I have a requirement where a (const) variable should be available throughout an entire cpp which consists of several classes. I have decided to use a namespace to solve the problem, but unsure about the following:
Do I need to define this variable as static?
Is it true that I can avoid making the variable static only if I go with an unnamed namespace?
You don't need to define the variable as static, or in an anonymous namespace. However, if you're not using this object outside of the file it's defined in, it's a good idea, to reduce namespace pollution and speed up links (by reducing how many symbols need to be considered by the linker).
If you declare a variable in an anonymous namespace, it will be effectively static. There's no need to actually make it static as well (although you can if you like). The advantage of anonymous namespaces is you can also define types (classes, structs, enums, typedefs) as well as static variables and functions.
Related
In other words: is it fair to say that a namespace has member variables and functions just like a class does?
Not really. A namespace is used to limit the scope of variables, functions and classes. Meaning that by putting them into a namespace, you can have the same named function appear in your code and distinguish them through the use of a namespace.
A class is meant to represent a self-contained set of data (the variables) with a set of operations (the functions) to be performed on that data.
While you can have classes that only have static members and only publicly accessible variables, you start to lose the advantages (and the point) of classes.
No.
The terms member, member variables, member functions and methods are commonly used for class (struct) variables and functions only.
Functions in a namespace are just functions in the scope of the namespace, like variables are simply variables in the scope of the namespace.
Besides the differences between namespaces and classes regarding access and other stuff, the most import difference here is that you can create multiple objects of a class which then each hold their data in their member variables, and the methods == member functions work on this data.
I have a TU whose global functions won't be used by any other TUs. I read that declaring them as static gives them internal linkage, and this is good from an optimization standpoint. But I want to know what are the correct situations in which I should use them. Should I always give global functions/variables internal linkage when I know they won't be used anywhere else in the program?
Put them in an unnamed namespace instead.
This is the idiomatic solution in C++ for functions that will be used only in the current TU.
In my current job, I am seeing variables declared in the unnamed namespace in the cpp file and used only by that class as if they are member variables.
I see it as an interesting way of keeping only interface information in .h and implmentation in .cpp and is less work than the usual pimpl idiom.
I see people using pimpl all the time but never this approach, is there any problem with it?
Variables declared in the unnamed namespace of a .cpp file are file scoped; this means that there is only one instance per execution of the program.
You can see this for yourself by creating two instances of your object and observing that they interfere with each other's variables in the unnamed namespace.
Show some example code please. AFAIK, you cannot declare member variables in the unnamed namespace (unless the class itself is declared in the unnamed namespace).
The unnamed namespace was introduced to replace the common practice of declaring variables as static that are used in just one compilation unit.
I've recently seen a bit on SO about the static keyword before a function and I'm wondering how to use it properly.
1) When should I write the keyword static before a non-member function?
2) Is it dangerous to define a static non-member function in the header? Why (not)?
(Side Question)
3) Is it possible to define a class in the header file in a certain way, so that it would only be available in the translation unit where you use it first?
(The reason that I'm asking this is because I'm learning STL and it might be a good solution for my predicates etc (possibly functors), since I don't like to define functions other than member-functions in the cpp file)
(Also, I think it is related in a way to the original question because according to my current reasoning, it would do the same thing as static before a function does)
EDIT
Another question that came up while seeing some answers:
4) Many people tell me I have to declare the static function in the header, and define it in the source file. But the static function is unique to the translation unit. How can the linker know which translation unit it is unique to, since header files do not directly relate to a source file (only when you include them)?
static, as I think you're using it, is a means of symbol hiding. Functions declared static are not given global visibility (a Unix-like nm will show these as 't' rather than 'T'). These functions cannot be called from other translation units.
For C++, static in this sense has been replaced, more or less, by the anonymous namespace, e.g.,
static int x = 0;
is pretty equivalent to
namespace {
int x = 0;
}
Note that the anonymous namespace is unique for every compilation unit.
Unlike static, the anonymous namespace also works for classes. You can say something like
namespace {
class Foo{};
}
and reuse that class name for unrelated classes in other translation units. I think this goes to your point 3.
The compiler actually gives each of the symbols you define this way a unique name (I think it includes the compilation time). These symbols are never available to another translation unit and will never collide with a symbol from another translation unit.
Note that all non-member functions declared to be inline are also by default static. That's the most common (and implicit) use of static. As to point 2, defining a static but not inline function in a header is a pretty corner case: it's not dangerous per se but it's so rarely useful it might be confusing. Such a function might or might not be emitted in every translation unit. A compiler might generate warnings if you never actually call the function in some TUs. And if that static function has within it a static variable, you get a separate variable per translation unit even with one definition in a single .h which might be confusing. There just aren't many (non-inline) use cases.
As to point 4, I suspect those people are conflating the static member function meaning of static with that of the linkage meaning of static. Which is as good a reason as any for using the anonymous namespace for the latter.
The keyword "static" is overloaded to mean several different things:
It can control visibility (both C and C++)
It can persist a variable between subroutine invocations (both C and C++)
... and ...
It can make a method or member apply to an entire class (rather than just a class instance: C++ only)
Short answer: it's best not to use ANY language facility unless
a) you're pretty sure you need it
b) you're pretty sure you know what you're doing (i.e. you know WHY you need it)
There's absolutely nothing wrong with declaring static variables or standalone functions in a .cpp file. Declaring a static variable or standalone function in a header is probably unwise. And, if you actually need "static" for a class function or class member, then a header is arguably the BEST place to define it.
