Writing Template function when called without template arguments

Question

So I was trying to write a default function version when there are no template arguments passed in the following code:

struct A{ int x; };

struct B : public A { int y;};

struct C {
    void set(const A& v) { v_ = v; }

    template <typename T>
    const T& get() { return reinterpret_cast<T&>(const_cast<A&>(v_)); }

    template <>
    const A& get() { return v_;}

    A v_;
};

int main() {
    B b;
    C c;
    c.set(b);
    (void)c.get<B>().y;
    (void)c.get<A>().x;
    //(void)c.get().x; << Error
}

But it doesn't work at the indicated Error line above with this error:

test.cpp:22:13: error: no matching member function for call to 'get'
    (void)c.get().x;
          ~~^~~
test.cpp:9:14: note: candidate template ignored: couldn't infer template argument 'T'
    const T& get() { return reinterpret_cast<T&>(const_cast<A&>(v_)); }
             ^
1 error generated.

I do understand the issue that since no template argument was passed in the client call, it was unable to infer the typename but is there a way to provide a default implementation like the one in the above code when that happens? Essentially I want to avoid writing client code as (void)c.get<A>().x; and instead, adopt the default route implicitly.

songyuanyao · Answer 1 · 2020-07-08T03:00:33.660

2

As an alternative solution, you can specify A as default template argument for parameter T, e.g.

struct C {
    void set(const A& v) { v_ = v; }

    template <typename T = A>
    const T& get() { return reinterpret_cast<T&>(const_cast<A&>(v_)); }

    template <>
    const A& get() { return v_;}

    A v_;
};

int main() {
    B b;
    C c;
    c.set(b);
    (void)c.get<B>().y; // call the primary template
    (void)c.get<A>().x; // call the specification
    (void)c.get().x;    // call the specification
}

edited Jul 08 '20 at 03:00

answered Jul 08 '20 at 02:45

songyuanyao

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This doesn't [compile](https://godbolt.org/z/gBYjq7). – cigien Jul 08 '20 at 02:47
@cigien This is another issue; [clang](https://wandbox.org/permlink/KKyUoksTw8epNNs1) compiles. – songyuanyao Jul 08 '20 at 02:48
1

@cigien [Here](https://stackoverflow.com/a/49707378/3309790)'re some informations. – songyuanyao Jul 08 '20 at 02:51

score 1 · Accepted Answer · answered Jul 08 '20 at 02:32

1

You can delete the template <> part.

struct C {
    void set(const A& v) { v_ = v; }

    template <typename T>
    const T& get() { return reinterpret_cast<T&>(const_cast<A&>(v_)); }

    const A& get() { return v_;}

    A v_;
};

answered Jul 08 '20 at 02:32

Stephen Newell

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I feel such ignominy directed at myself on seeing the answer. And here I was experimenting with `enable_if_t`. – Zoso Jul 08 '20 at 02:38
We all have those moments sometimes :) – Stephen Newell Jul 08 '20 at 02:38

Writing Template function when called without template arguments

2 Answers2