I understand that only data members which are static, const and int/enum (pre c++11) can be initialized inside the class declaration. "All other static data members must be defined at global namespace scope (i.e. outside the body of the class definition) and can be only initialized in those definitions".
Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?
If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?
Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?
Most likely because C++ has separate translation units. The compiler needs to pick an object file where the initialization logic for those symbols will be placed. Forcing this to be in a specific source file makes that decision easy for the compiler.
If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?
Because that's just how C++ does class members. This is no different than other class members like member functions:
Header file:
namespace example {
// Class declared in header
struct some_class
{
// Member variable
static float example;
// Member function
void DoStuff() const;
};
}
Source file:
namespace example {
// Implement member variable
float some_class::example = 3.14159;
// Implement member function
void some_class::DoStuff() const
{
//....
}
}
There's a specific exception to allow static const integral members to be initialized in the header because it allows the compiler to treat them as compile-time constants. That is, you can use them to define sizes of arrays or other similar bits in the class definition.
Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?
In general, all static objects require a definition, in one single translation unit, so that they have a well-defined address. As a special exception, static, constant, non-volatile class members don't need a definition if their address is not required, and they have a simple enough type that their value can be replaced by a compile-time constant.
Historically, "simple enough" was defined as an integral or enumeration type; C++11 extends that to include any literal type with a constexpr specifier.
If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?
They are not declared at the global namespace scope. They are declared and scoped within the class.
If you mean, why are they defined outside the class definition, that's because there must be only one definition of the static member in the whole program; but the class must be defined in each translation unit that uses it.
Why can't other static data members be initialized in the class
definition? Was there a specific reason this was forbidden?
A static data member is in many respects (and especially from the point of view of a compiler) similar to a namespace-scope data object with external linkage.
The declaration of a static data member is just a declaration, not a definition. It is similar to an extern declaration of a global object and must be included into any translation unit where the object may be used.
The definition must appear in exactly one translation unit and this is where the initializer expression belongs. Unless an expression fulfills the strict criteria of a constant expression, its value may well depend upon the time and context it is called. Having such an initializer expression occur in multiple translation units would make the execution context and time of the initialization and finally the initial value ambiguous.
A class-scoped compile-time constant was deemed sufficiently valuable to make an exception for certain kinds of constant static members (which then could be used to initialize enums or specify array dimensions, etc). With constant expressions it is at least more difficult to accidentally incur different initializer values in different translation units. This concept was extended in C++11 with constexpr members.
If the data members are specific to the class, why are they declared
at the global namespace scope and not some scope relevant to their
class?
The declaration is within the class scope. The non-definition declaration is literally within the class definition and the definition appears at namespace scope, just like any other out-of-class definition of a class member. The member name is qualified by the class name, so it is clearly denoted as a member of the class and the initializer expression is actually considered to be within the scope of the class (at least in C++11; I have no C++98/03 standard available here).
You have to look at it the other way around. Basically, static data members must be defined and initialized outside the class definition, in a source file. There's an exception for static const int because it avoids various ugly workarounds for defining the size of a member array.
They would be re-initialized every time the class was instantiated. Every time you create a new object of type Foo, the static variables for all Foos would be reset to their initial value, which is probably not what you want. Therefore, if you want to use static variables with your object, they either a) can't change their value, meaning that reinitializing them to the same value is safe, or b) can only be changed outside the context of an initializer function.
Related
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();
}
Given is a class with a static member.
class BaseClass
{
public:
static std::string bstring;
};
String has obviously to be default-initialized outside of the class.
std::string BaseClass::bstring {"."};
If I include the above line in the header along with the class, I get a symbol multiply defined error. It has to be defined in a separate cpp file, even with include guards or pragma once.
Isn't there a way to define it in the header?
You can't define a static member variable more than once. If you put variable definitions into a header, it is going to be defined in each translation unit where the header is included. Since the include guards are only affecting the compilation of one translation unit, they won't help, either.