Here's a good link:
http://www.cprogramming.com/tutorial/statickeyword.html
'Hope that helps
You should define non-member functions as static when they are only to be visible inside the code file they were declared in.
This same question was asked on cplusplus.com
I'm curious about the benefits/detriments of different constant declaration and definition options in C++. For the longest time, I've just been declaring them at the top of the header file before the class definition:
//.h
const int MyConst = 10;
const string MyStrConst = "String";
class MyClass {
...
};
While this pollutes the global namespace (which I know is a bad thing, but have never found a laundry list of reasons why it is bad), the constants will still be scoped to individual translation units, so files that don't include this header won't have access to these constants. But you can get name collisions if other classes define a constant of the same name, which is arguably not a bad thing as it may be a good indication of an area that could be refactored.
Recently, I decided that it would be better to declare class specific constants inside of the class definition itself:
//.h
class MyClass {
public:
static const int MyConst = 10;
...
private:
static const string MyStrConst;
...
};
//.cpp
const string MyClass::MyStrConst = "String";
The visibility of the constant would be adjusted depending on whether the constant is used only internally to the class or is needed for other objects that use the class. This is what I'm thinking is the best option right now, mainly because you can keep internal class constants private to the class and any other classes using the public constants would have a more detailed reference to the source of the constant (e.g. MyClass::MyConst). It also won't pollute the global namespace. Though it does have the detriment of requiring non-integral initialization in the cpp file.
I've also considered moving the constants into their own header file and wrapping them in a namespace in case some other class needs the constants, but not the whole class definition.
Just looking for opinions and possibly other options I hadn't considered yet.
Your claim that declaring a non-integral constant as a static class member "have the detriment of requiring non-integral initialization in the cpp file" is not exactly solid, so to say. It does require a definition in cpp file, but it is not a "detriment", it is a matter of your intent. Namespace-level const object in C++ has internal linkage by default, meaning that in your original variant the declaration
const string MyStrConst = "String";
is equivalent to
static const string MyStrConst = "String";
i.e. it will define an independent MyStrConst object in every translation unit into which this header file is included. Are you aware of this? Was this your intent or not?
In any case, if you don't specifically need a separate object in every translation unit, the declaration of MyStrConst constant in your original example is not a good practice. Normally, you'd only put a non-defining declaration in the header file
extern const string MyStrConst;
and provide a definition in the cpp file
const string MyStrConst = "String";
thus making sure that the entire program uses the same constant object. In other words, when it comes to non-integral constants, a normal practice is to define them in cpp file. So, regardless of how you declare it (in the class or out) you will normally always have to deal with the "detriment" of having to define it in cpp file. Of course, as I said above, with namespace constants you can get away with what you have in your first variant, but that would be just an example of "lazy coding".
Anyway, I don't think there is a reason to over-complicate the issue: if the constant has an obvious "attachment" to the class, it should be declared as a class member.
P.S. Access specifiers (public, protected, private) don't control visibility of the name. They only control its accessibility. The name remains visible in any case.
Pollution of the global namespace is bad because someone (e.g. the writer of a library you use) might want to use the name MyConst for another purpose. This can lead to severe problems (libraries that can't be used together etc.)
Your second solution is clearly the best if the constants are linked to a single class. If that isn't so easy (think of physical or math constants without ties to a class in your program), the namespace solution is better than that. BTW: if you must be compatible to older C++ compilers, remember some of them can't use integral initialization in a header file - you must initialize in the C++ file or use the old enum trick in this case.
I think there are no better options for constants - at least can't think of one at the moment...
Polluting the global namespace should be self-evidently bad. If I include a header file, I don't want to encounter or debug name collisions with constants declared in that header. These types of errors are really frustrating and sometimes hard to diagnose. For example, I once had to link against a project that had this defined in a header:
#define read _read
If your constants are namespace pollution, this is namespace nuclear waste. The manifestation of this was a a series of very odd compiler errors complaining about missing the _read function, but only when linking against that library. We eventually renamed the read functions to something else, which isn't difficult but should be unnecessary.
Your second solution is very reasonable as it puts the variable into scope. There's no reason that this has to be associated with a class, and if I need to share constants among classes I'll declare constants in their own namespace and header file. This isn't great for compile-time, but sometimes it's necessary.
I've also seen people put constants into their own class, which can be implemented as a singleton. This to me seems work without reward, the language provides you some facilities for declaring constants.
You can declare them as globals in the c++ file, as long as they are not referenced in the header. Then they are private to that class and won't pollute the global namespace.
Personally I use your second approach; I've used it for years, and it works well for me.
From a visibility point I would tend to make the private constants file level statics as nobody outside the implementation file needs to know they exist; this helps prevent chain reaction recompiles if you need to change their names or add new ones as their name scope is the same as their usage scope...
If only one class is going to use these constants, declare them as static const inside the class body. If a bunch of related classes are going to use the constants, declare them either inside a class/struct that only holds the constants and utility methods or inside a dedicated namespace. For example,
namespace MyAppAudioConstants
{
//declare constants here
}
If they are constants used by the whole application (or substantial chunks of it), declare them inside a namespace in a header that is (either implicitly or explicitly) included everywhere.
namespace MyAppGlobalConstants
{
//declare constants here
}
don't pollute global namespace, pollute local.
namespace Space
{
const int Pint;
class Class {};
};
But practically...
class Class
{
static int Bar() {return 357;}
};