The static keyword and its various uses in C++
In order to clarify the question, I would rather categorize the usage of 'static' keyword in three different forms:
(A). variables
(B). functions
(C). member variables/functions of classes
the explanation follows below for each of the sub headings:
(A) 'static' keyword for variables
This one can be little tricky however if explained and understood properly, it's pretty straightforward.
To explain this, first it is really useful to know about the scope, duration and linkage of variables, without which things are always difficult to see through the murky concept of staic keyword
1. Scope : Determines where in the file, the variable is accessible. It can be of two types: (i) Local or Block Scope. (ii) Global Scope
2. Duration : Determines when a variable is created and destroyed. Again it's of two types: (i) Automatic Storage Duration (for variables having Local or Block scope). (ii) Static Storage Duration (for variables having Global Scope or local variables (in a function or a in a code block) with static specifier).
3. Linkage: Determines whether a variable can be accessed (or linked ) in another file. Again ( and luckily) it is of two types: (i) Internal Linkage (for variables having Block Scope and Global Scope/File Scope/Global Namespace scope) (ii) External Linkage (for variables having only for Global Scope/File Scope/Global Namespace Scope)
Let's refer an example below for better understanding of plain global and local variables (no local variables with static storage duration) :
//main file
#include <iostream>
int global_var1; //has global scope
const global_var2(1.618); //has global scope
int main()
{
//these variables are local to the block main.
//they have automatic duration, i.e, they are created when the main() is
// executed and destroyed, when main goes out of scope
int local_var1(23);
const double local_var2(3.14);
{
/* this is yet another block, all variables declared within this block are
have local scope limited within this block. */
// all variables declared within this block too have automatic duration, i.e,
/*they are created at the point of definition within this block,
and destroyed as soon as this block ends */
char block_char1;
int local_var1(32) //NOTE: this has been re-declared within the block,
//it shadows the local_var1 declared outside
std::cout << local_var1 <<"\n"; //prints 32
}//end of block
//local_var1 declared inside goes out of scope
std::cout << local_var1 << "\n"; //prints 23
global_var1 = 29; //global_var1 has been declared outside main (global scope)
std::cout << global_var1 << "\n"; //prints 29
std::cout << global_var2 << "\n"; //prints 1.618
return 0;
} //local_var1, local_var2 go out of scope as main ends
//global_var1, global_var2 go out of scope as the program terminates
//(in this case program ends with end of main, so both local and global
//variable go out of scope together
Now comes the concept of Linkage. When a global variable defined in one file is intended to be used in another file, the linkage of the variable plays an important role.
The Linkage of global variables is specified by the keywords: (i) static , and, (ii) extern
( Now you get the explanation )
static keyword can be applied to variables with local and global scope, and in both the cases, they mean different things. I will first explain the usage of 'static' keyword in variables with global scope ( where I also clarify the usage of keyword 'extern') and later the for those with local scope.
1. Static Keyword for variables with global scope
Global variables have static duration, meaning they don't go out of scope when a particular block of code (for e.g main() ) in which it is used ends . Depending upon the linkage, they can be either accessed only within the same file where they are declared (for static global variable), or outside the file even outside the file in which they are declared (extern type global variables)
In the case of a global variable having extern specifier, and if this variable is being accessed outside the file in which it has been initialized, it has to be forward declared in the file where it's being used, just like a function has to be forward declared if it's definition is in a file different from where it's being used.
In contrast, if the global variable has static keyword, it cannot be used in a file outside of which it has been declared.
(see example below for clarification)
eg:
//main2.cpp
static int global_var3 = 23; /*static global variable, cannot be
accessed in anyother file */
extern double global_var4 = 71; /*can be accessed outside this file linked to main2.cpp */
int main() { return 0; }
main3.cpp
//main3.cpp
#include <iostream>
int main()
{
extern int gloabl_var4; /*this variable refers to the gloabal_var4
defined in the main2.cpp file */
std::cout << global_var4 << "\n"; //prints 71;
return 0;
}
now any variable in c++ can be either a const or a non-const and for each 'const-ness' we get two case of default c++ linkage, in case none is specified:
(i) If a global variable is non-const, its linkage is extern by default, i.e, the non-const global variable can be accessed in another .cpp file by forward declaration using the extern keyword (in other words, non const global variables have external linkage ( with static duration of course)). Also usage of extern keyword in the original file where it has been defined is redundant. In this case to make a non-const global variable inaccessible to external file, use the specifier 'static' before the type of the variable.
