Related
When I use static variables in C++, I often end up wanting to initialize one variable passing another to its constructor. In other words, I want to create static instances that depend on each other.
Within a single .cpp or .h file this is not a problem: the instances will be created in the order they are declared. However, when you want to initialize a static instance with an instance in another compilation unit, the order seems impossible to specify. The result is that, depending on the weather, it can happen that the instance that depends on another is constructed, and only afterwards the other instance is constructed. The result is that the first instance is initialized incorrectly.
Does anyone know how to ensure that static objects are created in the correct order? I have searched a long time for a solution, trying all of them (including the Schwarz Counter solution), but I begin to doubt there is one that really works.
One possibility is the trick with the static function member:
Type& globalObject()
{
static Type theOneAndOnlyInstance;
return theOneAndOnlyInstance;
}
Indeed, this does work. Regrettably, you have to write globalObject().MemberFunction(), instead of globalObject.MemberFunction(), resulting in somewhat confusing and inelegant client code.
Update: Thank you for your reactions. Regrettably, it indeed seems like I have answered my own question. I guess I'll have to learn to live with it...
You have answered your own question. Static initialization order is undefined, and the most elegant way around it (while still doing static initialization i.e. not refactoring it away completely) is to wrap the initialization in a function.
Read the C++ FAQ items starting from https://isocpp.org/wiki/faq/ctors#static-init-order
Maybe you should reconsider whether you need so many global static variables. While they can sometimes be useful, often it's much simpler to refactor them to a smaller local scope, especially if you find that some static variables depend on others.
But you're right, there's no way to ensure a particular order of initialization, and so if your heart is set on it, keeping the initialization in a function, like you mentioned, is probably the simplest way.
Most compilers (linkers) actually do support a (non-portable) way of specifying the order. For example, with visual studio you can use the init_seg pragma to arrange the initialization into several different groups. AFAIK there is no way to guarantee order WITHIN each group. Since this is non-portable you may want to consider if you can fix your design to not require it, but the option is out there.
Indeed, this does work. Regrettably, you have to write globalObject().MemberFunction(), instead of globalObject.MemberFunction(), resulting in somewhat confusing and inelegant client code.
But the most important thing is that it works, and that it is failure proof, ie. it is not easy to bypass the correct usage.
Program correctness should be your first priority. Also, IMHO, the () above is purely stylistic - ie. completely unimportant.
Depending on your platform, be careful of too much dynamic initialization. There is a relatively small amount of clean up that can take place for dynamic initializers (see here). You can solve this problem using a global object container that contains members different global objects. You therefore have:
Globals & getGlobals ()
{
static Globals cache;
return cache;
}
There is only one call to ~Globals() in order to clean up for all global objects in your program. In order to access a global you still have something like:
getGlobals().configuration.memberFunction ();
If you really wanted you could wrap this in a macro to save a tiny bit of typing using a macro:
#define GLOBAL(X) getGlobals().#X
GLOBAL(object).memberFunction ();
Although, this is just syntactic sugar on your initial solution.
dispite the age of this thread, I would like to propose the solution I've found.
As many have pointed out before of me, C++ doesn't provide any mechanism for static initialization ordering. What I propose is to encapsule each static member inside a static method of the class that in turn initialize the member and provide an access in an object-oriented fashion.
Let me give you an example, supposing we want to define the class named "Math" which, among the other members, contains "PI":
class Math {
public:
static const float Pi() {
static const float s_PI = 3.14f;
return s_PI;
}
}
s_PI will be initialized the first time Pi() method is invoked (in GCC). Be aware: the local objects with static storage have an implementation dependent lifecyle, for further detail check 6.7.4 in 2.
Static keyword, C++ Standard
Wrapping the static in a method will fix the order problem, but it isn't thread safe as others have pointed out but you can do this to also make it thread if that is a concern.
