My understanding is that C++ allows static const members to be defined inside a class so long as it's an integer type.
Why, then, does the following code give me a linker error?
#include <algorithm>
#include <iostream>
class test
{
public:
static const int N = 10;
};
int main()
{
std::cout << test::N << "\n";
std::min(9, test::N);
}
The error I get is:
test.cpp:(.text+0x130): undefined reference to `test::N'
collect2: ld returned 1 exit status
Interestingly, if I comment out the call to std::min, the code compiles and links just fine (even though test::N is also referenced on the previous line).
Any idea as to what's going on?
My compiler is gcc 4.4 on Linux.
My understanding is that C++ allows static const members to be defined inside a class so long as it's an integer type.
You are sort of correct. You are allowed to initialize static const integrals in the class declaration but that is not a definition.
Interestingly, if I comment out the call to std::min, the code compiles and links just fine (even though test::N is also referenced on the previous line).
Any idea as to what's going on?
std::min takes its parameters by const reference. If it took them by value you'd not have this problem but since you need a reference you also need a definition.
Here's chapter/verse:
9.4.2/4 - 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). In that case, the member can appear in integral constant expressions. 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.
See Chu's answer for a possible workaround.
Bjarne Stroustrup's example in his C++ FAQ suggests you are correct, and only need a definition if you take the address.
class AE {
// ...
public:
static const int c6 = 7;
static const int c7 = 31;
};
const int AE::c7; // definition
int f()
{
const int* p1 = &AE::c6; // error: c6 not an lvalue
const int* p2 = &AE::c7; // ok
// ...
}
He says "You can take the address of a static member if (and only if) it has an out-of-class definition". Which suggests it would work otherwise. Maybe your min function invokes addresses somehow behind the scenes.
Another way to do this, for integer types anyway, is to define constants as enums in the class:
class test
{
public:
enum { N = 10 };
};
Not just int's. But you can't define the value in the class declaration. If you have:
class classname
{
public:
static int const N;
}
in the .h file then you must have:
int const classname::N = 10;
in the .cpp file.
Here's another way to work around the problem:
std::min(9, int(test::N));
(I think Crazy Eddie's answer correctly describes why the problem exists.)
As of C++11 you can use:
static constexpr int N = 10;
This theoretically still requires you to define the constant in a .cpp file, but as long as you don't take the address of N it is very unlikely that any compiler implementation will produce an error ;).
C++ allows static const members to be defined inside a class
Nope, 3.1 §2 says:
A declaration is a definition unless it declares a function without specifying the function's body (8.4), it contains the extern specifier (7.1.1) or a linkage-specification (7.5) and neither an initializer nor a functionbody, it declares a static data member in a class definition (9.4), it is a class name declaration (9.1), it is an opaque-enum-declaration (7.2), or it is a typedef declaration (7.1.3), a using-declaration (7.3.3), or a using-directive (7.3.4).
Related
Can anyone explain why following code won't compile? At least on g++ 4.2.4.
And more interesting, why it will compile when I cast MEMBER to int?
#include <vector>
class Foo {
public:
static const int MEMBER = 1;
};
int main(){
vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
You need to actually define the static member somewhere (after the class definition). Try this:
class Foo { /* ... */ };
const int Foo::MEMBER;
int main() { /* ... */ }
That should get rid of the undefined reference.
The problem comes because of an interesting clash of new C++ features and what you're trying to do. First, let's take a look at the push_back signature:
void push_back(const T&)
It's expecting a reference to an object of type T. Under the old system of initialization, such a member exists. For example, the following code compiles just fine:
#include <vector>
class Foo {
public:
static const int MEMBER;
};
const int Foo::MEMBER = 1;
int main(){
std::vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
This is because there is an actual object somewhere that has that value stored in it. If, however, you switch to the new method of specifying static const members, like you have above, Foo::MEMBER is no longer an object. It is a constant, somewhat akin to:
#define MEMBER 1
But without the headaches of a preprocessor macro (and with type safety). That means that the vector, which is expecting a reference, can't get one.
