The following code compiles fine:
#include <type_traits>
template <typename T> struct dependent_true : std::true_type { };
template <typename T> struct dependent_false : std::false_type { };
template <bool B = false>
class X { static_assert(dependent_false<X>::value); };
template <>
class X<true> { static_assert(dependent_true<X>::value); };
int main() {
X<true> x;
}
That is, the static_assert in the primary template is not evaluated. On the contrary, if I switch to:
template <bool B = false>
class X { static_assert(dependent_true<X>::value); };
template <>
class X<true> { static_assert(dependent_false<X>::value); };
int main() {
X<false> x;
}
Then, the static assertion in template specialization fails, even if it is not instantiated. I just wonder why. I observed this behavior with GCC 8 and Clang 6 (-std=c++17).
Live demo: https://wandbox.org/permlink/MOWNLnGMgmuDA2Ht
template <> class X<true> {/* ... */}; - is not a template anymore.
[temp.expl.spec]/5
A member of an explicitly specialized class is not implicitly
instantiated from the member declaration of the class template;
instead, the member of the class template specialization shall itself
be explicitly defined if its definition is required. In this case, the
definition of the class template explicit specialization shall be in
scope at the point at which the member is defined. The definition of
an explicitly specialized class is unrelated to the definition of a
generated specialization. That is, its members need not have the same
names, types, etc. as the members of a generated specialization.
Members of an explicitly specialized class template are defined in the
same manner as members of normal classes, and not using the template<>
syntax. The same is true when defining a member of an explicitly
specialized member class. However, template<> is used in defining a
member of an explicitly specialized member class template that is
specialized as a class template.
The specialization is just like a regular class. It's not a template, and nothing is dependent. Therefore dependent_false<X>::value is just a constant expression that evaluates immediately to false. So the static assertion is immediately triggered.
Even non-instantiated template parts should be valid C++ code. static_assert(false) makes the program ill-formed. So you have your specialization with static_assert which is known on compile time to be false and your program becomes ill-formed. You have no non-resolved template parameters on your class which is used in static_assert to make compiler wonder; it knows exactly that it is false.
The same goes to if constexpr, you also can't use static_assert with expressions known to be false even if the part where this static_assert is located always gets discarded.
Related
template<class T> struct A {
struct B { };
template<class U> struct C {
void show();
};
};
template<>
template<>
void A<int>::C<int>::show(){ //#1
}
int main(){
}
Consider the above code, At #1, it's an explicit specialization definition for member of member class template. Some rules will be applied to it as the following specified:
temp.expl.spec#5
A member of an explicitly specialized class is not implicitly instantiated from the member declaration of the class template; instead, the member of the class template specialization shall itself be explicitly defined if its definition is required. In this case, the definition of the class template explicit specialization shall be in scope at the point at which the member is defined. The definition of an explicitly specialized class is unrelated to the definition of a generated specialization. That is, its members need not have the same names, types, etc. as the members of a generated specialization. Members of an explicitly specialized class template are defined in the same manner as members of normal classes, and not using the template<> syntax. The same is true when defining a member of an explicitly specialized member class. However, template<> is used in defining a member of an explicitly specialized member class template that is specialized as a class template.
Firstly, what's explicitly specialized class? Does it refer to the entity which has explicit specialization declaration? It seems to it doesn't mean it, please look at the example in Explicitly specialized class part
template<> template<> class A<int>::B<double>;
According to that example, A<int> within the explicit specialization for member can be called a explicitly specialized class. So, In my first example A<int> and C<int> are all explicitly specialized class? I'm not sure. I feel the phrase explicitly specialized class is not clear in this section.
Please note the emphasized part, it means the enclosing class template explicit specialization shall appear in the same scope as that of explicit specialization definition for its member. The member is defined in global scope but there's no any explicit specialization definition for A<int> or C<int> that appears in the global scope. How to interpret this?
By the way, as a opposite example:
template<class T> struct A {
struct B { };
template<class U> struct C {
void show();
};
};
template<>
template<typename U>
struct A<int>::C{ //#2
void show();
};
template<>
template<typename U>
void A<int>::C<U>::show(){ //#3
}
int main(){
}
why such code is required an explicit specialization for class template C before #3, what's the difference between such two examples?
Explicitly specialized class
The phrase "explicitly specialized class" is unclear in this section,
temp.expl.spec#15
A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a template<> for each enclosing class template that is explicitly specialized.
[ Example:
template<class T1> class A {
template<class T2> class B {
void mf();
};
};
template<> template<> class A<int>::B<double>;
template<> template<> void A<char>::B<char>::mf();
— end example ]
what's the explicitly specialized class mean, Is it refer to an entity that have a explicit specialization declaration or something others? It seems to no explicit specialization for A<int> in the above example.
There is no confusion there, you have to parse those statements (C++ and English) according to their grammatical structure. Source code is description of program in language understandable for humans. Programming language is a tool of human cooperation.
CWG529 removed need to understand intuitively by changing wording to explain order and content of template-ids.
