In the current draft of the C++ standard, there is the following paragraph ([temp] p.6]):
A template name has linkage. Specializations (explicit or implicit) of a template that has internal linkage are distinct from all specializations in other translation units. A template, a template explicit specialization, and a class template partial specialization shall not have C linkage. Use of a linkage specification other than "C" or "C++" with any of these constructs is conditionally-supported, with implementation-defined semantics. Template definitions shall obey the one-definition rule. [ Note: Default arguments for function templates and for member functions of class templates are considered definitions for the purpose of template instantiation ([temp.decls]) and must also obey the one-definition rule. — end note ]
I do not understand what the highlighted part means. How could I break the one definition rule using default arguments? Is there a way to "re-define" them?
The OP seems to be asking a few different questions:
What does the first part ("considered definitions for the purpose of template instantiation") mean?
What does the second part ("must also obey the one-definition rule") mean?
Isn't the part about the one-definition rule redundant with [basic.def.odr]/14.4 (which, at the time, was known as [basic.def.odr]/12.1)?
Assuming that the intent is e.g. to prohibit the same function template from having two different default arguments for the same parameter in different translation units such that the two default arguments don't have the same value, is this issue specific to templates? If not, then why does it need to be mentioned here at all?
Answer to question 1
That part of the note references [temp.decls], and [temp.decls.general]/3 elaborates:
For purposes of name lookup and instantiation, default arguments, type-constraints, requires-clauses (13.1), and noexcept-specifiers of function templates and of member functions of class templates are considered definitions; each default argument, type-constraint, requires-clause, or noexcept-specifier is a separate definition which is unrelated to the templated function definition or to any other default arguments, type-constraints, requires-clauses, or noexcept-specifiers. For the purpose of instantiation, the substatements of a constexpr if statement (8.5.2) are considered definitions.
Thus, it seems to be simply reminding the reader that default arguments are subject to instantiation separately from the function templates (or member functions of class templates) that they belong to.
Answer to question 2
The part about the one-definition rule appears to be reminding the reader that if the same default argument of a function template (or member function of class template) appears in multiple translation units, those multiple definitions must be "the same" in the sense of the one-definition rule ([basic.def.odr]/14). xskxzr already gave an example of how this could potentially be violated:
// TU 1
template <typename T> void foo(int = 0);
// TU 2
template <typename T> void foo(int = 1);
Answer to question 3
In the current standard, default arguments are a kind of "definable item" ([basic.def.odr]/1.6) so the rule that multiple definitions in different translation units must be identical applies to them just as it does to class definitions, inline function definitions, and so on ([basic.def.odr]/14). This does make the note redundant, which is fine. Notes are non-normative, so they ought to be redundant. Notes are often used to clarify normative text or to draw the reader's attention to a possibly surprising consequence of the normative text.
Answer to question 4
The first half, about instantiation, is obviously specific to templates. The second half is not now specific to templates, but it used to be. In C++20, it was made illegal for the same function parameter to be given different default arguments by different translation units; prior to C++17, it was legal. (This is [diff.cpp17.dcl.dcl]/2 in the "Compatibility" section of C++20.)
To be specific, from C++98 through C++17, therefore, it was permitted for non-template functions to have different default arguments in different translation units, but it was not permitted for function templates; therefore, the rule about default arguments being definitions that are subject to the one-definition rule was specific to function templates (and member functions of class templates). The wording of the note has not changed since C++98.
It might be more clear if the wording of the note were now changed to:
Default arguments for function templates and for member functions of class templates are considered definitions for the purpose of template instantiation ([temp.decls]) and, like default arguments of non-templated functions, must also obey the one-definition rule.
I don't think this change is necessary, though. The standard cannot be expected to be a particularly well-written document, as it is maintained by volunteers who already have their hands full with determining what the normative rules should be. You should expect that there will be times when you will look at something in the standard and wonder "why isn't this written in a different way?" And usually there will be some historical reason, but even if you can't figure out what that reason is, you shouldn't let it detract from your ability to understand what the standard is saying.
Related
[Disclaimer #1: I'm actually not referring to the C++ standard; I'm referring to cppreference's "Definitions and ODR (One Definition Rule)".]
