

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• Only draw visible objects to the camera in 2D

  - by Deukalion

  I have Map, each map has an array of Ground, each Ground consists of an array of VertexPositionTexture and a texture name reference so it renders a texture at these points (as a shape through triangulation). Now when I render my map I only want to get a list of all objects that are visible in the camera. (So I won't loop through more than I have to) Structs: public struct Map { public Ground[] Ground { get; set; } } public struct Ground { public int[] Indexes { get; set; } public VertexPositionNormalTexture[] Points { get; set; } public Vector3 TopLeft { get; set; } public Vector3 TopRight { get; set; } public Vector3 BottomLeft { get; set; } public Vector3 BottomRight { get; set; } } public struct RenderBoundaries<T> { public BoundingBox Box; public T Items; } when I load a map: foreach (Ground ground in CurrentMap.Ground) { Boundaries.Add(new RenderBoundaries<Ground>() { Box = BoundingBox.CreateFromPoints(new Vector3[] { ground.TopLeft, ground.TopRight, ground.BottomLeft, ground.BottomRight }), Items = ground }); } TopLeft, TopRight, BottomLeft, BottomRight are simply the locations of each corner that the shape make. A rectangle. When I try to loop through only the objects that are visible I do this in my Draw method: public int Draw(GraphicsDevice device, ICamera camera) { BoundingFrustum frustum = new BoundingFrustum(camera.View * camera.Projection); // Visible count int count = 0; EffectTexture.World = camera.World; EffectTexture.View = camera.View; EffectTexture.Projection = camera.Projection; foreach (EffectPass pass in EffectTexture.CurrentTechnique.Passes) { pass.Apply(); foreach (RenderBoundaries<Ground> render in Boundaries.Where(m => frustum.Contains(m.Box) != ContainmentType.Disjoint)) { // Draw ground count++; } } return count; } When I try adding just one ground, then moving the camera so the ground is out of frame it still returns 1 which means it still gets draw even though it's not within the camera's view. Am I doing something or wrong or can it be because of my Camera? Any ideas why it doesn't work?

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• Patterns for Handling Changing Property Sets in C++

  - by Bhargav Bhat

  I have a bunch "Property Sets" (which are simple structs containing POD members). I'd like to modify these property sets (eg: add a new member) at run time so that the definition of the property sets can be externalized and the code itself can be re-used with multiple versions/types of property sets with minimal/no changes. For example, a property set could look like this: struct PropSetA { bool activeFlag; int processingCount; /* snip few other such fields*/ }; But instead of setting its definition in stone at compile time, I'd like to create it dynamically at run time. Something like: class PropSet propSetA; propSetA("activeFlag",true); //overloading the function call operator propSetA("processingCount",0); And the code dependent on the property sets (possibly in some other library) will use the data like so: bool actvFlag = propSet["activeFlag"]; if(actvFlag == true) { //Do Stuff } The current implementation behind all of this is as follows: class PropValue { public: // Variant like class for holding multiple data-types // overloaded Conversion operator. Eg: operator bool() { return (baseType == BOOLEAN) ? this->ToBoolean() : false; } // And a method to create PropValues various base datatypes static FromBool(bool baseValue); }; class PropSet { public: // overloaded[] operator for adding properties void operator()(std::string propName, bool propVal) { propMap.insert(std::make_pair(propName, PropVal::FromBool(propVal))); } protected: // the property map std::map<std::string, PropValue> propMap; }; This problem at hand is similar to this question on SO and the current approach (described above) is based on this answer. But as noted over at SO this is more of a hack than a proper solution. The fundamental issues that I have with this approach are as follows: Extending this for supporting new types will require significant code change. At the bare minimum overloaded operators need to be extended to support the new type. Supporting complex properties (eg: struct containing struct) is tricky. Supporting a reference mechanism (needed for an optimization of not duplicating identical property sets) is tricky. This also applies to supporting pointers and multi-dimensional arrays in general. Are there any known patterns for dealing with this scenario? Essentially, I'm looking for the equivalent of the visitor pattern, but for extending class properties rather than methods. Edit: Modified problem statement for clarity and added some more code from current implementation.

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• C# Dev - I've tried Lisps, but I don't get it.