The C++ standard requires a definition for your static const member if the definition is somehow needed.
The definition is required, for example if it's address is used. push_back takes its parameter by const reference, and so strictly the compiler needs the address of your member and you need to define it in the namespace.
When you explicitly cast the constant, you're creating a temporary and it's this temporary which is bound to the reference (under special rules in the standard).
This is a really interesting case, and I actually think it's worth raising an issue so that the std be changed to have the same behaviour for your constant member!
Although, in a weird kind of way this could be seen as a legitimate use of the unary '+' operator. Basically the result of the unary + is an rvalue and so the rules for binding of rvalues to const references apply and we don't use the address of our static const member:
v.push_back( +Foo::MEMBER );
Aaa.h
class Aaa {
protected:
static Aaa *defaultAaa;
};
Aaa.cpp
// You must define an actual variable in your program for the static members of the classes
static Aaa *Aaa::defaultAaa;
In C++17, there is an easier solution using inline variables:
struct Foo{
inline static int member;
};
This is a definition of member, not just its declaration. Similar to inline functions, multiple identical definitions in different translation units do not violate ODR. There is no longer any need to pick a favourite .cpp file for the definition.
Just some additional info:
C++ allows to "define" const static types of integral and enumeration types as class members. But this is actually not a definition, just an "initializiation-marker"
You should still write a definition of your member outside of the class.
9.4.2/4 - 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). In that case, the member can appear in integral constant expressions. 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.
No idea why the cast works, but Foo::MEMBER isn't allocated until the first time Foo is loaded, and since you're never loading it, it's never allocated. If you had a reference to a Foo somewhere, it would probably work.
With C++11, the above would be possible for basic types as
class Foo {
public:
static constexpr int MEMBER = 1;
};
The constexpr part creates a static expression as opposed to a static variable - and that behaves just like an extremely simple inline method definition. The approach proved a bit wobbly with C-string constexprs inside template classes, though.
Regarding the second question: push_ref takes reference as a parameter, and you cannot have a reference to static const memeber of a class/struct. Once you call static_cast, a temporary variable is created. And a reference to this object can be passed, everything works just fine.
Or at least my colleague who resolved this said so.
Can anyone explain why following code won't compile? At least on g++ 4.2.4.
And more interesting, why it will compile when I cast MEMBER to int?
#include <vector>
class Foo {
public:
static const int MEMBER = 1;
};
int main(){
vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
You need to actually define the static member somewhere (after the class definition). Try this:
class Foo { /* ... */ };
const int Foo::MEMBER;
int main() { /* ... */ }
That should get rid of the undefined reference.
The problem comes because of an interesting clash of new C++ features and what you're trying to do. First, let's take a look at the push_back signature:
void push_back(const T&)
It's expecting a reference to an object of type T. Under the old system of initialization, such a member exists. For example, the following code compiles just fine:
#include <vector>
class Foo {
public:
static const int MEMBER;
};
const int Foo::MEMBER = 1;
int main(){
std::vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
This is because there is an actual object somewhere that has that value stored in it. If, however, you switch to the new method of specifying static const members, like you have above, Foo::MEMBER is no longer an object. It is a constant, somewhat akin to:
#define MEMBER 1
But without the headaches of a preprocessor macro (and with type safety). That means that the vector, which is expecting a reference, can't get one.
The C++ standard requires a definition for your static const member if the definition is somehow needed.
The definition is required, for example if it's address is used. push_back takes its parameter by const reference, and so strictly the compiler needs the address of your member and you need to define it in the namespace.
When you explicitly cast the constant, you're creating a temporary and it's this temporary which is bound to the reference (under special rules in the standard).
This is a really interesting case, and I actually think it's worth raising an issue so that the std be changed to have the same behaviour for your constant member!