However, you can define static member functions! Now, at first sight that may not look as if it could help except, of course, that function can have local static variable and returning a reference to one of these behaves nearly like a static member variable:
static std::string& bstring() { static std::string rc{"."}; return rc; }
The local static variable will be initialized the first time this function is called. That is, the construction is delayed until the function is accessed the first time. Of course, if you use this function to initialize other global objects it may also make sure that the object is constructed in time. If you use multiple threads this may look like a potential data race but it isn't (unless you use C++03): the initialization of the function local static variable is thread-safe.
In C++17 you can use inline variables, which you can use even outside classes.
The inline specifier, when used in a decl-specifier-seq of a variable with static storage duration (static class member or namespace-scope variable), declares the variable to be an inline variable.
A static member variable (but not a namespace-scope variable) declared constexpr is implicitly an inline variable.⁽¹⁾
For example:
class Someclass {
public:
inline static int someVar = 1;
};
Or,
namespace SomeNamespace {
inline static int someVar = 1;
}
⁽¹⁾ https://en.cppreference.com/w/cpp/language/inline
Regarding
” Isn't there a way to define [the static data member] in the header?
Yes there is.
template< class Dummy >
struct BaseClass_statics
{
static std::string bstring;
};
template< class Dummy >
std::string BaseClass_statics<Dummy>::bstring = ".";
class BaseClass
: public BaseClass_statics<void>
{};
An alternative is to use a function, as Dietmar suggested. Essentially that is a Meyers' singleton (google it).
Edit: Also, since this answer was posted we've got the inline object proposal, which I think is accepted for C++17.
Anyway, think twice about the design here. Globals variables are Evil™. This is essentially a global.
To keep the definition of a static value with the declaration in C++11
a nested static structure can be used. In this case the static member
is a structure and has to be defined in a .cpp file, but the values
are in the header.
class BaseClass
{
public:
static struct _Static {
std::string bstring {"."};
} global;
};
Instead of initializing individual members the whole static structure is initialized:
BaseClass::_Static BaseClass::global;
The values are accessed with
BaseClass::global.bstring;
Note that this solution still suffers from the problem of the order of
initialization of the static variables. When a static value is used to
initialize another static variable, the first may not be initialized,
yet.
// file.h
class File {
public:
static struct _Extensions {
const std::string h{ ".h" };
const std::string hpp{ ".hpp" };
const std::string c{ ".c" };
const std::string cpp{ ".cpp" };
} extension;
};
// file.cpp
File::_Extensions File::extension;
// module.cpp
static std::set<std::string> headers{ File::extension.h, File::extension.hpp };
In this case the static variable headers will contain either { "" }
or { ".h", ".hpp" }, depending on the order of initialization created by the linker.
§3.2.6 and the following paragraphs from the current c++ 17 draft (n4296) define the rules when more than one definition can be present in different translation units:
There can be more than one definition of a class type (Clause 9), enumeration type (7.2), inline function with
external linkage (7.1.2), class template (Clause 14), non-static function template (14.5.6), static data member
of a class template (14.5.1.3), member function of a class template (14.5.1.1), or template specialization for
which some template parameters are not specified (14.7, 14.5.5) in a program provided that each definition
appears in a different translation unit, and provided the definitions satisfy the following requirements. Given
such an entity named D defined in more than one translation unit, then [...]
Obviously definitions of static data members of class type are not considered to appear in multiple translations units. Thus, according to the standard, it is not allowed.
The suggested answers from Cheers and hth. - Alf and Dietmar are more kind of a "hack", exploiting that definitions of
static data member of a class template (14.5.1.3)
and
inline function with external linkage (7.1.2)
are allowed in multiple TU ( FYI: static functions defined inside a class definition have external linkage and are implicitly defined as inline ) .
No, it can't be done in a header - at least not if the header is included more than once in your source-files, which appears to be the case, or you wouldn't get an error like that. Just stick it in one of the .cpp files and be done with it.