  - by Jonathan Mitchem

  After a few months of learning about and playing with lisps, both CL and a bit of Clojure, I'm still not seeing a compelling reason to write anything in it instead of C#. I would really like some compelling reasons, or for someone to point out that I'm missing something really big. The strengths of a Lisp (per my research): Compact, expressive notation - More so than C#, yes... but I seem to be able to express those ideas in C# too. Implicit support for functional programming - C# with LINQ extension methods: mapcar = .Select( lambda ) mapcan = .Select( lambda ).Aggregate( (a,b) = a.Union(b) ) car/first = .First() cdr/rest = .Skip(1) .... etc. Lambda and higher-order function support - C# has this, and the syntax is arguably simpler: "(lambda (x) ( body ))" versus "x = ( body )" "#(" with "%", "%1", "%2" is nice in Clojure Method dispatch separated from the objects - C# has this through extension methods Multimethod dispatch - C# does not have this natively, but I could implement it as a function call in a few hours Code is Data (and Macros) - Maybe I haven't "gotten" macros, but I haven't seen a single example where the idea of a macro couldn't be implemented as a function; it doesn't change the "language", but I'm not sure that's a strength DSLs - Can only do it through function composition... but it works Untyped "exploratory" programming - for structs/classes, C#'s autoproperties and "object" work quite well, and you can easily escalate into stronger typing as you go along Runs on non-Windows hardware - Yeah, so? Outside of college, I've only known one person who doesn't run Windows at home, or at least a VM of Windows on *nix/Mac. (Then again, maybe this is more important than I thought and I've just been brainwashed...) The REPL for bottom-up design - Ok, I admit this is really really nice, and I miss it in C#. Things I'm missing in a Lisp (due to a mix of C#, .NET, Visual Studio, Resharper): Namespaces. Even with static methods, I like to tie them to a "class" to categorize their context (Clojure seems to have this, CL doesn't seem to.) Great compile and design-time support the type system allows me to determine "correctness" of the datastructures I pass around anything misspelled is underlined realtime; I don't have to wait until runtime to know code improvements (such as using an FP approach instead of an imperative one) are autosuggested GUI development tools: WinForms and WPF (I know Clojure has access to the Java GUI libraries, but they're entirely foreign to me.) GUI Debugging tools: breakpoints, step-in, step-over, value inspectors (text, xml, custom), watches, debug-by-thread, conditional breakpoints, call-stack window with the ability to jump to the code at any level in the stack (To be fair, my stint with Emacs+Slime seemed to provide some of this, but I'm partial to the VS GUI-driven approach) I really like the hype surrounding Lisps and I gave it a chance. But is there anything I can do in a Lisp that I can't do as well in C#? It might be a bit more verbose in C#, but I also have autocomplete. What am I missing? Why should I use Clojure/CL?

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• ConsumeStructuredBuffer, what am I doing wrong?

