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I have templated class which has a function (myfunc()) which does the same on every case of T except some cases (eg. bool). I have a working solution, based on this question:

template <class T> class opt_arg{
    private: void myfunc(){
     /*Do generic stuff */
    }

/* Can I insert here the bool specialization? */

};

/* The specialization "inserted outside of the class body": */
template<>
 inline void opt_arg<bool>::myfunc(){
    /* Do bool specific stuff*/
 }

As I have mentioned, it is working fine. I am just wondering, that can I insert the function specialization inside the "class body"?

I have templated class which has a function (myfunc()) which does the same on every case of T except some cases (eg. bool). I have a working solution, based on this question:

template <class T> class opt_arg{
    private: void myfunc(){
     /*Do generic stuff */
    }

/* Can I insert here the bool specialization? */

};

/* The specialization "inserted outside of the class body": */
template<>
 inline void opt_arg<bool>::myfunc(){
    /* Do bool specific stuff*/
 }

As I have mentioned, it is working fine. I am just wondering, that can I insert the function specialization inside the "class body"?

• c++
• templates
• specialization

Share Improve this question Follow edited Feb 25 at 10:32 wohlstad 29.2k1616 gold badges5959 silver badges9090 bronze badges asked Feb 25 at 10:21 Tom SolidTom Solid 2,48611 gold badge1616 silver badges3434 bronze badges 2

• 1 If I get it right, I think you can't under timsong-cpp.github.io/cppwp/n4861/temp.expl.spec#5: " may be explicitly specialized for a class specialization that is implicitly instantiated; in this case, the definition of the class template shall precede the explicit specialization for the member of the class template" – Oersted Commented Feb 25 at 10:53
• 2 I feel compelled to amend my previous comment (if it was correct). The impossibility I mentioned would be with a similar syntax. However, functionally, there is indeed the solution proposed by @wohlstad. – Oersted Commented Feb 25 at 11:43

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1 Answer 1

Sorted by: Reset to default 16

From C++17 you can use if constexpr (which is resolved at compile time) to achieve a similar result:

template <class T> class opt_arg {
private: 
    void myfunc() {
        if constexpr (std::same_as<T, bool>) {
            // specialized stuff
        }
        else {
            // generic stuff
        }
    }
};

Live demo

As @JeremyRichards commented below, with if constexpr in this case the code in the false branch might not even compile (thanks to the compile time resolve) which can be useful.
More info about this can be found here: Why does the false branch of "if constexpr" get compiled? (summary: this is true only for if constexpr within a template, and the code should anyway not be ill-formed).

Note the usage of std::same_as concept which is available from C++20. For C++17, you can use the type trait std::is_same (and the helper std::is_same_v).

You can add more if constexpr branches for other types if you need (or treat several types in the same branch).

This way the logic for both cases is in one place, similarly to what you wanted.

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