What is the reasoning to why static const members cannot exist in local classes? It seems like a rather silly restriction.
Example:
void foo() {
struct bar {
int baz() { return 0; } // allowed
static const int qux = 0; // not allowed?!?
};
}
struct non_local_bar {
int baz() { return 0; } // allowed
static const int qux = 0; // allowed
};
Quote from standard (9.8.4):
A local class shall not have static data members.
From the standard section 9.4.2:
If a static data member is of const integral or const enumeration
type, its declaration in the class definition can specify a
constant-initializer which shall be an integral constant expression.
In that case, the member can appear in integral constant expressions
within its scope. The member shall still be defined in a namespace
scope if it is used in the program and the namespace scope definition
shall not contain an initializer.
Basically, local classes have no linkage, and static data members require a linkage.
Since there's no way to define a static data member of a local class in namespace scope (a declaration with an initializer is not a definition), they are not allowed, whether they are of const integral type or not. On the surface it may seem like the compiler should just be able to inline the value, but then what happens if you try to access a pointer to the member? With namespace scoped classes you'd just get a linker error, but local classes have no linkage.
I guess in theory they could just allow you to use static const integral types in local classes as long as they are only used in integral constant expressions, but it would probably just put too much of a burden on the standards body and compiler vendors to differentiate for very little practical value; local static variables are accessible from local classes, so using a local static const should be just as good.
I dont think there is a.reason. Normal static datamembers are disallowed because there is no way to define them after being declared.
Also dont forget you can create a local const variable outside the.class that you can use inside the class as long as you only read its value (that is, as long as you dont take.its.address).
Static members of a class need to be defined in global scope, e.g.
abc.h
class myClass {
static int number;
};
abc.cpp
int myClass::number = 314;
Now, since the scope inside void abc(int x) is not global, there is no scope to define the static member.
As things progress, we now have C++11 and with that you can define integral constants variable members in your classes.
class test
{
public:
const int FOO = 123;
[...snip...]
};
That works when you compile with C++11. Notice that the static keyword is not used. When compiling with optimizations turned on, those variables will likely all get optimized out. In debug, though, they appear in your structure as regular variable members.
Note, however, that the size of the class/structure will still include that variable. So here it is likely 4 bytes for the variable FOO.
However, in most cases, classes defined in a function will completely be optimized out so this is a great way of doing things (a good 50% of my classes have such variable members!)
Related
I'm new to C++, and I just can't understand that why I can't assign a value to a non-const static member inside a class (like we do in java static int x = 12; ) even thought I can
declare a non-const static member (static int x;)
declare a static const member (static const x;)
assign a static const member (static const int x = 12;)
note: my class and my main() function are in the same file
In general
A static variable inside a class, just like everything else, is just a declaration by default. C++ then requires you to provide a single definition per entity that requires one, that's the One Definition Rule. The definition is where the initializer (which is not an assignment, but a construction) goes, since there should be only one as well. It is typically placed in a .cpp file so that it can't be accidentally duplicated by #includes.
The constant case
When a static member is a constant integer or enumeration, and is initialized with a compile-time expression, the initializer is allowed to be placed on the declaration, and the definition skipped. This is a result of a combination of old rules and isn't really interesting today IMO.
Proper inline initialization
Since C++17 introduced inline variables, you can use them as static members. The rules are roughly the same as inline functions, and are pretty sensible: you can provide multiple definition of an inline entity, you are responsible for ensuring that they are all strictly identical, and the implementation is responsible for collapsing them into a single definition with its initializer. Thus, what you're looking for is written:
struct Foo {
static inline int bar = 42;
// ^^^^^^
};
You have to initialise the static member outside the class definition like in this example:
class Box {
public:
static int x;
};
// Initialize static member of class Box outside the class definition
int Box::x = 12;
int main(void) {
...
}
Why must static data member initialization be outside the class?
class X
{
public:
int normalValue = 5; //NSDMI
static int i;
};
int X::i = 0;
Why is the static data member (here "i") only a declaration, not a definition?
It's important to distinguish the initializer which says what its initial value is, and the definition. This modified code is valid, with the initializer in the class definition:
class X
{
public:
int normalValue = 5;
static const int i = 0; // declaration, with initializer
};
const int X::i; // definition
i.e. What must be outside the class is a definition, not the initialization.
That's because a variable must have an address in memory (unless it's only used in limited situations, such as in compile-time constant expressions.)
A non-static member variable exists inside the object it is a member of, so its address depends on the address of the object that contains it. Every time you create a new X you also create a new X::normalValue variable. The non-static data member's lifetime begins with the class' constructor. NSDMI syntax doesn't have anything to do with the variable's address in memory, it just allows you to provide an initial value in one place, instead of repeating it in every constructor with an explicit constructor initializer list.