Although, in a weird kind of way this could be seen as a legitimate use of the unary '+' operator. Basically the result of the unary + is an rvalue and so the rules for binding of rvalues to const references apply and we don't use the address of our static const member:
v.push_back( +Foo::MEMBER );
Aaa.h
class Aaa {
protected:
static Aaa *defaultAaa;
};
Aaa.cpp
// You must define an actual variable in your program for the static members of the classes
static Aaa *Aaa::defaultAaa;
In C++17, there is an easier solution using inline variables:
struct Foo{
inline static int member;
};
This is a definition of member, not just its declaration. Similar to inline functions, multiple identical definitions in different translation units do not violate ODR. There is no longer any need to pick a favourite .cpp file for the definition.
Just some additional info:
C++ allows to "define" const static types of integral and enumeration types as class members. But this is actually not a definition, just an "initializiation-marker"
You should still write a definition of your member outside of the class.
9.4.2/4 - 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). In that case, the member can appear in integral constant expressions. 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.
No idea why the cast works, but Foo::MEMBER isn't allocated until the first time Foo is loaded, and since you're never loading it, it's never allocated. If you had a reference to a Foo somewhere, it would probably work.
With C++11, the above would be possible for basic types as
class Foo {
public:
static constexpr int MEMBER = 1;
};
The constexpr part creates a static expression as opposed to a static variable - and that behaves just like an extremely simple inline method definition. The approach proved a bit wobbly with C-string constexprs inside template classes, though.
Regarding the second question: push_ref takes reference as a parameter, and you cannot have a reference to static const memeber of a class/struct. Once you call static_cast, a temporary variable is created. And a reference to this object can be passed, everything works just fine.
Or at least my colleague who resolved this said so.
Can anyone explain why following code won't compile? At least on g++ 4.2.4.
And more interesting, why it will compile when I cast MEMBER to int?
#include <vector>
class Foo {
public:
static const int MEMBER = 1;
};
int main(){
vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
You need to actually define the static member somewhere (after the class definition). Try this:
class Foo { /* ... */ };
const int Foo::MEMBER;
int main() { /* ... */ }
That should get rid of the undefined reference.
The problem comes because of an interesting clash of new C++ features and what you're trying to do. First, let's take a look at the push_back signature:
void push_back(const T&)
It's expecting a reference to an object of type T. Under the old system of initialization, such a member exists. For example, the following code compiles just fine:
#include <vector>
class Foo {
public:
static const int MEMBER;
};
const int Foo::MEMBER = 1;
int main(){
std::vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
This is because there is an actual object somewhere that has that value stored in it. If, however, you switch to the new method of specifying static const members, like you have above, Foo::MEMBER is no longer an object. It is a constant, somewhat akin to:
#define MEMBER 1
But without the headaches of a preprocessor macro (and with type safety). That means that the vector, which is expecting a reference, can't get one.
The C++ standard requires a definition for your static const member if the definition is somehow needed.
The definition is required, for example if it's address is used. push_back takes its parameter by const reference, and so strictly the compiler needs the address of your member and you need to define it in the namespace.
When you explicitly cast the constant, you're creating a temporary and it's this temporary which is bound to the reference (under special rules in the standard).
This is a really interesting case, and I actually think it's worth raising an issue so that the std be changed to have the same behaviour for your constant member!
Although, in a weird kind of way this could be seen as a legitimate use of the unary '+' operator. Basically the result of the unary + is an rvalue and so the rules for binding of rvalues to const references apply and we don't use the address of our static const member:
v.push_back( +Foo::MEMBER );
Aaa.h
class Aaa {
protected:
static Aaa *defaultAaa;
};
Aaa.cpp
// You must define an actual variable in your program for the static members of the classes
static Aaa *Aaa::defaultAaa;
In C++17, there is an easier solution using inline variables:
struct Foo{
inline static int member;
};
This is a definition of member, not just its declaration. Similar to inline functions, multiple identical definitions in different translation units do not violate ODR. There is no longer any need to pick a favourite .cpp file for the definition.