Here you declared template of class A with template parameter T, which contains class B and a nested declaration of template class C with template parameter U, which contains a method show():
template<class T> struct A {
struct B { };
template<class U> struct C {
void show();
};
};
Here you declared, that for explicitly specialized template class A (which required to have it declared first) with T = int that there is a template class C which contains method show()
template<>
template<typename U>
struct A<int>::C{ //#2
void show();
};
This declaration doesn't contradict previous one, but because it's a specialization of class A, it may expand it! You can do this:
template<>
template<typename U>
struct A<int>::C{ //#4
void hide();
};
Which means that for any A with T=int, there is a template class C that got member hide(). But other A's will have template class C with member show(). What previous statement did that it removed any doubts about content of C for this A's specialization.
Now this only defines member function show() for all C's contained in A<int>:
template<>
template<typename U>
void A<int>::C<U>::show(){ //#3
}
You don't have an explicit specialization of C here, which is an enclosing class for show(). the memeber id show() is preceded by an unspecialized template-id template<typename U> ... C<U>. There is only a definition of member function but it requires a visible declaration of that template C - the #2 part. The visibility may be attained by various means and "the scope" mentioned is generalized description of it.
Omitting part #2 would be a semantic equivalent of writing:
class C;
void C::show() { // ill-formed - C is an incomplete type.
}
We would know that all of A's contain some class C, but we don't have a complete definition for that particular C in specialized A<int> (and it may be different).
This statement actually states that specialization C<int>nested in specialization A<int> contains show()
template<>
template<>
void A<int>::C<int>::show(){ //#1
}
Any possibility of contradiction, ambiguity or uncertainty (aside from undefined behavior) are leading to ill-formed code and rules are meant to form the framework of limitations against which the code should be checked. With absence of #2, the #3 at some point could be followed by #4, then the #3 statement would become illegal and thus it is deemed as such. #2 and #4 at same time would be two definitions of same thing which also leads either to ill-formed code if they are present in same unit, or they would lead to undefined behavior if they are present in separate units within program.
template<typename T>
struct A{
void method1(){}
};
template<>
struct A<int>{
void method2(){}
};
Will A<int> have both method1 and method2? And A<float> will only have method1 ?
Each specialization brings an entirely new data type into existence (or an entirely new template, if the specialization is only partial). From the Standard (C++11):
(§14.5.5/2) Each class template partial specialization is a distinct template and definitions shall be provided for the members of a template partial specialization (14.5.5.3).
And:
(§14.5.5.3/1) [...] The members of the class template partial specialization are unrelated to the members of the primary template. Class template partial specialization members that are used in a way that requires a definition shall be defined; the definitions of members of the primary template are never used as definitions for members of a class template partial specialization. [...]
The above is stated in the context of partial specializations, but it applies to explicit specializations (as in your case) as well, although the Standard does not say this very clearly.
Also note that you need not only declare all member functions that you want in a specialization, but you need to define them, too (here, the Standard is very clear even about explicit specializations):
(14.7.3/5) A member of an explicitly specialized class is not implicitly instantiated from the member declaration of the class template; instead, the member of the class template specialization shall itself be explicitly defined if its definition is required. In this case, the definition of the class template explicit specialization shall be in scope
at the point at which the member is defined. The definition of an explicitly specialized class is unrelated to the definition of a generated specialization. That is, its members need not have the same names, types, etc. as the members of a generated specialization. [...]
So, indeed, A<int> will only have method2(), and A<float> will only have method1() as member. Furthermore, if you were to introduce method1() in the A<int> specialization as well, it needs not have the same argument types or return type as A<float>::method1().
See #aschepler's answer for possible ways to avoid having to rewrite the template definition for the int case.
#jogojapan's answer explains what the language does. Here's a couple workarounds if you do want to add new members for a specific specialization:
template<typename T>
struct A_Base {
void method1() {}
};
template<typename T>
struct A : public A_Base<T> {};
template<>
struct A<int>
: public A_Base<int>
{
void method2() {}
};
Now A<int> has members method1 and method2, but A<float> has no method2.
OR (if you can modify the primary template)...
#include <type_traits>
template<typename T>
struct A {
void method1() {}
template<int N=0>
auto method2() ->
typename std::enable_if<std::is_same<T, int>::value && N==N>::type
{}
};
The template<int N> and N==N parts make sure std::enable_if has a dependent value and therefore doesn't complain until somebody actually tries to use A<T>::method2 with an incorrect T parameter.
And since this question and answer seem to still be getting attention, a much later edit to add that in C++20, you can simply do:
#include <type_traits>
template<typename T>
struct A {
void method1() {}
void method2() requires std::is_same_v<T, int> {}
};
The specialisation replaces the generic template. So A<int> will only have method2() and, of course, A<double> will only have method1().
Look at the code:
template <class x> struct Foo
{
int getX(x *p) { return(0); }
enum E12 { a };
};
template <> int Foo<int>::getX(int*)
{
return(-15);
}
template <> enum Foo<int>::E12
{
a, b, c
}
As it was discussed in Cannot overload function, the first specialization is legal and even works in MSVC. While the second specialization for enum does not even want to compile, saying "error C2988: unrecognizable template declaration/definition".