[Disclaimer #2: I'm actually too lazy to try it, which I defend in my mind by referring to the standard C++ disclaimer that ODR violations are not required to be diagnosed, so I probably won't learn anything by trying it.]
The C++ standard (as disclaimed in #1 above) says that one of the requirements for not being an ODR violation is "each definition consists of the same sequence of tokens".
Which seems pretty clear, except, I'm curious:
In declaring the type template parameters of a template, is substituting typename for class in two different definitions (in two different translation units, etc.) disallowed as well?
Because those keywords are generally interchangeable.
(I know about the special case that you can't substitute typename for class in a template template parameter in any event.)
typename and class are not the same token. So yes, if two definitions for the same entity differ on that, then it is an ODR violation.
The ODR rule is written with the expectation that template/inline definitions are only written out once in a header file which is then included in other files, so that such variations shouldn't happen.
struct A{
int a;
};
struct B{
int b;
};
auto&& [x] = A{}; //#1
auto&& [x] = B{}; //#2
int main(){
}
In this example, all compilers give an error that the x at #2 conflicts with that introduced at #1. However, IIUC, there's no rule in the post-C++20 working draft standard which can interpret what's the reason.
First, in my opinion, the declaration at #2 and the declaration at #1 declare the same entity. They correspond due to:
basic.scope#scope-3
Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless
[...]
They declare the same entity per basic.link#8
Two declarations of entities declare the same entity if, considering declarations of unnamed types to introduce their names for linkage purposes, if any ([dcl.typedef], [dcl.enum]), they correspond ([basic.scope.scope]), have the same target scope that is not a function or template parameter scope, and either
they appear in the same translation unit, or
[...]
So, as far as now, they declare the same entity and they shouldn't be considered as potentially conflict per basic.scope#scope-4
Two declarations potentially conflict if they correspond and cause their shared name to denote different entities([basic.link]). The program is ill-formed if, in any scope, a name is bound to two declarations that potentially conflict and one precedes the other ([basic.lookup]).
Since they denote the same entity, as aforementioned, they do not potentially conflict.
They still do not violate this rule:
basic.link#11
For any two declarations of an entity E:
If one declares E to be a variable or function, the other shall declare E as one of the same type.
[...]
Since structured bindings are not mentioned in this list, they do not violate this rule. Similar, they do not violate One-definition rule
No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, template, default argument for a parameter (for a function in a given scope), or default template argument.
At least, according to what the relevant rules say, the two declarations in this example shouldn't result in any program ill-formed. If I don't miss some other rules, Can it be considered as vague in the standard which cannot interpret why two structured binding declarations conflict with each other in this case? This case is more underspecified in the N4861
This is just a missing case in [basic.link]/11: that if one (of the two declarations) declares a structured binding, the program is ill-formed. (One could alternatively merely require that the other also declare a structured binding and then extend the list in [basic.def.odr]/1, but that’s more complicated and suggests that it might be possible to redefine it in another translation unit.)
You are citing text from a working draft of a post-C++20 version of the language. As such, the behavior it describes is not likely implemented by any compiler currently existing. As it is a working draft, it likely contains a number of language defects and/or bugs, so trying to learn from it is not a productive activity.
All of the "correspond" language you cite is adopted from P1787, which is not part of any C++ standard actual compilers implement. As such, compilers are providing you with the C++20 functionality, and under those rules, these clearly conflict.
There may be some defective wording in P1787, but that's expected with complex proposals and working drafts of a standard. File a defect report on it.
I'm looking for a way to automatically make default template parameter be unique each time a template is instantiated. Since unnamed function objects created by lambda expressions have different types I thought of adopting them somehow. With recent changes to standard daft removing "A lambda-expression shall not appear in ... a template-argument" restriction (see Wording for lambdas in unevaluated contexts) it seemed like a good idea. So I wrote the following kinda working snippet that compiles on recent gcc and clang:
#include <type_traits>
template<void ( * ) (void) = [](){}> class
unique final {};
static_assert(false == ::std::is_same_v<unique<>, unique<>>);
int main()
{
return 0;
}
Is this a viable approach or one of those "ill-formed, no diagnostic is required" cases?