// File scope static pointer is thread safe and is initialized first.
static Type * theOneAndOnlyInstance = 0;
Type& globalObject()
{
if(theOneAndOnlyInstance == 0)
{
// Put mutex lock here for thread safety
theOneAndOnlyInstance = new Type();
}
return *theOneAndOnlyInstance;
}
I have a question of "good design practices" in C++. I am writing a numerical library in C++11 and I use at lot of metaprogramming and template-based technique. But I have a very basic question :
Consider a function that can have a two very close behaviours excepted an option that can be activated by a boolean flag. I consider only a flag that can be set/unset by the developer and not a flag that can be set/unset at runtime. There are 3 possibilities of design :
1) Write two functions with the explicit option in their name :
myFunctionFlag1(...);
myFunctionFlag2(...);
2) Use a template parameter :
template<bool Flag> myFunction(...);
3) Use a variable parameter :
myFunction(..., const bool flag);
In terms of good design practices which solution is acceptable/unacceptable ? If there is a best solution, which one is it and why ? If there is a worst solution, which one is it and why ?
EDIT : for the considered functions the runtime overhead could be considered as negligible, so this is not the most critical point.
EDIT 2 : I know that all the three work. But as my library will have users, it needs to have a reliable/good design.
Is the option 2 common (because it seems to me to be a good compromise) ?
None of the above. Boolean flags are terrible for code readability. If the flag-controlled functionality is sufficiently small that it makes sense to use a single function, then use a single function, but don't use bool as the type of the flag. Instead, use an enumeration with enumerators that have useful names:
enum class MyFunctionMode { EnableFoo, DisableFoo };
void myFunction(..., MyFunctionMode mode);
This pattern makes it easy to understand at the call site what options are being provided to the function. Multiple options may be combined using flags.
My recommendation would be to use the option 1.
Comments:
The most simple and clear. It does not have run time overhead.
I do not see enough justification (that might be still present if more details are given) for using this approach. It is more complex than option 1.
This variant has run time overhead. It can be still used in a non time/CPU critical code. If your function is large (100+ lines), then this variant becomes more attractive,
My 2 cents.
Your first solution duplicates code, and that is something nobody wants to do.
In the third one is acceptable, but the if statement will be evaluate at run time, and implies a lose of time.
The template one seems to be appropriate.
The if statement will be eather if (true) or if (false) at compile time, and the compiler will optimize it, as if they were no if statement.
You can do all three you know.
void fun_that_does_stuff();
void fun_that_does_something_else();
template < bool F >
void fun_that_does_stuff_or_something();
template < >
void fun_that_does_stuff_or_something<true>() { fun_that_does_stuff(); }
... blah blah.
Duplicating non-trivial code is the worst option. If the two functions do little more than dispatching to the actual interesting function it may be the right option. Of course, the two functions could dispatch to the templated version but they could give a better indication of what the difference is than true or false could do.
Another concern is how the functions are actually used: If they are themselves called from functions having the same configurations or configurations from which the choice of flags can be derived, using different names becomes rather annoying or even not maintainable: you'd end up with having to call functions conditionally:
(flag? &myFunctionFlag1: &myFunctionFlag2)(...);
The choice between the other two versions cones down to the question: does the check matter? If it does, the template is the only choice. If it doesn't you can choose either version and I'd bet that an anonymous poll would vote for the run-time version: the eyes of the majority of my colleagues glace over when I show them a trivial template.
Sometimes when fixing a defect in an existing code base I might (often out of laziness) decide to change a method from:
void
MyClass::foo(uint32_t aBar)
{
// Do something with aBar...
}
to:
void
MyClass::foo(uint32_t aBar, bool aSomeCondition)
{
if (aSomeCondition)
{
// Do something with aBar...
}
}
During a code review a colleague mentioned that a better approach would be to sub-class MyClass to provide this specialized functionality.
However, I would argue that as long as aSomeCondition doesn't violate the purpose or cohesion of MyClass it is an acceptable pattern to use. Only if the code became infiltrated with flags and if statements would inheritance be a better option, otherwise we would be potentially be entering architecture astronaut territory.
What's the tipping point here?
Note: I just saw this related answer which suggests that an enum may be a better
choice than a bool, but I think my question still applies in this case.
There is not only one solution for this kind of problem.
Boolean has a very low semantic. If you want to add in the future a new condition you will have to add a new parameter...
After four years of maintenance your method may have half a dozen of parameters, if these parameters are all boolean it is very nice trap for maintainers.
Enum is a good choice if cases are exclusive.
Enums can be easily migrated to a bit-mask or a context object.