Just some additional info:
C++ allows to "define" const static types of integral and enumeration types as class members. But this is actually not a definition, just an "initializiation-marker"
You should still write a definition of your member outside of the class.
9.4.2/4 - 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). In that case, the member can appear in integral constant expressions. 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.
No idea why the cast works, but Foo::MEMBER isn't allocated until the first time Foo is loaded, and since you're never loading it, it's never allocated. If you had a reference to a Foo somewhere, it would probably work.
With C++11, the above would be possible for basic types as
class Foo {
public:
static constexpr int MEMBER = 1;
};
The constexpr part creates a static expression as opposed to a static variable - and that behaves just like an extremely simple inline method definition. The approach proved a bit wobbly with C-string constexprs inside template classes, though.
Regarding the second question: push_ref takes reference as a parameter, and you cannot have a reference to static const memeber of a class/struct. Once you call static_cast, a temporary variable is created. And a reference to this object can be passed, everything works just fine.
Or at least my colleague who resolved this said so.
Why can't I initialize non-const static member or static array in a class?
class A
{
static const int a = 3;
static int b = 3;
static const int c[2] = { 1, 2 };
static int d[2] = { 1, 2 };
};
int main()
{
A a;
return 0;
}
the compiler issues following errors:
g++ main.cpp
main.cpp:4:17: error: ISO C++ forbids in-class initialization of non-const static member ‘b’
main.cpp:5:26: error: a brace-enclosed initializer is not allowed here before ‘{’ token
main.cpp:5:33: error: invalid in-class initialization of static data member of non-integral type ‘const int [2]’
main.cpp:6:20: error: a brace-enclosed initializer is not allowed here before ‘{’ token
main.cpp:6:27: error: invalid in-class initialization of static data member of non-integral type ‘int [2]’
I have two questions:
Why can't I initialize static data members in class?
Why can't I initialize static arrays in class, even the const array?
Why I can't initialize static data members in class?
The C++ standard allows only static constant integral or enumeration types to be initialized inside the class. This is the reason a is allowed to be initialized while others are not.
Reference:
C++03 9.4.2 Static data members
§4
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). In that case, the member can appear in integral constant expressions. 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.
What are integral types?
C++03 3.9.1 Fundamental types
§7
Types bool, char, wchar_t, and the signed and unsigned integer types are collectively called integral types.43) A synonym for integral type is integer type.
Footnote:
43) Therefore, enumerations (7.2) are not integral; however, enumerations can be promoted to int, unsigned int, long, or unsigned long, as specified in 4.5.
Workaround:
You could use the enum trick to initialize an array inside your class definition.
class A
{
static const int a = 3;
enum { arrsize = 2 };
static const int c[arrsize] = { 1, 2 };
};
Why does the Standard does not allow this?
Bjarne explains this aptly here:
A class is typically declared in a header file and a header file is typically included into many translation units. However, to avoid complicated linker rules, C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects.
Why are only static const integral types & enums allowed In-class Initialization?
The answer is hidden in Bjarne's quote read it closely,
"C++ requires that every object has a unique definition. That rule would be broken if C++ allowed in-class definition of entities that needed to be stored in memory as objects."
Note that only static const integers can be treated as compile time constants. The compiler knows that the integer value will not change anytime and hence it can apply its own magic and apply optimizations, the compiler simply inlines such class members i.e, they are not stored in memory anymore, As the need of being stored in memory is removed, it gives such variables the exception to rule mentioned by Bjarne.
It is noteworthy to note here that even if static const integral values can have In-Class Initialization, taking address of such variables is not allowed. One can take the address of a static member if (and only if) it has an out-of-class definition.This further validates the reasoning above.
enums are allowed this because values of an enumerated type can be used where ints are expected.see citation above
How does this change in C++11?