  - by John

  I'm trying to implement the 3rd exercise in chapter 12 of Introduction to 3D Game Programming with DirectX 11, that is: Implement a Compute Shader to calculate the length of 64 vectors. Previous exercises ask you to do the same with typed buffers and regular structured buffers and I had no problems with them. For what I've read, [Consume|Append]StructuredBuffers are bound to the pipeline using UnorderedAccessViews (as long as they use the D3D11_BUFFER_UAV_FLAG_APPEND, and the buffers have both D3D11_BIND_SHADER_RESOURCE and D3D11_BIND_UNORDERED_ACCESS bind flags). Problem is: my AppendStructuredBuffer works, since I can append data to it and retrieve it from the application to write to a results file, but the ConsumeStructuredBuffer always returns zeroed data. Data is in the buffer, since if I change the UAV to a ShaderResourceView and to a StructuredBuffer in the HLSL side it works. I don't know what I am missing: Should I initialize the ConsumeStructuredBuffer on the GPU, or can I do it when I create the buffer (as I amb currently doing). Is it OK to bind the buffer with a UAV as described above? Do I need to bind it as a ShaderResourceView somehow? Maybe I am missing some step? This is the declaration of buffers in the Compute Shader: struct Data { float3 v; }; struct Result { float l; }; ConsumeStructuredBuffer<Data> gInput; AppendStructuredBuffer<Result> gOutput; And here the creation of the buffer and UAV for input data: D3D11_BUFFER_DESC inputDesc; inputDesc.Usage = D3D11_USAGE_DEFAULT; inputDesc.ByteWidth = sizeof(Data) * mNumElements; inputDesc.BindFlags = D3D11_BIND_SHADER_RESOURCE | D3D11_BIND_UNORDERED_ACCESS; inputDesc.CPUAccessFlags = 0; inputDesc.StructureByteStride = sizeof(Data); inputDesc.MiscFlags = D3D11_RESOURCE_MISC_BUFFER_STRUCTURED; D3D11_SUBRESOURCE_DATA vinitData; vinitData.pSysMem = &data[0]; HR(md3dDevice->CreateBuffer(&inputDesc, &vinitData, &mInputBuffer)); D3D11_UNORDERED_ACCESS_VIEW_DESC uavDesc; uavDesc.Format = DXGI_FORMAT_UNKNOWN; uavDesc.ViewDimension = D3D11_UAV_DIMENSION_BUFFER; uavDesc.Buffer.FirstElement = 0; uavDesc.Buffer.Flags = D3D11_BUFFER_UAV_FLAG_APPEND; uavDesc.Buffer.NumElements = mNumElements; md3dDevice->CreateUnorderedAccessView(mInputBuffer, &uavDesc, &mInputUAV); Initial data is an array of Data structs, which contain a XMFLOAT3 with random data. I bind the UAV to the shader using the Effects framework: ID3DX11EffectUnorderedAccessViewVariable* Input = mFX->GetVariableByName("gInput")->AsUnorderedAccessView(); Input->SetUnorderedAccessView(uav); // uav is mInputUAV Any ideas? Thank you.

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• Is 2 lines of push/pop code for each pre-draw-state too many?

  - by Griffin

  I'm trying to simplify vector graphics management in XNA; currently by incorporating state preservation. 2X lines of push/pop code for X states feels like too many, and it just feels wrong to have 2 lines of code that look identical except for one being push() and the other being pop(). The goal is to eradicate this repetitiveness,and I hoped to do so by creating an interface in which a client can give class/struct refs in which he wants restored after the rendering calls. Also note that many beginner-programmers will be using this, so forcing lambda expressions or other advanced C# features to be used in client code is not a good idea. I attempted to accomplish my goal by using Daniel Earwicker's Ptr class: public class Ptr<T> { Func<T> getter; Action<T> setter; public Ptr(Func<T> g, Action<T> s) { getter = g; setter = s; } public T Deref { get { return getter(); } set { setter(value); } } } an extension method: //doesn't work for structs since this is just syntatic sugar public static Ptr<T> GetPtr <T> (this T obj) { return new Ptr<T>( ()=> obj, v=> obj=v ); } and a Push Function: //returns a Pop Action for later calling public static Action Push <T> (ref T structure) where T: struct { T pushedValue = structure; //copies the struct data Ptr<T> p = structure.GetPtr(); return new Action( ()=> {p.Deref = pushedValue;} ); } However this doesn't work as stated in the code. How might I accomplish my goal? Example of code to be refactored: protected override void RenderLocally (GraphicsDevice device) { if (!(bool)isCompiled) {Compile();} //TODO: make sure state settings don't implicitly delete any buffers/resources RasterizerState oldRasterState = device.RasterizerState; DepthFormat oldFormat = device.PresentationParameters.DepthStencilFormat; DepthStencilState oldBufferState = device.DepthStencilState; { //Rendering code } device.RasterizerState = oldRasterState; device.DepthStencilState = oldBufferState; device.PresentationParameters.DepthStencilFormat = oldFormat; }

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• SQL 2014 does data the way developers want