Bit mask : C++ includes C language, you can use some plain old practices. Sometime a bit mask on an unsigned int is a good choice (but you loose type checking) and you can pass by mistake an incorrect mask. It is a convenient way to move smoothly from a boolean or an enum argument to this kind of pattern.
Bit mask can be migrated with some effort to a context-object. You may have to implement some kind of bitwise arithmetics such as operator | and operator & if you have to keep a buildtime compatibility.
Inheritence is sometime a good choice if the split of behavior is big and this behavior IS RELATED to the lifecycle of the instance. Note that you also have to use polymorphism and this is may slow down the method if this method is heavily used.
And finally inheritence induce change in all your factory code... And what will you do if you have several methods to change in an exclusive fashion ? You will clutter your code of specific classes...
In fact, I think that this generally not a very good idea.
Method split : Another solution is sometime to split the method in several private and provide two or more public methods.
Context object : C++ and C lack of named parameter can be bypassed by adding a context parameter. I use this pattern very often, especially when I have to pass many data across level of a complex framework.
class Context{
public:
// usually not a good idea to add public data member but to my opinion this is an exception
bool setup:1;
bool foo:1;
bool bar:1;
...
Context() : setup(0), foo(0), bar(0) ... {}
};
...
Context ctx;
ctx.setup = true; ...
MyObj.foo(ctx);
Note:
That this is also useful to minimize access (or use) of static data or query to singleton object, TLS ...
Context object can contain a lot more of caching data related to an algorithm.
...
I let your imagination run free...
Anti patterns
I add here several anti pattern (to prevent some change of signature):
*NEVER DO THIS *
*NEVER DO THIS * use a static int/bool for argument passing (some people that do that, and this is a nightmare to remove this kind of stuff). Break at least multithreading...
*NEVER DO THIS * add a data member to pass parameter to method.
Unfortunately, I don't think there is a clear answer to the problem (and it's one I encounter quite frequently in my own code). With the boolean:
foo( x, true );
the call is hard to understand .
With an enum:
foo( x, UseHigherAccuracy );
it is easy to understand but you tend to end up with code like this:
foo( x, something == someval ? UseHigherAccuracy : UseLowerAccuracy );
which is hardly an improvement. And with multiple functions:
if ( something == someval ) {
AccurateFoo( x );
}
else {
InaccurateFoo( x );
}
you end up with a lot more code. But I guess this is the easiest to read, and what I'd tend to use, but I still don't completely like it :-(
One thing I definitely would NOT do however, is subclass. Inheritance should be the last tool you ever reach for.
The primary question is if the flag affects the behaviour of the class, or of that one function. Function-local changes should be parameters, not subclasses. Run-time inheritance should be one of the last tools reached for.
The general guideline I use is: if aSomeCondition changes the nature of the function in a major way, then I consider subclassing.
Subclassing is a relatively large effort compared to adding a flag that has only a minor effect.
Some examples:
if it's a flag that changes the direction in which a sorted collection is returned to the caller, that's a minor change in nature (flag).
if it's a one-shot flag (something that affects the current call rather than a persistent change to the object), it should probably not be a subclass (since going down that track is likely to lead to a massive number of classes).
if it's a enumeration that changes the underlying data structure of your class from array to linked list or balanced tree, that's a complex change (subclass).
Of course, that last one may be better handled by totally hiding the underlying data structure but I'm assuming here that you want to be able to select one of many, for reasons such as performance.
IMHO, aSomeCondition flag changes or depends on the state of current instance, therefore, under certain conditions this class should change its state and handle mentioned operation differently. In this case, I can suggest the usage of State Pattern. Hope it helps.
I would just change code:
void MyClass::foo(uint32_t aBar, bool aSomeCondition)
{
if (aSomeCondition)
{
// Do something with aBar...
}
}
to:
void MyClass::foo(uint32_t aBar)
{
if (this->aSomeCondition)
{
// Do something with aBar...
}
}
I always omit bool as function parameter and prefer to put into struct, even if I would have to call
myClass->enableCondition();
I'm looking for a solution for this problem in C or C++.
edit: To clarify. This is on a linux system. Linux-specific solutions are absolutely fine. Cross-plaform is not a concern.