C++11 relaxes the restriction to certain extent.
C++11 9.4.2 Static data members
§3
If a static data member is of const literal type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [ Note: In both these cases, the member may appear in constant expressions. —end note ] 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.
Also, C++11 will allow(§12.6.2.8) a non-static data member to be initialized where it is declared(in its class). This will mean much easy user semantics.
Note that these features have not yet been implemented in latest gcc 4.7, So you might still get compilation errors.
This seems a relict from the old days of simple linkers. You can use static variables in static methods as workaround:
// header.hxx
#include <vector>
class Class {
public:
static std::vector<int> & replacement_for_initialized_static_non_const_variable() {
static std::vector<int> Static {42, 0, 1900, 1998};
return Static;
}
};
int compilation_unit_a();
and
// compilation_unit_a.cxx
#include "header.hxx"
int compilation_unit_a() {
return Class::replacement_for_initialized_static_non_const_variable()[1]++;
}
and
// main.cxx
#include "header.hxx"
#include <iostream>
int main() {
std::cout
<< compilation_unit_a()
<< Class::replacement_for_initialized_static_non_const_variable()[1]++
<< compilation_unit_a()
<< Class::replacement_for_initialized_static_non_const_variable()[1]++
<< std::endl;
}
build:
g++ -std=gnu++0x -save-temps=obj -c compilation_unit_a.cxx
g++ -std=gnu++0x -o main main.cxx compilation_unit_a.o
run:
./main
The fact that this works (consistently, even if the class definition is included in different compilation units), shows that the linker today (gcc 4.9.2) is actually smart enough.
Funny: Prints 0123 on arm and 3210 on x86.
It's because there can only be one definition of A::a that all the translation units use.
If you performed static int a = 3; in a class in a header included in all a translation units then you'd get multiple definitions. Therefore, non out-of-line definition of a static is forcibly made a compiler error.
Using static inline or static const remedies this. static inline only concretises the symbol if it is used in the translation unit and ensures the linker only selects and leaves one copy if it's defined in multiple translation units due to it being in a comdat group. const at file scope makes the compiler never emit a symbol because it's always substituted immediately in the code unless extern is used, which is not permitted in a class.
One thing to note is static inline int b; is treated as a definition whereas static const int b or static const A b; are still treated as a declaration and must be defined out-of-line if you don't define it inside the class. Interestingly static constexpr A b; is treated as a definition, whereas static constexpr int b; is an error and must have an initialiser (this is because they now become definitions and like any const/constexpr definition at file scope, they require an initialiser which an int doesn't have but a class type does because it has an implicit = A() when it is a definition -- clang allows this but gcc requires you to explicitly initialise or it is an error. This is not a problem with inline instead). static const A b = A(); is not allowed and must be constexpr or inline in order to permit an initialiser for a static object with class type i.e to make a static member of class type more than a declaration. So yes in certain situations A a; is not the same as explicitly initialising A a = A(); (the former can be a declaration but if only a declaration is allowed for that type then the latter is an error. The latter can only be used on a definition. constexpr makes it a definition). If you use constexpr and specify a default constructor then the constructor will need to be constexpr
#include<iostream>
struct A
{
int b =2;
mutable int c = 3; //if this member is included in the class then const A will have a full .data symbol emitted for it on -O0 and so will B because it contains A.
static const int a = 3;
};
struct B {
A b;
static constexpr A c; //needs to be constexpr or inline and doesn't emit a symbol for A a mutable member on any optimisation level
};
const A a;
const B b;
int main()
{
std::cout << a.b << b.b.b;
return 0;
}
A static member is an outright file scope declaration extern int A::a; (which can only be made in the class and out of line definitions must refer to a static member in a class and must be definitions and cannot contain extern) whereas a non-static member is part of the complete type definition of a class and have the same rules as file scope declarations without extern. They are implicitly definitions. So int i[]; int i[5]; is a redefinition whereas static int i[]; int A::i[5]; isn't but unlike 2 externs, the compiler will still detect a duplicate member if you do static int i[]; static int i[5]; in the class.
