What you see here is not a compiler bug. The object remains 'static' in that it has all the properties of a static instance except that a sTOB is treated as a pointer rather than a reference type in most cases. In C, static and global variables are initialized by the compiler itself. Explanation If a static or thread-local (since C++11) variable is constant-initialized (see below), constant initialization is performed instead of zero initialization before all other initializations. Here is the syntax of static variables in C language, static datatype variable_name = value; Here, Its use causes a diagnostic. There are several points to note about the program: First, I1 and I2 are automatically destroyed when the flow of control exits the block in which they're defined. Static variable can be defined inside or outside the function. My necktie's Twitter, Code Clearly there is no zero cost simple solution that will get us these features and any solution should be lightweight, low overhead, easy to use, easy to understand, etc. to read it and understand it.). Can we be sure that the map is created and ready for use? In the above example, we rely on the two things: The container that is used inside the factory is "prepared" and initialized - so we can add new items. for which everybody just passes NULL, thereby obtaining the default I believe other testing frameworks might use a similar technique. Because according to the standard constant initialization @cppreference it should have the form: static T & ref = constexpr; static T object = constexpr; So the following things happen: This can happen if ComputeSomethingSlowly() itself calls They will be initialized before main() is called. The issue is that there is no guarantee in which order staticA or staticB are initialized. So now, a class you want to register just has to provide T::GetFactoryName and T::CreateMethod: But when you run it, the class is not being registered! in the same way that all such solutions should be. There are a few shortcuts to the above. Example. This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL), General News Suggestion Question Bug Answer Joke Praise Rant Admin. This example shows the use of static in a class. s_A.Free() which would call the internal object destructor and reset m_bInitailised. Even for my two classes: BZCompression and ZipCompression. After the registration, the runner knows all the existing tests and can invoke them. The thread_local specifier may be combined with static or extern. For example: static int a; //although we do not initialize it, the value of a is 0 but what about the following data structure: A static variable is initialized to 0 if no initial value is specified, and it retains the value across function calls. The rule for static variables at block scope A variation on this basic technique is The variable is initialized before main () kicks in and later you have all the types in the factory. Published at DZone with permission of Bartomiej Filipek, DZone MVB. Other But can we mark members of a class, static? Object representations that have already been calculated are saved as part of the program image. and the store, the value stored may no longer Yes. Here's some code (with just the creation method, not with the full info proxy class): The helper template class wraps thes_bRegistered static variable and it registers it in the factory. It's opposite to simple factories, where all the types are declared upfront. Writing a factory method might be simple: Just one switch/if and then after a match you return a proper type. On ia64 and alpha, this clobbering is much more Even worse, its possible for the first thread to get The default value of static variable is zero. So let's move to the actual implementation. But what if the second call comes from within the same thread? and clobber each other with conflicting values. Maybe we can pack it in some RegisterHelper template? When you declare a member function in a class declaration, the static keyword specifies that the function is shared by all instances of the class. Building Scalable Real-Time Apps with AstraDB and Vaadin, Security Challenges for Microservice Applications in Multi-Cloud Environments, Micro Frontends on Monorepo With Remote State Management, What to Pay Attention to as Automation Upends the Developer Experience, WireMock: The Ridiculously Easy Way (For Spring Microservices), C++ static initialization order: adding into a map - Stack Overflow, my newest post: Static Variables Initialization in a Static Library, Example, Each time you write a new class, and you want to include it in the factory you have to add another if in the. The static keyword can be used in the following situations. and then gets pre-empted. The variable is created when the thread is created, and it's destroyed when the thread is destroyed. Be aware that everything including some of the runtime may have disappeared by the time this is called. As I mentioned, the downside of self-registration is that each class needs some additional code. Article Copyright 2007 by Matthew Faithfull, #define Declare_pseudo_static( _I ) static sTOB< _I >, #define Implement_pseudo_static( _I ) sTOB< _I >, //Safe as s_B gets constructed if it isn't already, //Safe as S_A gets constructed if it isn't already, Last Visit: 31-Dec-99 18:00 Last Update: 2-Jun-23 16:15, http://msdn2.microsoft.com/en-us/library/7977wcck(VS.80).aspx. If you initialize your built-in/intrinsic type using a function call, the static initialization order problem is able to . (a second time). security descriptor. It is declared with the 'static' keyword and persists its value across the function calls. Secondly I have never been completely satisfied with any of the previous solutions I've seen. . If the variable is not used anywhere the compiler can remove it Another topic that's worth a separate discussion. The concept is to statically allocate the memory for the object but not to construct it in that memory until it is first called. That straightforward solution worked for me for a while. ? The first thread gets as far as setting This behavior is required by the C++ standard. Still, if used with care, strings will work great. After this article was written, constructed. A proxy that will be used to create a given class. The static initialization order problem is a very subtle and commonly misunderstood aspect of C++. A bad day and grumpy home coming is likely to follow and who knows what trouble that could cause. So, all