On the other hand, a static member variable is not contained within an instance of the class, it exists independently of any single instance and exists from the start of the program, at a fixed address. In order for a static member variable (or any other global object) to get a unique address the linker must see exactly one definition of the static variable, in exactly one object file, and assign it an address.
Because a static variable needs exactly one definition in exactly one object file, it doesn't make sense to allow that definition to be provided in the class, since class definitions typically exist in header files and are included in multiple object files. So although you can provide an initializer in the class, you still need to define the static data member somewhere.
You can also look at it like declaring an extern variable:
namespace X {
extern int i;
}
This declares the variable, but there must be a definition somewhere in the program:
int X::i = 0;
You need to supply a separate definition for a static data member (if its odr-used, as defined in C++11) simply because that definition shall reside somewhere - in one and only one translation unit. Static class data members are basically global objects (global variables) declared in class scope. The compiler wants you to choose a specific translation unit that will hold the actual "body" of each global object. It is you who has to decide which translation unit to place the actual object to.
"static" class member is like a globally allocated variable (it is not related to the single class instance), so it must reside in some object file (and to be declared in the ".cpp" file) as a symbol just like any global variable.
Simple class member (non-static) resides in the memory block allocated for the class instance.
The simple reason is because classes are usually declared in header files, which often are included in multiple cpp files. Static data members have external linkage and must be declared in exactly one translation unit which makes them unfit for being defined inside a class.
As juanchopanza points out the following is allowed:
struct A
{
const static int i = 1;
};
However, this is only a declaration not a definition. You still need to define it if you are going to use i's address somewhere.
For example:
f(int);
g(int&);
X<A::i> x; // Okay without definition for template arguments
char a[A::i]; // Okay without definition, just using value as constant expression
&A::i; // Need a definition because I'm taking the address
f(A::i); // Okay without definition for pass by value
g(A::i); // Need a definition with pass by reference
Bear in mind that is is possible to initialize the static data member at the point of declaration if it is of const integral type of const enumeration type:
From the C++03 standard, §9.4.2
If a static data member is of const integral or const enumeration type, its declaration in the class
definition can specify a constant-initializer which shall be an integral constant expression (5.19)
struct Foo {
static const int j = 42; // OK
};
When the compiler generate binary code from a unit (extreme simplification: a cpp file and all its included headers) it will emit a symbol for the static variable and eventually initialization code for that variable.
It is okay for a static variable symbol to be declared in multiple units, but it is not okay for it to be initialized multiple times.
So you must make sure that the initialization code is only emitted for a single unit.
This mean that the static variable must be defined in exactly one unit.
Static Data Member
#include<iostream.h>
#include<conio.h>
class static_var
{
static int count; //static member of class
public :
void incr_staticvar()
{
count++;
}
void outputc()
{
cout<<"Value of Static variable Count :- "<<count<<endl;
}
};
int static_var::count;
void main()
{
clrscr();
static_var obj1,obj2,obj3,obj4;
obj1.incr_staticvar();
obj2.incr_staticvar();
obj3.incr_staticvar();
obj4.incr_staticvar();
cout<<"\nAfter Increment of static variable by Four Different objects is :-\n";
obj1.outputc ( );
obj2.outputc ( );
obj3.outputc ( );
obj4.outputc ( );
getch();
}
Why must static data member initialization be outside the class?
class X
{
public:
int normalValue = 5; //NSDMI
static int i;
};
int X::i = 0;
Why is the static data member (here "i") only a declaration, not a definition?
It's important to distinguish the initializer which says what its initial value is, and the definition. This modified code is valid, with the initializer in the class definition:
class X
{
public:
int normalValue = 5;
static const int i = 0; // declaration, with initializer
};
const int X::i; // definition
i.e. What must be outside the class is a definition, not the initialization.
That's because a variable must have an address in memory (unless it's only used in limited situations, such as in compile-time constant expressions.)
A non-static member variable exists inside the object it is a member of, so its address depends on the address of the object that contains it. Every time you create a new X you also create a new X::normalValue variable. The non-static data member's lifetime begins with the class' constructor. NSDMI syntax doesn't have anything to do with the variable's address in memory, it just allows you to provide an initial value in one place, instead of repeating it in every constructor with an explicit constructor initializer list.
On the other hand, a static member variable is not contained within an instance of the class, it exists independently of any single instance and exists from the start of the program, at a fixed address. In order for a static member variable (or any other global object) to get a unique address the linker must see exactly one definition of the static variable, in exactly one object file, and assign it an address.
Because a static variable needs exactly one definition in exactly one object file, it doesn't make sense to allow that definition to be provided in the class, since class definitions typically exist in header files and are included in multiple object files. So although you can provide an initializer in the class, you still need to define the static data member somewhere.