Some additional context: I want to use this to implement Ada-style strong type definitions that should work in a single translation unit without manually inventing unique tags that would be otherwise unused:
struct _tag_WowInt {};
using Int = type<int, _tag_WowInt>;
struct _tag_SoUnique {};
using DifferentInt = type<int, _tag_SoUnique>;
Upd1: I would like to mention that approaches involving __COUNTER__ or similar macros won't work in general case because they will be expanded by preprocessor only once and won't yield unique types when used inside of template for example.
I believe that you are right, it seems to me that is "ill-formed, no diagnostic required". I think this is covered by
[temp.res/8.4] and [temp.res/8.5]:
(8.4) ― a hypothetical instantiation of a template immediately following its definition would be ill-formed due to
a construct that does not depend on a template parameter, or
(8.5) ― the interpretation of such a construct in the hypothetical instantiation is different from the interpretation
of the corresponding construct in any actual instantiation of the template. [Note: This can happen in situations including the following:
(8.5.1) ― a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is performed, or
(8.5.2) ― lookup for a name in the template definition found a using-declaration, but the lookup in the corresponding scope in the instantiation does not find any declarations because the using-declaration was a pack expansion and the corresponding pack is empty, or
(8.5.3) ― an instantiation uses a default argument or default template argument that had not been defined at the point at which the template was defined, or
(8.5.4) ― constant expression evaluation within the template instantiation uses
(8.5.4.1) ― the value of a const object of integral or unscoped enumeration type or
(8.5.4.2) ― the value of a constexpr object or
(8.5.4.3) ― the value of a reference or
(8.5.4.4) ― the definition of a constexpr function,
and that entity was not defined when the template was defined, or
(8.5.5) ― a class template specialization or variable template specialization that is specified by a non-dependent simple-template-id is used by the template, and either it is instantiated from a partial specialization that was not defined when the template was defined or it names an explicit specialization that was not declared when the template was defined.
— end note]
Even though your use case is not explicitly listed in the examples of the note, in my understanding the requirement implies that unique<> must refer to the same thing throughout the whole program, otherwise it is ill-formed, no diagnostic required.
This was CWG1850. The Committee appear to dislike this
kind of stateful meta-programming. The constexpr counter no longer works
in newer versions of the compilers.
Can the following be written in a header file:
inline void f () { std::function<void ()> func = [] {}; }
or
class C { std::function<void ()> func = [] {}; C () {} };
I guess in each source file, the lambda's type may be different and therefore the contained type in std::function (target_type's results will differ).
Is this an ODR (One Definition Rule) violation, despite looking like a common pattern and a reasonable thing to do? Does the second sample violate the ODR every time or only if at least one constructor is in a header file?
This boils down to whether or not a lambda's type differs across translation units. If it does, it may affect template argument deduction and potentially cause different functions to be called - in what are meant to be consistent definitions. That would violate the ODR (see below).
However, that isn't intended. In fact, this problem has already been touched on a while ago by core issue 765, which specifically names inline functions with external linkage - such as f:
7.1.2 [dcl.fct.spec] paragraph 4 specifies that local static variables and string literals appearing in the body of an inline function with
external linkage must be the same entities in every translation unit
in the program. Nothing is said, however, about whether local types
are likewise required to be the same.
Although a conforming program could always have determined this by use
of typeid, recent changes to C++ (allowing local types as template
type arguments, lambda expression closure classes) make this question
more pressing.
Notes from the July, 2009 meeting:
The types are intended to be the same.
Now, the resolution incorporated the following wording into [dcl.fct.spec]/4:
A type defined within the body of an extern inline function is the same type in every translation unit.
(NB: MSVC isn't regarding the above wording yet, although it might in the next release).
Lambdas inside such functions' bodies are therefore safe, since the closure type's definition is indeed at block scope ([expr.prim.lambda]/3).
Hence multiple definitions of f were ever well-defined.
This resolution certainly doesn't cover all scenarios, as there are many more kinds of entities with external linkage that can make use of lambdas, function templates in particular - this should be covered by another core issue.
In the meantime, Itanium already contains appropriate rules to ensure that such lambdas' types coincide in more situations, hence Clang and GCC should already mostly behave as intended.