I have a service that runs in its own thread. This service is a class with several methods, some of which need to run in the own service's thread rather than in the caller's thread.
Currently I'm using wrapper methods that create a structure with input and output parameters, insert the structure on a queue and either return (if a "command" is asynchronous) or wait for its execution (if a "command" is synchronous).
On the thread side, the service wakes, pops a structure from the queue, figures out what to execute and calls the appropriate method.
This implementation works but adding new methods is quite cumbersome: define wrapper, structure with parameters, and handler. I was wondering if there is a more straightforward means of coding this kind of model: a class method that executes on the class's own thread, instead of in the caller's thread.
edit - kind of conclusion:
It seems that there's no de facto way to implement what I asked that doesn't involve extra coding effort.
I'll stick with what I came up with, it ensures type safeness, minimizes locking, allows sync and async calls and the overhead it fairly modest.
On the other hand it requires a bit of extra coding and the dispatch mechanism may become bloated as the number of methods increases. Registering the dispatch methods on construction, or having the wrappers do that work seem to solve the issue, remove a bit of overhead and also remove some code.
My standard reference for this problem is here.
Implementing a Thread-Safe Queue using Condition Variables
As #John noted, this uses Boost.Thread.
I'd be careful about the synchronous case you described here. It's easy to get perf problems if the producer (the sending thread) waits for a result from the consumer (the service thread). What happens if you get 1000 async calls, filling up the queue with a backlog, followed by a sync call from each of your producer threads? Your system will 'play dead' until the queue backlog clears, freeing up those sync callers. Try to decouple them using async only, if you can.
There are several ways to achieve this, depending upon the complexity you want to accept. Complexity of the code is directly proportional to the flexibility desired. Here's a simple one (and quite well used):
Define a classes corresponding to each functionality your server exposes.
Each of these classes implements a function called execute and take a basic structure called input args and output args.
Inside the service register these methods classes at the time of initialization.
Once a request comes to the thread, it will have only two args, Input and Ouput, Which are the base classes for more specialized arguments, required by different method classes.
Then you write you service class as mere delegation which takes the incoming request and passes on to the respective method class based on ID or the name of the method (used during initial registration).
I hope it make sense, a very good example of this approach is in the XmlRpc++ (a c++ implementation of XmlRpc, you can get the source code from sourceforge).
To recap:
struct Input {
virtual ~Input () = 0;
};
struct Ouput {
virtual ~Output () = 0;
};
struct MethodInterface {
virtual int32_t execute (Input* __input, Output* __output) = 0;
};
// Write specialized method classes and taking specialized input, output classes
class MyService {
void registerMethod (std::string __method_name, MethodInterface* __method);
//external i/f
int32_t execute (std::string __method, Input* __input, Output* __output);
};
You will still be using the queue mechanism, but you won't need any wrappers.
IMHO, If you want to decouple method execution and thread context, you should use Active Object Pattern (AOP)
However, you need to use ACE Framework, which supports many OSes, e.g. Windows, Linux, VxWorks
You can find detailed information here
Also, AOP is a combination of Command, Proxy and Observer Patterns, if you know the details of them, you may implement your own AOP. Hope it helps
In addition to using Boost.Thread, I would look at boost::function and boost::bind. That said, it seems fair to have untyped (void) arguments passed to the target methods, and let those methods cast to the correct type (a typical idiom for languages like C#).
Hey now Rajivji, I think you have it upside-down. Complexity of code is inversely proportional to flexibility. The more complex your data structures and algorithms are, the more restrictions you are placing on acceptable inputs and behaviour.
To the OP: your description seems perfectly general and the only solution, although there are different encodings of it. The simplest may be to derive a class from:
struct Xqt { virtual void xqt(){} virtual ~Xqt(){} };
and then have a thread-safe queue of pointers to Xqt. The service thread then just pops the queue to px and calls px->xqt(), and then delete px. The most important derived class is this one:
struct Dxqt : Xqt {
xqt *delegate;
Dxqt(xqt *d) : delegate(d) {}
void xqt() { delegate->xqt(); }
};
because "all problems in Computer Science can be solved by one more level of indirection" and in particular this class doesn't delete the delegate. This is much better than using a flag, for example, to determine if the closure object should be deleted by the server thread.
