CODEWITHSUNDEEP
c++templatesc++11type-traitstypename
HostileFork says dont trust SE
HostileFork says dont trust SE

Reputation: 33607

Using operations like std::remove_pointer to build derivative types in templates

So when I have code like:

shared_ptr<Foo> bar (my_normal_operator<Foo>(mumble));

Even though the type Foo is coming out of left field, it works as the return type is produced solely through an "additive" pattern to what is given:

template <typename Target, typename Source>
shared_ptr<Target> my_normal_operator(Source src)
{
    /* ... whatever ... */
}

But what if the situation instead looked something like this:

shared_ptr<Foo> bar (my_pointer_operator<Foo*>(mumble));

It needs some way to pull the pointer off the type. I dug around and found std::remove_pointer, but a naive application gives a "type/value mismatch":

template <typename Target, typename Source>
shared_ptr< std::remove_pointer<Target>::type > my_pointer_operator(Source src)
{
    /* ... whatever ... */
}

I didn't actually expect it to work...but I'm putting it here as an expression of the what-I'm-looking for intent!

Sigh. Every time I step into any new territory with templates and traits I feel like one of those "I have no idea what I'm doing" meme animals. :-/

Upvotes: 2

Views: 1452

Answers (1)

GManNickG
GManNickG

Reputation: 503933

You need typename:

template <typename Target, typename Source>
shared_ptr< typename std::remove_pointer<Target>::type >
    my_pointer_operator(Source src)
{
    /* ... whatever ... */
}

Because the type of std::remove_pointer<Target>::type depends on a template argument.

Personally, I would leave Target as Foo and within the definition of my_pointer_operator use typename std::add_pointer<Target>::type, so the caller can specify the return value more directly. The function name gives away the difference in implementation.

Upvotes: 3

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