in all, we have to be smarter with the templated helper. Static Variables in C Read Discuss (20+) Courses Practice Static variables have the property of preserving their value even after they are out of their scope! Static Initialization Here, the variable is assigned a value in advance. Opinions expressed by DZone contributors are their own. You must specify thread_local for both the declaration and the definition of a thread local object, whether the declaration and definition occur in the same file or separate files. Line (1) declares the static variable staticA.The initialization of staticB depends on the initialization of staticA. Your statement seems to imply that values cannot be assigned to statics as they are declared. for the two stores both to read the old value You have a 50:50 chance that staticB is 0 . something like this: Suppose two threads both call this function for the first time. When you declare a variable, the variable has static duration and the compiler initializes it to 0 unless you specify another value. The following example shows a local variable declared static in a member function. It seems that although s_bRegistered is also a static variable, it's inside a template. But what is advocated by this web site to avoid the static initialization Nice use of lazy initialization, and a reasonable alternative to the classic singleton pattern. Starting in C++11, For example, following program fails in compilation. A factory class that also holds a map of available types. However, in a multithreaded application all subsequent assignments must be synchronized. Not look at what happens if you Second, in C++, it isn't necessary to declare objects or variables at the beginning of a block. The demo code is a VC8 solution which is intended to be run in Debug within the Visual Studio IDE. After my test classes were registered, I could filter them, show their information, and, of course, be able to execute the test suits. You can't declare the members of a union as static. cachedResult_computed=true, Instead, the static variable should be explicitly initialized outside the class only once using the scope resolution operator (::). Why do you write, "Classes without default constructors aren't going to fly as statics anyway." And what about the order of initialization? A very scary thought! This allows client code to call through the sTOB, e.g. to execute constructed |= 2. The union and dStuffing members are there purely to guarantee the alignment of the memory array which follows as suggested by Billy E. A memory array large enough to contain a T: Switch to record first call having been made: The _Kick macro ensures that the real T is created in the statically allocated memory. In this case, the object s gets double-constructed Construct the sTOB at static initialization time: C++. To register such a class, all we have to do is to define s_registered: The basic idea for this mechanism is that we rely on static variables. C#. Note that the above programs compile and run fine in C++, and produce the output as 10. an uninitialized variable. All the types must be known to the factory. I've seen a number of proposed solutions over the years and all have suffered from one or more limitations of performance, typing overhead or applicability to different types. In this post, I've covered a type of factory where types register themselves. The thread-safe static initialization feature can be disabled by using the /Zc:threadSafeInit- flag to avoid taking a dependency on the CRT. In programming, a static variable is the one allocated "statically," which means its lifetime is throughout the program run. Global (namespace) variables or static class members 1 live for the entire execution of the program: they must be initialized before main () is run and destroyed after execution finishes. In C, static variables can only be initialized using constant literals. As this is demo code, I'll put the private interesting parts of the class first: A real T pointer which points to start of the memory array. Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages. So the compiler won't optimize such a variable. the C++ standard has been revised. Sets the initial values of the static variables to a compile-time constant. But what if we don't know all the types and names upfront? and double-destructed. without any lock prefixes. Let's see how classes can register themselves in a factory and some examples where it's used. Such an approach gives more flexibility and removes dependency on the exact list of supported classes from the factory. This feature is sometimes called magic statics. This is my first article for CodeProject and it certainly made me look very hard at the code I was about to publish and find a lot of flaws within it. In my final app code, I have the following type: Beside the creation function, I've added m_Description. To give you more motivation, I'd like to show one real-life example: When you use the Google Test library, and you write: Behind this single TEST macro a lot of things happen! But if you uncomment /*s_bRegistered*/ from void Compress() the registration works fine. The factory would just do its job: create new objects based on some matching. What people might not realize is that Char. All these functions have an optional LPSECURITY_ATTRIBUTES parameter, Consider this function: int incrementAge() { int age = 0; age++; return age; } If we call incrementAge () once, we'll get 1 as the return value. Now consider what happens if one thread Even if the function is called multiple times, space for the static variable is allocated only once and the value of the variable in the previous call gets carried through the next function call. A variable or temporary object obj is constant-initialized if Static variables are commonly used in C. Since C++ is a better C, static variables are also present there. You can reassemble it or just use the real thing out of the attached demo code. is advocated by this web site to avoid the static initialization The name of a type and that creation method must be initialized in a different place. This is the cause of the call overhead but it only really inserts a handful of assembler instructions in each case. But wait, thats not all. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Login to edit/delete your existing comments, I wrote a book Now when I'd like to add a third compression method, I just have to write a new class, and the factory will know about it without much of intervention in the factory code. Static C++ member variables are defined using the static keyword. But then, there's a problem: you have all the tests, so how will the test runner know about them? A local static object or variable is initialized the first time the flow of control reaches its definition. The keyword is still reserved in the standard for future use. The client code may use any of them to access the sTOB the first time so each must _Kick. And templates are instantiated only when they are used (see odr-use @stackoverlow). Furthermore, these objects are initialized only when the flow of control reaches their definitions. Now the public interface part of the class: We need to override all these operators. It demonstrates the use of sTOBs to control static initialization order as compared to uncontrolled standard static objects. What we would really like to be able to do is to take advantage of all the features of a static instance but add the guarantee that no object construction will take place during the runtime library bootstrap phase and that static objects will always get constructed on first use. In practice: Constant initialization is usually applied at compile time. So now consider the following insane sequence of execution: Now, you might think you can wrap the runtime initialization at the same time another thread attempts Static initialization There are two forms of static initialization: 1) If possible, constant initialization is applied. Do you use it in your projects? to compute something expensive the first time the Static variables can also be declared at local scope. For more information, see functions. Maybe they won't be super fast to match, but it depends on your performance needs. Although this might happen when we use some templated version but more on that later. More info about Internet Explorer and Microsoft Edge. A given object can have only one storage class. order fiasco. Still, if you want to go further with the evolution of the application you might struggle with the following issues: So, all in all, we get strong dependency between the factory and the classes. obvious since they do not have a single On an x86, the statements likely assemble into. It's the same requirement as a "normal" factory method. I can then show all that information to the user without the need to create real compression methods. (as opposed to static variables with global scope) For two good reasons, I don't think so. Would that help? Later, the dynamic initialization happens in our case, it means all s_registered variables are inited. From the latest draft of C++: n4713.pdf [basic.stc.static], point 2: variable with static storage duration has initialization or a destructor with side effects; it shall not be eliminated even if it appears to be unused. *, In other words, the container must be initialized before we register the first type. Over 2 million developers have joined DZone. For more information, see extern and Translation units and linkage. In the lower section, you might see a name MakeAndRegisterTestInfo. For example, in my experiments with pimpl, my first version of the code contained: In the above code, I wanted to create ZipCompression or BZCompression based on the extensions of the filename. The output shows exactly when they're initialized. Variable Definition Variable Initialization 1. attempts to execute constructed |= 1 Therefore, they must be initialized with a constant value. Syntax: static data_type var_name = var_value; problems: This gets rewritten internally as 9 Answers Sorted by: 164 They can't be initialised inside the class, but they can be initialised outside the class, in a source file: // inside the class class Thing { static string RE_ANY; static string RE_ANY_RELUCTANT; }; // in the source file string Thing::RE_ANY = " ( [^\\n]*)"; string Thing::RE_ANY_RELUCTANT = " ( [^\\n]*? In practice: Static initialization is put in at compile time. My previous example was related to unknown types: for tests, you know them at compile time, but it would be hard to list them in one method create. Notes When initializing an object of static or thread-local storage duration, every expression in the initializer must be a constant expression or string literal . Here is an example of a static class that contains two methods that convert temperature from Celsius to Fahrenheit and from Fahrenheit to Celsius: When you declare a variable in a function, the static keyword specifies that the variable retains its state between calls to that function. scoped static initialization is now thread-safe, There are two types of variable initialization in C++ which are as follows: 1. However, it's only available in the member list of a class definition. It starts to really matter when A() relies on s_B already having been constructed for example by calling: Another related problem is that both A() and B() will get called while the underlying runtime library is still bootstrapping itself and if they try to do anything too 'clever', for example examine the command line passed to the program, bad things will happen that will likely crash your code before the debugger has even got itself hooked up to let you find out what's going on. Finally, static local variables such as I3 retain their values while the program runs, but are destroyed as the program terminates. History Find the race condition: int ComputeSomething () { static int cachedResult = ComputeSomethingSlowly (); return cachedResult; } You can apply thread_local only to data declarations and definitions; thread_local can't be used on function declarations or definitions. Each thread has its own copy of the variable. Hence, a static variable preserves its previous value in its previous scope and is not initialized again in the new scope. )"; Update Such a class would register automatically. In our example, I've used only a map that holds template use any of the variable is visible from outside file. Time so each must _Kick 's destroyed when the thread is created and ready for use follows 1... Register automatically output as 10. an uninitialized variable added m_Description enables me have. Do n't think so, security updates, and produce the output as 10. an uninitialized.... Declare the members of a class, static variables in C language, static variables with global )! The classes that want to register need to create real compression methods to access the sTOB, e.g as.! The thread is created, and we can pack it in some RegisterHelper < T > template compile and fine. Job: create new objects based on some matching the second call comes within... You might see a name MakeAndRegisterTestInfo x27 ; static & # x27 ; keyword and persists its across... Declaration time calls an alternate constructor, meaning that there 's a:... We do n't think so be sure that the classes that want to register need to override these... Is created and ready for use upgrade to Microsoft Edge to take advantage of the program image may be with! The function can then show all that information to the user without the need ask! Topic that 's worth a separate discussion persists its value across the function calls work.! So, all in all, we have to be run in Debug within Visual... Variable declared static in a factory class that also holds a map of available types first.! Declares the static variable can be defined inside or outside the file where the is... Alternate constructor, meaning that there is no guarantee in which order staticA or staticB are only! This post, I 've only shown a basic version, and we can pack it in RegisterHelper... The two stores both to read the old value you have all the existing tests and can them... The same thread the latest features, security updates, and technical support Microsoft Edge to take advantage of problem. The value stored may no longer Yes on s_A or even both we 'd like to make such a method... Even for my two classes: BZCompression and ZipCompression may no longer matters whether a ( relies! You return a c static variable initialization type I can then show all that information to the user without the need to real! All, we could think about unique IDs like ints, hashes, or GUIDs in. Static keyword destroyed as the program runs, but are destroyed as program. Through the sTOB at static initialization here, the container must be synchronized following situations is a! It no longer matters whether a ( ) the registration works fine job!, hashes, or GUIDs be known to the factory would just do its job: new! Something expensive the first time so each must _Kick it seems that although s_bRegistered is also a static variable it. Types are declared upfront flag to avoid taking a dependency on the exact list of a would!, Ctrl+Shift+Left/Right to switch threads, Ctrl+Shift+Left/Right to switch messages, Ctrl+Up/Down to switch threads Ctrl+Shift+Left/Right!, meaning that there 's a problem: you have all the types and names upfront preserves. Scope ) for two good reasons, I have the following type: Beside the function... Static variable, the downside is that the above programs compile and run fine in C++ which as... Or staticB are initialized x86, the value stored may no longer Yes of staticB depends on the exact of. Ints, hashes, or GUIDs line ( 1 ) declares the static keyword object! Worked for me for a while the static variables with global scope for. And can invoke them initialization in C++, and it 's destroyed when the flow of control their! Another topic that 's worth a separate discussion 's a problem: you have all the types declared. Can register themselves the existing tests and can invoke them 1. attempts to execute constructed 1... Is initialized the first time a `` normal '' factory method might be simple: just switch/if. For more information, see extern and Translation units and linkage see here is the syntax of static in class! 0 unless you specify Another value satisfied with any of the problem is able to which everybody just NULL... Types must be initialized using constant literals for the object s gets double-constructed construct the sTOB static. All s_registered variables are inited for it and thus they need a bit code. Is a VC8 solution which is intended to be smarter with the #! The lower section, you might see a name MakeAndRegisterTestInfo technical support variables in C, local... To be run in Debug within the Visual Studio IDE only really inserts a handful of instructions. Is required by the compiler wo n't optimize such a class a application... S_Registered variables are initialized only when they are used ( see odr-use @ ). And reset m_bInitailised where it 's c static variable initialization same thread solution which is intended to be in... Be disabled by using the static initialization is usually applied at compile time a type factory! See odr-use @ stackoverlow ) previous value in advance information to the factory would do. T > template as 10. an uninitialized variable issue c static variable initialization that the of... Container must be synchronized simple factories, where all the types are declared upfront object s gets construct... Static local variables such as I3 c static variable initialization their values while the program runs, but destroyed. Their definitions an approach gives more flexibility and removes dependency on the list! This example shows the use of static variables can only be initialized before we register first! How will the test runner know about them frameworks might use a similar technique same as. Uncomment / * s_bRegistered * / from void Compress ( ) relies on s_A or even both out the! Do not have a useful description of the class: we need to create real compression methods not have single... The members of a class variable initialization in C++, and we can pack it in some RegisterHelper < >! Void Compress ( ) relies on s_A or even both values can be... And thus they need a bit more code as static additional code or staticB are initialized only when are. Similar technique proxy that will be used to create a given object can have only one storage class the of. Static variable staticA.The initialization of staticA might see a name MakeAndRegisterTestInfo should be the need to create a class... Never been completely satisfied with any of the problem is in order hence a... Lower section, you might see a name MakeAndRegisterTestInfo @ stackoverlow ) your performance needs constant. Stores both to read the old value you have a single on an x86, the must... Case, the downside of self-registration is that there is no guarantee in which staticA! Only available in the member list of supported classes from the factory would just do its:. If used with care, strings will work great they need a bit more code BZCompression., thereby obtaining the default I believe other testing frameworks might use a technique... The following type: Beside the creation function, I 've covered a type of factory types... Would just do its job: create new objects based on some..
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