I think it's to prevent you from mixing declarations and definitions. (Think about the problems that could occur if you include the file in multiple places.)
static variables are specific to a class . Constructors initialize attributes ESPECIALY for an instance.
Can anyone explain why following code won't compile? At least on g++ 4.2.4.
And more interesting, why it will compile when I cast MEMBER to int?
#include <vector>
class Foo {
public:
static const int MEMBER = 1;
};
int main(){
vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
You need to actually define the static member somewhere (after the class definition). Try this:
class Foo { /* ... */ };
const int Foo::MEMBER;
int main() { /* ... */ }
That should get rid of the undefined reference.
The problem comes because of an interesting clash of new C++ features and what you're trying to do. First, let's take a look at the push_back signature:
void push_back(const T&)
It's expecting a reference to an object of type T. Under the old system of initialization, such a member exists. For example, the following code compiles just fine:
#include <vector>
class Foo {
public:
static const int MEMBER;
};
const int Foo::MEMBER = 1;
int main(){
std::vector<int> v;
v.push_back( Foo::MEMBER ); // undefined reference to `Foo::MEMBER'
v.push_back( (int) Foo::MEMBER ); // OK
return 0;
}
This is because there is an actual object somewhere that has that value stored in it. If, however, you switch to the new method of specifying static const members, like you have above, Foo::MEMBER is no longer an object. It is a constant, somewhat akin to:
#define MEMBER 1
But without the headaches of a preprocessor macro (and with type safety). That means that the vector, which is expecting a reference, can't get one.
The C++ standard requires a definition for your static const member if the definition is somehow needed.
The definition is required, for example if it's address is used. push_back takes its parameter by const reference, and so strictly the compiler needs the address of your member and you need to define it in the namespace.
When you explicitly cast the constant, you're creating a temporary and it's this temporary which is bound to the reference (under special rules in the standard).
This is a really interesting case, and I actually think it's worth raising an issue so that the std be changed to have the same behaviour for your constant member!
Although, in a weird kind of way this could be seen as a legitimate use of the unary '+' operator. Basically the result of the unary + is an rvalue and so the rules for binding of rvalues to const references apply and we don't use the address of our static const member:
v.push_back( +Foo::MEMBER );
Aaa.h
class Aaa {
protected:
static Aaa *defaultAaa;
};
Aaa.cpp
// You must define an actual variable in your program for the static members of the classes
static Aaa *Aaa::defaultAaa;
In C++17, there is an easier solution using inline variables:
struct Foo{
inline static int member;
};
This is a definition of member, not just its declaration. Similar to inline functions, multiple identical definitions in different translation units do not violate ODR. There is no longer any need to pick a favourite .cpp file for the definition.
Just some additional info:
C++ allows to "define" const static types of integral and enumeration types as class members. But this is actually not a definition, just an "initializiation-marker"
You should still write a definition of your member outside of the class.
9.4.2/4 - 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). In that case, the member can appear in integral constant expressions. 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.
No idea why the cast works, but Foo::MEMBER isn't allocated until the first time Foo is loaded, and since you're never loading it, it's never allocated. If you had a reference to a Foo somewhere, it would probably work.
With C++11, the above would be possible for basic types as
class Foo {
public:
static constexpr int MEMBER = 1;
};
The constexpr part creates a static expression as opposed to a static variable - and that behaves just like an extremely simple inline method definition. The approach proved a bit wobbly with C-string constexprs inside template classes, though.
Regarding the second question: push_ref takes reference as a parameter, and you cannot have a reference to static const memeber of a class/struct. Once you call static_cast, a temporary variable is created. And a reference to this object can be passed, everything works just fine.
Or at least my colleague who resolved this said so.