UPDATE: My answer below explains why this cannot be done in the way suggested by the question. There are at least two answers circumventing this; they may or may not solve the problem.
The bstring static member has to be linked to a specific memory address. For this to happen, it has to appear in a single object file, therefore it has to appear in a single cpp file. Unless you're playing with #ifdef's to make sure this happens, what you want cannot be done in the header file, as your header file may be included by more than one cpp files.
I understand that only data members which are static, const and int/enum (pre c++11) can be initialized inside the class declaration. "All other static data members must be defined at global namespace scope (i.e. outside the body of the class definition) and can be only initialized in those definitions".
Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?
If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?
Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?
Most likely because C++ has separate translation units. The compiler needs to pick an object file where the initialization logic for those symbols will be placed. Forcing this to be in a specific source file makes that decision easy for the compiler.
If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?
Because that's just how C++ does class members. This is no different than other class members like member functions:
Header file:
namespace example {
// Class declared in header
struct some_class
{
// Member variable
static float example;
// Member function
void DoStuff() const;
};
}
Source file:
namespace example {
// Implement member variable
float some_class::example = 3.14159;
// Implement member function
void some_class::DoStuff() const
{
//....
}
}
There's a specific exception to allow static const integral members to be initialized in the header because it allows the compiler to treat them as compile-time constants. That is, you can use them to define sizes of arrays or other similar bits in the class definition.
Why can't other static data members be initialized in the class definition? Was there a specific reason this was forbidden?
In general, all static objects require a definition, in one single translation unit, so that they have a well-defined address. As a special exception, static, constant, non-volatile class members don't need a definition if their address is not required, and they have a simple enough type that their value can be replaced by a compile-time constant.
Historically, "simple enough" was defined as an integral or enumeration type; C++11 extends that to include any literal type with a constexpr specifier.
If the data members are specific to the class, why are they declared at the global namespace scope and not some scope relevant to their class?
They are not declared at the global namespace scope. They are declared and scoped within the class.
If you mean, why are they defined outside the class definition, that's because there must be only one definition of the static member in the whole program; but the class must be defined in each translation unit that uses it.
Why can't other static data members be initialized in the class
definition? Was there a specific reason this was forbidden?
A static data member is in many respects (and especially from the point of view of a compiler) similar to a namespace-scope data object with external linkage.
The declaration of a static data member is just a declaration, not a definition. It is similar to an extern declaration of a global object and must be included into any translation unit where the object may be used.
The definition must appear in exactly one translation unit and this is where the initializer expression belongs. Unless an expression fulfills the strict criteria of a constant expression, its value may well depend upon the time and context it is called. Having such an initializer expression occur in multiple translation units would make the execution context and time of the initialization and finally the initial value ambiguous.
A class-scoped compile-time constant was deemed sufficiently valuable to make an exception for certain kinds of constant static members (which then could be used to initialize enums or specify array dimensions, etc). With constant expressions it is at least more difficult to accidentally incur different initializer values in different translation units. This concept was extended in C++11 with constexpr members.
If the data members are specific to the class, why are they declared
at the global namespace scope and not some scope relevant to their
class?
The declaration is within the class scope. The non-definition declaration is literally within the class definition and the definition appears at namespace scope, just like any other out-of-class definition of a class member. The member name is qualified by the class name, so it is clearly denoted as a member of the class and the initializer expression is actually considered to be within the scope of the class (at least in C++11; I have no C++98/03 standard available here).
You have to look at it the other way around. Basically, static data members must be defined and initialized outside the class definition, in a source file. There's an exception for static const int because it avoids various ugly workarounds for defining the size of a member array.
They would be re-initialized every time the class was instantiated. Every time you create a new object of type Foo, the static variables for all Foos would be reset to their initial value, which is probably not what you want. Therefore, if you want to use static variables with your object, they either a) can't change their value, meaning that reinitializing them to the same value is safe, or b) can only be changed outside the context of an initializer function.
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();
}