You can also look at it like declaring an extern variable:
namespace X {
extern int i;
}
This declares the variable, but there must be a definition somewhere in the program:
int X::i = 0;
You need to supply a separate definition for a static data member (if its odr-used, as defined in C++11) simply because that definition shall reside somewhere - in one and only one translation unit. Static class data members are basically global objects (global variables) declared in class scope. The compiler wants you to choose a specific translation unit that will hold the actual "body" of each global object. It is you who has to decide which translation unit to place the actual object to.
"static" class member is like a globally allocated variable (it is not related to the single class instance), so it must reside in some object file (and to be declared in the ".cpp" file) as a symbol just like any global variable.
Simple class member (non-static) resides in the memory block allocated for the class instance.
The simple reason is because classes are usually declared in header files, which often are included in multiple cpp files. Static data members have external linkage and must be declared in exactly one translation unit which makes them unfit for being defined inside a class.
As juanchopanza points out the following is allowed:
struct A
{
const static int i = 1;
};
However, this is only a declaration not a definition. You still need to define it if you are going to use i's address somewhere.
For example:
f(int);
g(int&);
X<A::i> x; // Okay without definition for template arguments
char a[A::i]; // Okay without definition, just using value as constant expression
&A::i; // Need a definition because I'm taking the address
f(A::i); // Okay without definition for pass by value
g(A::i); // Need a definition with pass by reference
Bear in mind that is is possible to initialize the static data member at the point of declaration if it is of const integral type of const enumeration type:
From the C++03 standard, §9.4.2
If a static data member is of const integral or const enumeration type, its declaration in the class
definition can specify a constant-initializer which shall be an integral constant expression (5.19)
struct Foo {
static const int j = 42; // OK
};
When the compiler generate binary code from a unit (extreme simplification: a cpp file and all its included headers) it will emit a symbol for the static variable and eventually initialization code for that variable.
It is okay for a static variable symbol to be declared in multiple units, but it is not okay for it to be initialized multiple times.
So you must make sure that the initialization code is only emitted for a single unit.
This mean that the static variable must be defined in exactly one unit.
Static Data Member
#include<iostream.h>
#include<conio.h>
class static_var
{
static int count; //static member of class
public :
void incr_staticvar()
{
count++;
}
void outputc()
{
cout<<"Value of Static variable Count :- "<<count<<endl;
}
};
int static_var::count;
void main()
{
clrscr();
static_var obj1,obj2,obj3,obj4;
obj1.incr_staticvar();
obj2.incr_staticvar();
obj3.incr_staticvar();
obj4.incr_staticvar();
cout<<"\nAfter Increment of static variable by Four Different objects is :-\n";
obj1.outputc ( );
obj2.outputc ( );
obj3.outputc ( );
obj4.outputc ( );
getch();
}
What is the reasoning to why static const members cannot exist in local classes? It seems like a rather silly restriction.
Example:
void foo() {
struct bar {
int baz() { return 0; } // allowed
static const int qux = 0; // not allowed?!?
};
}
struct non_local_bar {
int baz() { return 0; } // allowed
static const int qux = 0; // allowed
};
Quote from standard (9.8.4):
A local class shall not have static data members.
From the standard section 9.4.2:
If a static data member is of const integral or const enumeration
type, its declaration in the class definition can specify a
constant-initializer which shall be an integral constant expression.
In that case, the member can appear in integral constant expressions
within its scope. The member shall still be defined in a namespace
scope if it is used in the program and the namespace scope definition
shall not contain an initializer.
Basically, local classes have no linkage, and static data members require a linkage.
Since there's no way to define a static data member of a local class in namespace scope (a declaration with an initializer is not a definition), they are not allowed, whether they are of const integral type or not. On the surface it may seem like the compiler should just be able to inline the value, but then what happens if you try to access a pointer to the member? With namespace scoped classes you'd just get a linker error, but local classes have no linkage.
I guess in theory they could just allow you to use static const integral types in local classes as long as they are only used in integral constant expressions, but it would probably just put too much of a burden on the standards body and compiler vendors to differentiate for very little practical value; local static variables are accessible from local classes, so using a local static const should be just as good.
I dont think there is a.reason. Normal static datamembers are disallowed because there is no way to define them after being declared.
Also dont forget you can create a local const variable outside the.class that you can use inside the class as long as you only read its value (that is, as long as you dont take.its.address).
Static members of a class need to be defined in global scope, e.g.
abc.h
class myClass {
static int number;
};
abc.cpp
int myClass::number = 314;
Now, since the scope inside void abc(int x) is not global, there is no scope to define the static member.
As things progress, we now have C++11 and with that you can define integral constants variable members in your classes.
class test
{
public:
const int FOO = 123;
[...snip...]