(ii) If a global variable is const, its linkage is static by default, i.e a const global variable cannot be accessed in a file other than where it is defined, (in other words, const global variables have internal linkage (with static duration of course)). Also usage of static keyword to prevent a const global variable from being accessed in another file is redundant. Here, to make a const global variable have an external linkage, use the specifier 'extern' before the type of the variable
Here's a summary for global scope variables with various linkages
//globalVariables1.cpp
// defining uninitialized vairbles
int globalVar1; // uninitialized global variable with external linkage
static int globalVar2; // uninitialized global variable with internal linkage
const int globalVar3; // error, since const variables must be initialized upon declaration
const int globalVar4 = 23; //correct, but with static linkage (cannot be accessed outside the file where it has been declared*/
extern const double globalVar5 = 1.57; //this const variable ca be accessed outside the file where it has been declared
Next we investigate how the above global variables behave when accessed in a different file.
//using_globalVariables1.cpp (eg for the usage of global variables above)
// Forward declaration via extern keyword:
extern int globalVar1; // correct since globalVar1 is not a const or static
extern int globalVar2; //incorrect since globalVar2 has internal linkage
extern const int globalVar4; /* incorrect since globalVar4 has no extern
specifier, limited to internal linkage by
default (static specifier for const variables) */
extern const double globalVar5; /*correct since in the previous file, it
has extern specifier, no need to initialize the
const variable here, since it has already been
legitimately defined perviously */
- Static Keyword for variables with Local Scope
Updates (August 2019) on static keyword for variables in local scope
This further can be subdivided in two categories :
(i) static keyword for variables within a function block, and (ii) static keyword for variables within a unnamed local block.
(i) static keyword for variables within a function block.
Earlier, I mentioned that variables with local scope have automatic duration, i.e they come to exist when the block is entered ( be it a normal block, be it a function block) and cease to exist when the block ends, long story short, variables with local scope have automatic duration and automatic duration variables (and objects) have no linkage meaning they are not visible outside the code block.
If static specifier is applied to a local variable within a function block, it changes the duration of the variable from automatic to static and its life time is the entire duration of the program which means it has a fixed memory location and its value is initialized only once prior to program start up as mentioned in cpp reference(initialization should not be confused with assignment)
lets take a look at an example.
//localVarDemo1.cpp
int localNextID()
{
int tempID = 1; //tempID created here
return tempID++; //copy of tempID returned and tempID incremented to 2
} //tempID destroyed here, hence value of tempID lost
int newNextID()
{
static int newID = 0;//newID has static duration, with internal linkage
return newID++; //copy of newID returned and newID incremented by 1
} //newID doesn't get destroyed here :-)
int main()
{
int employeeID1 = localNextID(); //employeeID1 = 1
int employeeID2 = localNextID(); // employeeID2 = 1 again (not desired)
int employeeID3 = newNextID(); //employeeID3 = 0;
int employeeID4 = newNextID(); //employeeID4 = 1;
int employeeID5 = newNextID(); //employeeID5 = 2;
return 0;
}
Looking at the above criterion for static local variables and static global variables, one might be tempted to ask, what the difference between them could be. While global variables are accessible at any point in within the code (in same as well as different translation unit depending upon the const-ness and extern-ness), a static variable defined within a function block is not directly accessible. The variable has to be returned by the function value or reference. Lets demonstrate this by an example:
//localVarDemo2.cpp
//static storage duration with global scope
//note this variable can be accessed from outside the file
//in a different compilation unit by using `extern` specifier
//which might not be desirable for certain use case.
static int globalId = 0;
int newNextID()
{
static int newID = 0;//newID has static duration, with internal linkage
return newID++; //copy of newID returned and newID incremented by 1
} //newID doesn't get destroyed here
int main()
{
//since globalId is accessible we use it directly
const int globalEmployee1Id = globalId++; //globalEmployeeId1 = 0;
const int globalEmployee2Id = globalId++; //globalEmployeeId1 = 1;
//const int employeeID1 = newID++; //this will lead to compilation error since newID++ is not accessible direcly.
int employeeID2 = newNextID(); //employeeID3 = 0;
int employeeID2 = newNextID(); //employeeID3 = 1;
return 0;
}
More explaination about choice of static global and static local variable could be found on this stackoverflow thread
(ii) static keyword for variables within a unnamed local block.
static variables within a local block (not a function block) cannot be accessed outside the block once the local block goes out of scope. No caveats to this rule.
//localVarDemo3.cpp
int main()
{
{
const static int static_local_scoped_variable {99};
}//static_local_scoped_variable goes out of scope
//the line below causes compilation error
//do_something is an arbitrary function
do_something(static_local_scoped_variable);
return 0;
}
C++11 introduced the keyword constexpr which guarantees the evaluation of an expression at compile time and allows compiler to optimize the code. Now if the value of a static const variable within a scope is known at compile time, the code is optimized in a manner similar to the one with constexpr. Here's a small example
I recommend readers also to look up the difference between constexprand static const for variables in this stackoverflow thread.
this concludes my explanation for the static keyword applied to variables.
