HasNoExceptMemFun type trait

July 29, 2014

When creating policy based classes we can use std::enable_if with SFINAE and type traits as a mostly-works substitute for real concepts in order to enforce certain things about those policies. This is somewhat like the override keyword that clients can have to ensure that they conform exactly to part of an interface, including const ‘ness and noexcept .There does not appear to be any type traits to determine if a type’s member function is const or noexcept . This is important if you want to call a template argument’s member function and need to maintain a const or noexcept guarantee. noexcept in particular is particularly easy to get wrong as one could inadvertently call a function which throws an exception inside another noexcept func, risking calls to std::terminate .By mucking about with std::declval and perfect forwarding we can test a member function to see if it is noexcept .

template <class Object, class MemFun, class... Args> struct HasNoExceptMemFun {
  static constexpr bool value = noexcept (((std::declval <Object> ()).*(MemFun ()))(std::declval <Args> ()...));

template <class Object, class MemFun> struct EnableIfHasNoExceptMemFun :
  public std::enable_if <HasNoExceptMemFun <Object, MemFun>::value>::type

template <class T, typename = typename EnableIfHasNoExceptMemFun <T, decltype (&T::MyFunc)>::type> struct MyTemplate {
  void MyFunc () noexcept {
    T t;
    t.MyFunc ();

One might be tempted to say gesundheit after seeing something like that, but it can be decomposed fairly easily:

class... Args

is what’s known as a parameter pack and effectively allows you to have an arbitrary number of parameters to a template. We use this to grab the types of the arguments to the member function.

noexcept ()

Generates a const-expression based on whether or not a particular operation is noexcept. If any operation within noexcept () is not noexcept (true) then noexcept () will return constexpr false.

std::declval <Object>

Is the magic that allows us to skip having to “construct” Object in order to “call” MemFun inside the noexcept block. It simply turns a type into an rvalue reference. One might be posed to ask why such a seemingly unsafe construct (turn nothing into an rvalue reference?!) made its way into the standard, but it is effectively a function that does nothing in practice and is only there to help the compiler. With our new eval reference, we can go ahead and call MemFun

((std::declval <Object> ()).*(MemFun ()))

Since the rvalue reference returned by std::declval is a temporary, we need to put it in brackets. .* is the pointer-to-member operator which allows us to access some member by providing a pointer to it. Since MemFun is a type and not a pointer we need to “construct” it and also put it in braces since it is a temporary. Finally the entire operation needs to be put in brackets as operator() takes precedence over operator.*.

(std::declval <Args>...)

Turns out that std::declval can be used with parameter packs too, so we’re going to get “free” rvalue references to all of MemFun’s parameters.

Now that we’ve “called” MemFun with Object the value will be stored in the static bool const-expression value.

From there we can use it with std::enable_if.

std::enable_if <HasNoExceptMemFun <Object, MemFun>::value>::type

std::enable_if is basically one of the closest things we have to concepts at the moment. Effectively, it creates an erroneous substitution if the value passed as its first argument is false and a valid substitution if the value passed as its first argument is true. The second argument, if specified, represents the type that the nested typedef type will be.

Remember that HasNoExceptMemFun will be true if the arguments are a no-except member function of that type and false if they aren’t. Thus, it is only in the true situation that we’re able to inherit from type.

In order to use this, we define a small helper as follows (helps to avoid really long lines):

template <class Object, class MemFun> struct EnableIfHasNoExceptMemFun : public std::enable_if <HasNoExceptMemFun <Object, MemFun>::value>::type { };

And then we use that helper like so

template <class T, typename = typename EnableIfHasNoExceptMemFun <T, decltype (&T::MyFunc)>::type>

Remember that the MemFun template parameter is a type and not a pointer so we need to convert the actual member function pointer which we wish to check to its type. We can do that using decltype.

From there, we can finally be sure that in calling that member function of the template parameter that we’re not running the risk of having exceptions thrown, because we can guarantee that the function is noexcept.

Feel free to play with it here.

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Written by Sam Spilsbury an Australian PhD student living in Helsinki.