It seems to me that C++ is making relaitively unlogical exception for methods. Enum is just an example. The same thing can be applied to member classes, typedefs, etc.
I will be happy is some body will comment on this.
This is a very obscure new feature of C++11. File a bug report with Microsoft, although it is unlikely it will be given priority as almost nobody is aware this is allowed. The correct syntax would be
template <class x> struct Foo
{
int getX(x *p) { return(0); }
enum E12 { a };
};
template <> int Foo<int>::getX(int*)
{
return(-15);
}
template <> enum Foo<int>::E12
{
a, b, c
};
I've filed a bug with GCC. Can someone test on recent Clang?
In C++03, only classes and functions may be explicitly specialized. From the standard, C++03 14.7.3/1:
An explicit specialization of any of the following:
function template
class template
member function of a class template
static data member of a class template
member class of a class template
member class template of a class or class template
member function template of a class or class template
can be declared by a declaration introduced by template<>
A member enum is not such a case. (Generally speaking, an enum type is always defined only once at its first declaration.)
To obtain a templated enum or typedef, you can wrap it in a class template. In your case, it would be a member class template of Foo. Such a construct is called a metafunction.
C++11 also has alias templates, which are like templated typedefs, but they cannot be explicitly specialized.
The policy of only allowing classes and functions to be specialized, and then allowing such templates to encapsulate other things like enum and typedef, seems more consistent to me than allowing direct specialization of enum. But, perhaps the language is going in your preferred direction.
This code:
template <class T>
class Foo {};
typedef Foo<void*> Bar;
template <class T>
class Foo<T*> : public Bar {};
// use Foo<int*> somewhere.
Compiles and works fine in MSVC 9.0, but doesn't compile in GCC 4.1.1 or GCC 4.3.4, with the error:
error: invalid use of undefined type 'class Bar'
Is this illegal C++ that MSVC accepts incorrectly, or a limitation of GCC?
Either way, how can I work around this get the desired behaviour: pointer specialisations of Foo that inherit from unspecialised Foo<void*>?
You cannot do that, except by writing the specialization for all T*, except when T is void. Otherwise, you will derive the class from itself, which for obvious reasons can't work.
Instantiating the primary class template for arguments that have an explicit or partial specialization is not possible. If you try to, by provoking an instantiation before the explicit or partial specialization is visible (note that your code did not provoke such an instantiation), your program is ill-formed, with no diagnostic being required (which effectively renders the behavior undefined).
To achieve the above work-around, you can use SFINAE
template <class T>
struct isnt_void { typedef void type; };
template<> struct isnt_void<void> { };
template <class T, class = void>
class Foo {};
template <class T>
class Foo<T*, typename isnt_void<T>::type> : public Foo<void*> {};
The typedef is a red herring.
The following code is equivalent:
template <class T>
class Foo {};
template <class T>
class Foo<T*> : public Foo<void*> {};
It should be clear that, although Foo<T*> is declared at this point, it is not defined. And thus you may not use it as a base.
[class.name] (2003 wording, 9.1/2):
A class definition introduces the class name into the scope where it is defined
[class.mem] (2003 wording, 9.2/2):
A class is considered a
completely-defined object type (3.9)
(or complete type) at the closing } of
the class-specifier. Within the class
member-specification, the class is
regarded as complete within function
bodies, default arguments and
constructor ctor-initializers
(including such things in nested
classes). Otherwise it is regarded as
incomplete within its own class
member-specification.
[class.derived] (2003 wording, 10/1):
The class-name in a base-specifier shall not be an incompletely defined class (clause 9);
A superior solution would be to compose of Foo<void*>. After all, you don't want the raw void* interface cluttering up your stuff, and you don't want a Foo<T*> to be convertible to a Foo<void*>.
Alternatively, you could fully specialize Foo<void*> beforehand.
Assuming, of course, that you're doing this for type folding, instead of because you actually want inheritance.
template <typename T>
struct A
{
template <typename U>
struct B;
template <>
struct B<int> {static const int tag = 1;}; // Works fine in VS2010
};
How can I specialize B the same way, but outside of A. I tried this with no success :
template <typename T> template <>
struct A<T>::B<int> {static const int tag = 1;};
I get:
error C3212: 'A<T>::B<int>' : an explicit specialization of a template member must be a member of an explicit specialization
It does not make sense since I can do exactly that by defining it inside the class
VS2010 problem? Wrong syntax?
Thanks
PS: This one (which should be wrong anyway, crashes VS2010):
template <> template <typename T>
struct A<T>::B<int> {static const int tag = 1;};
To quote the C++ spec, §14.17.3.18:
In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialzed, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. [...]
(my emphasis)
This suggests that you can't specialize a template class nested inside another template class unless the outer template class is specialized as well. So it looks like VS2010 has this behavior wrong and g++ has it right.
It just doesn't work that way.:-(
You cannot specialize a function inside the class declaration, even though msvc accepts this with its default settings.
You also cannot specialize a member function without also specializing the enclosing class. Most compilers avred on this (as does the language standard).