B. 'static' keyword used for functions
in terms of functions, the static keyword has a straightforward meaning. Here, it refers to linkage of the function Normally all functions declared within a cpp file have external linkage by default, i.e a function defined in one file can be used in another cpp file by forward declaration.
using a static keyword before the function declaration limits its linkage to internal , i.e a static function cannot be used within a file outside of its definition.
C. Staitc Keyword used for member variables and functions of classes
1. 'static' keyword for member variables of classes
I start directly with an example here
#include <iostream>
class DesignNumber
{
private:
static int m_designNum; //design number
int m_iteration; // number of iterations performed for the design
public:
DesignNumber() { } //default constructor
int getItrNum() //get the iteration number of design
{
m_iteration = m_designNum++;
return m_iteration;
}
static int m_anyNumber; //public static variable
};
int DesignNumber::m_designNum = 0; // starting with design id = 0
// note : no need of static keyword here
//causes compiler error if static keyword used
int DesignNumber::m_anyNumber = 99; /* initialization of inclass public
static member */
enter code here
int main()
{
DesignNumber firstDesign, secondDesign, thirdDesign;
std::cout << firstDesign.getItrNum() << "\n"; //prints 0
std::cout << secondDesign.getItrNum() << "\n"; //prints 1
std::cout << thirdDesign.getItrNum() << "\n"; //prints 2
std::cout << DesignNumber::m_anyNumber++ << "\n"; /* no object
associated with m_anyNumber */
std::cout << DesignNumber::m_anyNumber++ << "\n"; //prints 100
std::cout << DesignNumber::m_anyNumber++ << "\n"; //prints 101
return 0;
}
In this example, the static variable m_designNum retains its value and this single private member variable (because it's static) is shared b/w all the variables of the object type DesignNumber
Also like other member variables, static member variables of a class are not associated with any class object, which is demonstrated by the printing of anyNumber in the main function
const vs non-const static member variables in class
(i) non-const class static member variables In the previous example the static members (both public and private) were non constants. ISO standard forbids non-const static members to be initialized in the class. Hence as in previous example, they must be initalized after the class definition, with the caveat that the static keyword needs to be omitted
(ii) const-static member variables of class this is straightforward and goes with the convention of other const member variable initialization, i.e the const static member variables of a class can be initialized at the point of declaration and they can be initialized at the end of the class declaration with one caveat that the keyword const needs to be added to the static member when being initialized after the class definition.
I would however, recommend to initialize the const static member variables at the point of declaration. This goes with the standard C++ convention and makes the code look cleaner
for more examples on static member variables in a class look up the following link from learncpp.com http://www.learncpp.com/cpp-tutorial/811-static-member-variables/
2. 'static' keyword for member function of classes
Just like member variables of classes can ,be static, so can member functions of classes. Normal member functions of classes are always associated with a object of the class type. In contrast, static member functions of a class are not associated with any object of the class, i.e they have no *this pointer.
Secondly since the static member functions of the class have no *this pointer, they can be called using the class name and scope resolution operator in the main function (ClassName::functionName(); )
Thirdly static member functions of a class can only access static member variables of a class, since non-static member variables of a class must belong to a class object.
for more examples on static member functions in a class look up the following link from learncpp.com
http://www.learncpp.com/cpp-tutorial/812-static-member-functions/
Update April 2021: static keyword and lambda expressions
Lambda expressions follow normal name lookup rules and hence the scope (local vs global) and storage class (static vs automatic) affect how the variables are available to the lambda expressions
- non-static global variables is available to lambda expression within a local scope.
//global member
int i=10;
int main(){
[]{std::cout << i;}();
//prints 10
}
- Non-Static local variable is not available to lambda expression that appears within the same or a different scope. In this case, as we're mostly used to, the variables have to be captured by value or by reference
int main(){
int i{11};
[]{std::cout << i;}(); //compiler error
[i]{std::cout << i;}(); //capture by value; correct
//or
[&i]{std::cout << i;}(); //capture by reference; correct
}
- Static variables locally scoped static variables can be captured by lambda expressions within the same or lower/child scope
int main(){
static int i{12};
[]{std::cout << i;}(); //prints 12
{
[]{std::cout << i;}();//also prints 12
}
}
However static variables in unnamed scope cannot be accessed outside the scope as explained previously
It's actually quite simple. If you declare a variable as static in the scope of a function, its value is preserved between successive calls to that function. So:
int myFun()
{
static int i=5;
i++;
return i;
}
int main()
{
printf("%d", myFun());
printf("%d", myFun());
printf("%d", myFun());
}
will show 678 instead of 666, because it remembers the incremented value.
