Trait introspection and concept creator for C++14
Tick provides a mechanism for easily defining and using traits in C++14. It is based on ideas developed by Eric Niebler in this blog post.
For example we could create an is_incrementable trait, like this:
TICK_TRAIT(is_incrementable)
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
x++,
++x
);
};And then we can use a simple requires clause in our functions:
template<class T, TICK_REQUIRES(is_incrementable<T>())>
void increment(T& x)
{
x++;
}So if we pass something that is not incrementable to increment:
struct foo {};
foo f;
increment(f);Then we get an error like this in clang:
demo.cpp:25:2: error: no matching function for call to 'increment'
increment(f);
^~~~~~~~~
demo.cpp:14:19: note: candidate template ignored: disabled by 'enable_if' [with T = foo]
template<class T, TICK_REQUIRES(is_incrementable<T>())>
^
This is pretty concise and gives enough information for most commons cases, however, sometimes we may want more information. In that case the TICK_TRAIT_CHECK can be used. For example, say we had a is_incrementable type defined like this:
TICK_TRAIT(is_incrementable, std::is_integral<_>)
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
x++,
++x
);
};
Then if we use TICK_TRAIT_CHECK, we can see why int* is not incrementable:
TICK_TRAIT_CHECK(is_incrementable<int*>);
Which will produce this error:
../tick/trait_check.h:95:38: error: implicit instantiation of undefined template 'tick::TRAIT_CHECK_FAILURE<std::is_integral<int *>, is_incrementable<int *> >'
Which shows the traits that failed including any refinements. So we can see that it failed because std::is_integral<int *> is not true.
This macro will build a boolean type trait for you. Each trait requires a requires member function of the form:
TICK_TRAIT(my_trait)
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
...
);
};The parameters to the trait are based on the parameters to the requires function. Then inside the TICK_VALID macro, all the expressions are placed that will be check for. If one of the expressions is not valid, the the trait will return false.
Refinements can be expressed after the name. Each refinement is a placeholder expression, where each placeholder(ie _1, _2, etc) are replaced by their corresponding type passed into the trait. In the case of traits that accept a single parameter the unnamed placeholder(_) can be used, for example:
TICK_TRAIT(is_incrementable, std::is_default_constructible<_>)
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
x++,
++x
);
};This trait will be true when x++ and ++x are valid expressions and x is default constructible.
When a trait has multiple parameters, its best to use named placeholders. For example:
TICK_TRAIT(is_equality_comparable,
std::is_default_constructible<_1>,
std::is_default_constructible<_2>)
{
template<class T, class U>
auto requires(T&& x, U&& y) -> TICK_VALID(
x == y,
x != y
);
};This trait will be true when x == y and x != y are valid expressions and both x and y are default constructible.
These can be used to query more information about the types then just valid expressions.
When a type is matched, it can either be convertible to the type given, or the evaluated placeholder expression must be true. Placeholder expressions can be given so the type can be matched against other traits.
The returns query can check if the result of the expressions matches the type. For example,
TICK_TRAIT(is_incrementable)
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
returns<int>(x++)
);
};This trait will be true if the expressions x++ is valid and is convertible to int.
Here's an example using placeholder expressions as well:
TICK_TRAIT(is_incrementable)
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
returns<std::is_integral<_>>(x++)
);
};This trait will be true if the expressions x++ is valid and returns a type that is_integral.
The has_type query can check if a type exist and if the type matches. For example:
TICK_TRAIT(has_nested_type)
{
template<class T>
auto requires(const T& x) -> TICK_VALID(
has_type<typename T::type>()
);
};This trait will be true if T has a nested type called type.
Also, an optionally second parameter can be provided to check if the type matches. Here's an example:
TICK_TRAIT(has_nested_int_type)
{
template<class T>
auto requires(const T& x) -> TICK_VALID(
has_type<typename T::type, std::is_integral<_>>()
);
};This trait will be true if T has a nested type called type which is an integral type.
The has_template query can check if a template exist. For example:
TICK_TRAIT(has_nested_result)
{
template<class T>
auto requires(const T& x) -> TICK_VALID(
has_template<T::template result>()
);
};This trait will be true if T has a nested template called result.
The is_true and is_false queries can check if a trait is true or false. Using refinements is the preferred way of checking for additional traits, but this can be useful if the evaluation of some trait can't be used lazily with placeholder expressions. So the is_true and is_false can be used instead, for example:
TICK_TRAIT(is_2d_array)
{
template<class T>
auto requires(const T& x) -> TICK_VALID(
is_true<std::is_same<std::rank<T>::type, std::integral_constant<std::size_t, 2>> >()
);
};The traits can be built using the TICK_TRAIT macros. Heres how to build them. First, build a class for the requires functions and inherit from tick::ops to bring in all the query operations:
struct is_incrementable_r : tick::ops
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
x++,
++x
);
};Next, turn it into a trait using tick::trait:
template<class... Ts>
struct is_incrementable
: tick::trait<is_incrementable_r(Ts...)>
{};Refinements can be used by using the tick::refines class:
struct is_incrementable_r
: tick::ops, tick::refines<std::is_default_constructible<tick::_>>
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
x++,
++x
);
};Notice, the placeholders have to be fully qualified here.
Three macros are provided to help improve the readability of template constraints.
The TICK_REQUIRES can be used on template parameters. For example,
template<class T, TICK_REQUIRES(is_incrementable<T>())>
void increment(T& x)
{
x++;
}This can also be used with classes.
The TICK_CLASS_REQUIRES can be used when template specialization is done on classes. For example,
template<class T, class=void>
struct foo
{
...
};
template<class T>
struct foo<T, TICK_CLASS_REQUIRES(is_incrementable<T>() and not std::is_integral<T>())>
{
...
};
template<class T>
struct foo<T, TICK_CLASS_REQUIRES(std::is_integral<T>())>
{
...
};The TICK_FUNCTION_REQUIRES can be used on functions. This requires placing parenthesis around the return type:
template<class T>
TICK_FUNCTION_REQUIRES(is_incrementable<T>())
(void) increment(T& x)
{
x++;
}Note: The TICK_REQUIRES should be preferred.
The TICK_TRAIT_CHECK macro will statically assert the list of traits are true but it will show what traits failed including base traits. This can be useful to show more informative messages about why a trait is false.
This requires a C++14 compiler. There a no third-party dependencies. This has been tested on clang 3.4 and gcc 4.9.
ZLang is supported for some of the macros. The macros are in the tick namespace. For example,
$(trait is_incrementable)
{
template<class T>
auto requires(T&& x) -> TICK_VALID(
x++,
++x
);
};