};
That works when you compile with C++11. Notice that the static keyword is not used. When compiling with optimizations turned on, those variables will likely all get optimized out. In debug, though, they appear in your structure as regular variable members.
Note, however, that the size of the class/structure will still include that variable. So here it is likely 4 bytes for the variable FOO.
However, in most cases, classes defined in a function will completely be optimized out so this is a great way of doing things (a good 50% of my classes have such variable members!)
Why must static data member initialization be outside the class?
class X
{
public:
int normalValue = 5; //NSDMI
static int i;
};
int X::i = 0;
Why is the static data member (here "i") only a declaration, not a definition?
It's important to distinguish the initializer which says what its initial value is, and the definition. This modified code is valid, with the initializer in the class definition:
class X
{
public:
int normalValue = 5;
static const int i = 0; // declaration, with initializer
};
const int X::i; // definition
i.e. What must be outside the class is a definition, not the initialization.
That's because a variable must have an address in memory (unless it's only used in limited situations, such as in compile-time constant expressions.)
A non-static member variable exists inside the object it is a member of, so its address depends on the address of the object that contains it. Every time you create a new X you also create a new X::normalValue variable. The non-static data member's lifetime begins with the class' constructor. NSDMI syntax doesn't have anything to do with the variable's address in memory, it just allows you to provide an initial value in one place, instead of repeating it in every constructor with an explicit constructor initializer list.
On the other hand, a static member variable is not contained within an instance of the class, it exists independently of any single instance and exists from the start of the program, at a fixed address. In order for a static member variable (or any other global object) to get a unique address the linker must see exactly one definition of the static variable, in exactly one object file, and assign it an address.
Because a static variable needs exactly one definition in exactly one object file, it doesn't make sense to allow that definition to be provided in the class, since class definitions typically exist in header files and are included in multiple object files. So although you can provide an initializer in the class, you still need to define the static data member somewhere.
You can also look at it like declaring an extern variable:
namespace X {
extern int i;
}
This declares the variable, but there must be a definition somewhere in the program:
int X::i = 0;
You need to supply a separate definition for a static data member (if its odr-used, as defined in C++11) simply because that definition shall reside somewhere - in one and only one translation unit. Static class data members are basically global objects (global variables) declared in class scope. The compiler wants you to choose a specific translation unit that will hold the actual "body" of each global object. It is you who has to decide which translation unit to place the actual object to.
"static" class member is like a globally allocated variable (it is not related to the single class instance), so it must reside in some object file (and to be declared in the ".cpp" file) as a symbol just like any global variable.
Simple class member (non-static) resides in the memory block allocated for the class instance.
The simple reason is because classes are usually declared in header files, which often are included in multiple cpp files. Static data members have external linkage and must be declared in exactly one translation unit which makes them unfit for being defined inside a class.
As juanchopanza points out the following is allowed:
struct A
{
const static int i = 1;
};
However, this is only a declaration not a definition. You still need to define it if you are going to use i's address somewhere.
For example:
f(int);
g(int&);
X<A::i> x; // Okay without definition for template arguments
char a[A::i]; // Okay without definition, just using value as constant expression
&A::i; // Need a definition because I'm taking the address
f(A::i); // Okay without definition for pass by value
g(A::i); // Need a definition with pass by reference
Bear in mind that is is possible to initialize the static data member at the point of declaration if it is of const integral type of const enumeration type:
From the C++03 standard, §9.4.2
If a static data member is of const integral or const enumeration type, its declaration in the class
definition can specify a constant-initializer which shall be an integral constant expression (5.19)
struct Foo {
static const int j = 42; // OK
};
When the compiler generate binary code from a unit (extreme simplification: a cpp file and all its included headers) it will emit a symbol for the static variable and eventually initialization code for that variable.
It is okay for a static variable symbol to be declared in multiple units, but it is not okay for it to be initialized multiple times.
So you must make sure that the initialization code is only emitted for a single unit.
This mean that the static variable must be defined in exactly one unit.
Static Data Member
#include<iostream.h>
#include<conio.h>
class static_var
{
static int count; //static member of class
public :
void incr_staticvar()
{
count++;
}
void outputc()
{
cout<<"Value of Static variable Count :- "<<count<<endl;
}
};
int static_var::count;
void main()
{
clrscr();
static_var obj1,obj2,obj3,obj4;
obj1.incr_staticvar();
obj2.incr_staticvar();
obj3.incr_staticvar();
obj4.incr_staticvar();
cout<<"\nAfter Increment of static variable by Four Different objects is :-\n";
obj1.outputc ( );
obj2.outputc ( );
obj3.outputc ( );
obj4.outputc ( );
getch();
}