Tick

Trait introspection and concept creator for C++11

Getting Started

Tick provides a mechanism for easily defining and using traits in C++11. For example, if we defined a generic increment function, like this:

template<class T> void increment(T& x) { x++; }

If we pass something that does not have the ++ operator to increment , we will get an error inside of the increment function. This can make it unclear whether the error is due to a mistake by the user of the function or by the implementor of the function. Instead we want to check the type requirements of the function.

Using Tick we can create an is_incrementable trait, like this:

TICK_TRAIT(is_incrementable) { template<class T> auto require(T&& x) -> valid< decltype(x++), decltype(++x) >; };

And then we can use a simple requires clause in our function to check the type requirements:

template<class T, TICK_REQUIRES(is_incrementable<T>())> void increment(T& x) { x++; }

So, now, 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 gives an error at the call to increment rather than inside the function, and then pointes to the type requirements of the function. This 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 the is_incrementable trait defined like this:

TICK_TRAIT(is_incrementable, std::is_integral<_>) { template<class T> auto require(T&& x) -> valid< decltype(x++), decltype(++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 *> >'