  - by Rob Farley

  A post I’ve been meaning to write for a while, good that it fits with this month’s T-SQL Tuesday, hosted by Joey D’Antoni (@jdanton) Ever since I got into databases, I’ve been a fan. I studied Pure Maths at university (as well as Computer Science), and am very comfortable with Set Theory, which undergirds relational database concepts. But I’ve also spent a long time as a developer, and appreciate that that databases don’t exactly fit within the stuff I learned in my first year of uni, particularly the “Algorithms and Data Structures” subject, in which we studied concepts like linked lists. Writing in languages like C, we used pointers to quickly move around data, without a database in sight. Of course, if we had a power failure all this data was lost, as it was only persisted in RAM. Perhaps it’s why I’m a fan of database internals, of indexes, latches, execution plans, and so on – the developer in me wants to be reassured that we’re getting to the data as efficiently as possible. Back when SQL Server 2005 was approaching, one of the big stories was around CLR. Many were saying that T-SQL stored procedures would be a thing of the past because we now had CLR, and that obviously going to be much faster than using the abstracted T-SQL. Around the same time, we were seeing technologies like Linq-to-SQL produce poor T-SQL equivalents, and developers had had a gutful. They wanted to move away from T-SQL, having lost trust in it. I was never one of those developers, because I’d looked under the covers and knew that despite being abstracted, T-SQL was still a good way of getting to data. It worked for me, appealing to both my Set Theory side and my Developer side. CLR hasn’t exactly become the default option for stored procedures, although there are plenty of situations where it can be useful for getting faster performance. SQL Server 2014 is different though, through Hekaton – its In-Memory OLTP environment. When you create a table using Hekaton (that is, a memory-optimized one), the table you create is the kind of thing you’d’ve made as a developer. It creates code in C leveraging structs and pointers and arrays, which it compiles into fast code. When you insert data into it, it creates a new instance of a struct in memory, and adds it to an array. When the insert is committed, a small write is made to the transaction to make sure it’s durable, but none of the locking and latching behaviour that typifies transactional systems is needed. Indexes are done using hashes and using bw-trees (which avoid locking through the use of pointers) and by handling each updates as a delete-and-insert. This is data the way that developers do it when they’re coding for performance – the way I was taught at university before I learned about databases. Being done in C, it compiles to very quick code, and although these tables don’t support every feature that regular SQL tables do, this is still an excellent direction that has been taken. @rob_farley

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• Is this a pattern? Should it be?

  - by Arkadiy

  The following is more of a statement than a question - it describes something that may be a pattern. The question is: is this a known pattern? Or, if it's not, should it be? I've had a situation where I had to iterate over two dissimilar multi-layer data structures and copy information from one to the other. Depending on particular use case, I had around eight different kinds of layers, combined in about eight different combinations: A-B-C B-C A-C D-E A-D-E and so on After a few unsuccessful attempts to factor out the repetition of per-layer iteration code, I realized that the key difficulty in this refactoring was the fact that the bottom level needed access to data gathered at higher levels. To explicitly accommodate this requirement, I introduced IterationContext class with a number of get() and set() methods for accumulating the necessary information. In the end, I had the following class structure: class Iterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const = 0; }; class RecursingIterator : public Iterator { RecursingIterator(const Iterator &below); }; class IterateOverA : public RecursingIterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const { // Iterate over members in dataStructure1 // locate corresponding item in dataStructure2 (passed via context) // and set it in the context // invoke the sub-iterator }; class IterateOverB : public RecursingIterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const { // iterate over members dataStructure2 (form context) // set dataStructure2's item in the context // locate corresponding item in dataStructure2 (passed via context) // invoke the sub-iterator }; void main() { class FinalCopy : public Iterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const { // copy data from structure 1 to structure 2 in the context, // using some data from higher levels as needed } } IterationContext ctx(dateStructure2); IterateOverA(IterateOverB(FinalCopy())).iterate(dataStructure1, ctx); } It so happens that dataStructure1 is a uniform data structure, similar to XML DOM in that respect, while dataStructure2 is a legacy data structure made of various structs and arrays. This allows me to pass dataStructure1 outside of the context for convenience. In general, either side of the iteration or both sides may be passed via context, as convenient. The key situation points are: complicated code that needs to be invoked in "layers", with multiple combinations of layer types possible at the bottom layer, the information from top layers needs to be visible. The key implementation points are: use of context class to access the data from all levels of iteration complicated iteration code encapsulated in implementation of pure virtual function two interfaces - one aware of underlying iterator, one not aware of it. use of const & to simplify the usage syntax.

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