Standardese on why differing closure types are an ODR violation follows. Consider bullet points (6.2) and (6.4) in [basic.def.odr]/6:
There can be more than one definition of […]. Given such an entity named D defined in more than one translation unit, then each definition of D shall consist of the
same sequence of tokens; and
(6.2) - in each definition of D, corresponding names, looked up
according to [basic.lookup], shall refer to an entity defined within
the definition of D, or shall refer to the same entity, after
overload resolution ([over.match]) and after matching of partial
template specialization ([temp.over]), […]; and
(6.4) - in each definition of D, the overloaded operators referred to,
the implicit calls to conversion functions, constructors,
operator new functions and operator delete functions, shall refer to
the same function, or to a function defined within the definition of
D; […]
What this effectively means is that any functions called in the entity's definition shall be the same in all translation units - or have been defined inside its definition, like local classes and their members. I.e. usage of a lambda per se is not problematic, but passing it to function templates clearly is, since these are defined outside the definition.
In your example with C, the closure type is defined within the class (whose scope is the smallest enclosing one). If the closure type differs in two TUs, which the standard may unintentionally imply with the uniqueness of a closure type, the constructor instantiates and calls different specializations of function's constructor template, violating (6.4) in the above quote.
UPDATED
After all I agree with #Columbo answer, but want to add the practical five cents :)
Although the ODR violation sounds dangerous, it's not really a serious problem in this particular case. The lambda classes created in different TUs are equivalent except their typeids. So unless you have to cope with the typeid of a header-defined lambda (or a type depending on the lambda), you are safe.
Now, when the ODR violation is reported as a bug, there is a big chance that it will be fixed in compilers that have the problem e.g. MSVC and probably some other ones which don't follow the Itanium ABI. Note that Itanium ABI conformant compilers (e.g. gcc and clang) are already producing ODR-correct code for header-defined lambdas.
In C++ it's OK to have a funcction that takes a function local type:
int main() {
struct S { static void M(const S& s) { } };
S s;
S::M(s);
}
but not OK to have a template that does:
template<typename T> void Foo(const T& t) { }
int main() {
struct S { } s;
Foo(s); // Line 5: error: no matching function for call to 'Foo(main()::S&)'
}
14.3.1 paragraph 2 in the c++ standard.
A type with no linkage [...] shall not be used as a template-argument for a template type-parameter
Why does C++ disallow that?
The best explanation I've heard so far it that inner types have no linkage and that this could imply that a function that takes them as an arg must have no linkage. But there is no reason I can see that a template instantiation must have linkage.
p.s. Please don't just say "thats not allowed because the standard says it's not"
I believe the difficulty that was foreseen was with two instantiations of Foo<T> actually meaning entirely different things, because T wasn't the same for both. Quite a few early implementations of templates (including cfront's) used a repository of template instantiations, so the compiler could automatically instantiate a template over a required type when/if it was found that an instantiation over that type wasn't already in the repository.
To make that work with local types, the repository wouldn't just be able to store the type over which the template was instantiated, but instead it would have to do something like creating a complete "path" to the type for the instantiation. While that's probably possible, I think it was seen as a lot of extra work for little (if any) real benefit.
Since then, the rules have changed enough that the compiler is already required to do something that's just about equivalent, finding (and coalescing) instantiations over the same type at different places (including across TUs) so that two instantiations of foo<int> (for example) don't violate the ODR. Based on that realization, the restriction has been loosened in (the current draft of) C++0x (you still can't instantiate a template class over a local type, but you can use a local type as parameter to a template function).
I'm guessing it is because it would require the template to be effectively instantiated within the scope of the function, since that is where such types are visible. However, at the same time, template instantiations are supposed to act as if they are in the scope in which the template is defined. I'm sure this it's possible to deal with that somehow, but if I'm right the standards body decided not to put that burden on compiler writers.
A similar decision was the reason vector<vector<int>> is invalid syntax per the standard; detecting that construction requires some interaction between compiler lexer and parser phases. However, that's changing, because the C++0x standards folk found that all the compilers are detecting it anyway to emit sane error messages.
I suspect that if it were to be demonstrated that allowing this construction was trivial to implement, and that it didn't introduce any ambiguities in the language scoping rules, you might someday see the standard changed here too.