As for the static members, they preserve their value across instances of the class. So the following code:
struct A
{
static int a;
};
int main()
{
A first;
A second;
first.a = 3;
second.a = 4;
printf("%d", first.a);
}
will print 4, because first.a and second.a are essentially the same variable. As for the initialization, see this question.
Variables:
static variables exist for the "lifetime" of the translation unit that it's defined in, and:
- If it's in a namespace scope (i.e. outside of functions and classes), then it can't be accessed from any other translation unit. This is known as "internal linkage" or "static storage duration". (Don't do this in headers except for
constexpr. Anything else, and you end up with a separate variable in each translation unit, which is crazy confusing) - If it's a variable in a function, it can't be accessed from outside of the function, just like any other local variable. (this is the local they mentioned)
- class members have no restricted scope due to
static, but can be addressed from the class as well as an instance (likestd::string::npos). [Note: you can declare static members in a class, but they should usually still be defined in a translation unit (cpp file), and as such, there's only one per class]
locations as code:
static std::string namespaceScope = "Hello";
void foo() {
static std::string functionScope= "World";
}
struct A {
static std::string classScope = "!";
};
Before any function in a translation unit is executed (possibly after main began execution), the variables with static storage duration (namespace scope) in that translation unit will be "constant initialized" (to constexpr where possible, or zero otherwise), and then non-locals are "dynamically initialized" properly in the order they are defined in the translation unit (for things like std::string="HI"; that aren't constexpr). Finally, function-local statics will be initialized the first time execution "reaches" the line where they are declared. All static variables all destroyed in the reverse order of initialization.
The easiest way to get all this right is to make all static variables that are not constexpr initialized into function static locals, which makes sure all of your statics/globals are initialized properly when you try to use them no matter what, thus preventing the static initialization order fiasco.
T& get_global() {
static T global = initial_value();
return global;
}
Be careful, because when the spec says namespace-scope variables have "static storage duration" by default, they mean the "lifetime of the translation unit" bit, but that does not mean it can't be accessed outside of the file.
Functions
Significantly more straightforward, static is often used as a class member function, and only very rarely used for a free-standing function.
A static member function differs from a regular member function in that it can be called without an instance of a class, and since it has no instance, it cannot access non-static members of the class. Static variables are useful when you want to have a function for a class that definitely absolutely does not refer to any instance members, or for managing static member variables.
struct A {
A() {++A_count;}
A(const A&) {++A_count;}
A(A&&) {++A_count;}
~A() {--A_count;}
static int get_count() {return A_count;}
private:
static int A_count;
}
int main() {
A var;
int c0 = var.get_count(); //some compilers give a warning, but it's ok.
int c1 = A::get_count(); //normal way
}
A static free-function means that the function will not be referred to by any other translation unit, and thus the linker can ignore it entirely. This has a small number of purposes:
- Can be used in a cpp file to guarantee that the function is never used from any other file.
- Can be put in a header and every file will have it's own copy of the function. Not useful, since inline does pretty much the same thing.
- Speeds up link time by reducing work
- Can put a function with the same name in each translation unit, and they can all do different things. For instance, you could put a
static void log(const char*) {}in each cpp file, and they could each all log in a different way.
Static storage duration means that the variable resides in the same place in memory through the lifetime of the program.
Linkage is orthogonal to this.
I think this is the most important distinction you can make. Understand this and the rest, as well as remembering it, should come easy (not addressing @Tony directly, but whoever might read this in the future).
The keyword static can be used to denote internal linkage and static storage, but in essence these are different.
What does it mean with local variable? Is that a function local variable?
Yes. Regardless of when the variable is initialized (on first call to the function and when execution path reaches the declaration point), it will reside in the same place in memory for the life of the program. In this case, static gives it static storage.
Now what about the case with static and file scope? Are all global variables considered to have static storage duration by default?
Yes, all globals have by definition static storage duration (now that we cleared up what that means). But namespace scoped variables aren't declared with static, because that would give them internal linkage, so a variable per translation unit.
How does static relate to the linkage of a variable?
It gives namespace-scoped variables internal linkage. It gives members and local variables static storage duration.
Let's expand on all this:
//
static int x; //internal linkage
//non-static storage - each translation unit will have its own copy of x
//NOT A TRUE GLOBAL!
int y; //static storage duration (can be used with extern)
//actual global
//external linkage
struct X
{
static int x; //static storage duration - shared between class instances
};
void foo()
{
static int x; //static storage duration - shared between calls
}
This whole static keyword is downright confusing
Definitely, unless you're familiar with it. :) Trying to avoid adding new keywords to the language, the committee re-used this one, IMO, to this effect - confusion. It's used to signify different things (might I say, probably opposing things).