CODEWITHSUNDEEP
c++templates
BPL
BPL

Reputation: 9863

How to pass a template function as template class argument?

Here's the thing, it seems it's possible to pass a templatized struct which contains some static functions like this:

template <class T> struct FunctionHolder {};

template <> struct FunctionHolder<string> {
    static void f(const string &s) {
        cout << "f call " << s << endl;
    }
};

template <class Key, class Value, class Holder = FunctionHolder<Key>>
class Foo {
  public:
    Foo(Key k) {
        Holder::f(k);
    }
};

int main(int argc, char *argv[]) {
    Foo<string, int> foo = Foo<string, int>("test_string");
}

But, would it be possible to pass directly templatized functions without been defined statically on templatized structs? I've tried this and it won't compile:

template <class string> static void f(const string &s) {
    cout << "f call " << k << endl;
}

template <class Key, class Value, typename func<Key>> class Foo {
  public:
    Foo(Key k) {
        func(k);
    }
};

int main(int argc, char *argv[]) {
    Foo<string, int> foo = Foo<string, int>("test_string");
}

Asking this cos it's not cool to be forced to create dummy structures (structures containing a bunch of static functions) to be used as template type of the main class.

Upvotes: 1

Views: 154

Answers (1)

songyuanyao
songyuanyao

Reputation: 173014

Unfortunately function templates can't be used as template template arguments; you can use function pointer as the non-type template parameter instead, e.g.

template <class Key, class Value, void(*func)(const Key&) = f<Key>> class Foo {
  public:
    Foo(Key k) {
        func(k);
    }
};

LIVE

Upvotes: 2

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