When I use static variables in C++, I often end up wanting to initialize one variable passing another to its constructor. In other words, I want to create static instances that depend on each other.
Within a single .cpp or .h file this is not a problem: the instances will be created in the order they are declared. However, when you want to initialize a static instance with an instance in another compilation unit, the order seems impossible to specify. The result is that, depending on the weather, it can happen that the instance that depends on another is constructed, and only afterwards the other instance is constructed. The result is that the first instance is initialized incorrectly.
Does anyone know how to ensure that static objects are created in the correct order? I have searched a long time for a solution, trying all of them (including the Schwarz Counter solution), but I begin to doubt there is one that really works.
One possibility is the trick with the static function member:
Type& globalObject()
{
static Type theOneAndOnlyInstance;
return theOneAndOnlyInstance;
}
Indeed, this does work. Regrettably, you have to write globalObject().MemberFunction(), instead of globalObject.MemberFunction(), resulting in somewhat confusing and inelegant client code.
Update: Thank you for your reactions. Regrettably, it indeed seems like I have answered my own question. I guess I'll have to learn to live with it...
You have answered your own question. Static initialization order is undefined, and the most elegant way around it (while still doing static initialization i.e. not refactoring it away completely) is to wrap the initialization in a function.
Read the C++ FAQ items starting from https://isocpp.org/wiki/faq/ctors#static-init-order
Maybe you should reconsider whether you need so many global static variables. While they can sometimes be useful, often it's much simpler to refactor them to a smaller local scope, especially if you find that some static variables depend on others.
But you're right, there's no way to ensure a particular order of initialization, and so if your heart is set on it, keeping the initialization in a function, like you mentioned, is probably the simplest way.
Most compilers (linkers) actually do support a (non-portable) way of specifying the order. For example, with visual studio you can use the init_seg pragma to arrange the initialization into several different groups. AFAIK there is no way to guarantee order WITHIN each group. Since this is non-portable you may want to consider if you can fix your design to not require it, but the option is out there.
Indeed, this does work. Regrettably, you have to write globalObject().MemberFunction(), instead of globalObject.MemberFunction(), resulting in somewhat confusing and inelegant client code.
But the most important thing is that it works, and that it is failure proof, ie. it is not easy to bypass the correct usage.
Program correctness should be your first priority. Also, IMHO, the () above is purely stylistic - ie. completely unimportant.
Depending on your platform, be careful of too much dynamic initialization. There is a relatively small amount of clean up that can take place for dynamic initializers (see here). You can solve this problem using a global object container that contains members different global objects. You therefore have:
Globals & getGlobals ()
{
static Globals cache;
return cache;
}
There is only one call to ~Globals() in order to clean up for all global objects in your program. In order to access a global you still have something like:
getGlobals().configuration.memberFunction ();
If you really wanted you could wrap this in a macro to save a tiny bit of typing using a macro:
#define GLOBAL(X) getGlobals().#X
GLOBAL(object).memberFunction ();
Although, this is just syntactic sugar on your initial solution.
dispite the age of this thread, I would like to propose the solution I've found.
As many have pointed out before of me, C++ doesn't provide any mechanism for static initialization ordering. What I propose is to encapsule each static member inside a static method of the class that in turn initialize the member and provide an access in an object-oriented fashion.
Let me give you an example, supposing we want to define the class named "Math" which, among the other members, contains "PI":
class Math {
public:
static const float Pi() {
static const float s_PI = 3.14f;
return s_PI;
}
}
s_PI will be initialized the first time Pi() method is invoked (in GCC). Be aware: the local objects with static storage have an implementation dependent lifecyle, for further detail check 6.7.4 in 2.
Static keyword, C++ Standard
Wrapping the static in a method will fix the order problem, but it isn't thread safe as others have pointed out but you can do this to also make it thread if that is a concern.
// File scope static pointer is thread safe and is initialized first.
static Type * theOneAndOnlyInstance = 0;
Type& globalObject()
{
if(theOneAndOnlyInstance == 0)
{
// Put mutex lock here for thread safety
theOneAndOnlyInstance = new Type();
}
return *theOneAndOnlyInstance;
}