Is there a way to check what datatype a template is?

Question

Say I have the following code:

template<typename K, typename V>
    int Hash<K, V>::hf(const K& key)
    {
        if(K == typeid(string))
        {
            return MurmurHash2(key.c_str(), key.size());
        }
        else
        {
            return key*2654435761;
        }

}

Is it possible to do this in some way? If not could you recommend a method to accomplish the same thing?

@isedev could you give an example of template specialization ive never used it before. — Iriketurtles, Mar 30 '16 at 21:29
@SergeyA: wouldn't overloading introduce unnecessary code when `K` is not a `string` (inlining and optimisations apart)? — isedev, Mar 30 '16 at 21:32
@Iriketurtles this may help explain what isedev writes of: http://en.cppreference.com/w/cpp/language/template_specialization — user4581301, Mar 30 '16 at 21:33
@lriketurtles: added a (hopefully correct) example as an answer. — isedev, Mar 30 '16 at 21:40

score 0 · Answer 1 · answered Mar 30 '16 at 21:40

0

You could use (partial) template specialisation:

// general case
template <typename K, typename V>
    int Hash<K, V>::hf(const K& key)
    {
        return key*2654435761;
    }

// string case
template <typename V>
    int Hash<std::string, V>::hf(const std::string& key)
    {
        return MurmurHash2(key.c_str(), key.size());
    }

answered Mar 30 '16 at 21:40

isedev

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Im getting some errors saying unable to match function definition to an existing deceleration – Iriketurtles Mar 30 '16 at 22:30

Rakete1111 · Answer 2 · 2016-03-31T16:33:36.043

0

Here are 2 ways of doing it:

1) Template specialization (making a special case for a template argument (here: std::string))

template<typename K, typename V>
int Hash(const K& key)
{
    return key * 2654435761;
}

template<typename V>
int Hash(const std::string& key)
{
    return MurmurHash2(key.c_str(), key.size());
}

2) Using typeid to compare the types

template<typename K, typename V>
int Hash(const K& key)
{
    if (typeid(K).hash_code() == typeid(std::string).hash_code())
        return MurmurHash2(key.c_str(), key.size());
    return key * 2654435761;
}

edited Mar 31 '16 at 16:33

answered Mar 30 '16 at 22:27

Rakete1111

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1

This is different from actual question, when hf is member of the class. There is also no guarantee that comparing `typeid()` of the same type will yield true. – SergeyA Mar 31 '16 at 13:06
Thanks! `hash_code()` is guaranteed to be the same for the same types. – Rakete1111 Mar 31 '16 at 16:34

Is there a way to check what datatype a template is?

2 Answers2