Specify template argument by subclassing

Question

When I inherit from a templated class and provide the argument during the inheritence, how do I have to handle functions from the parent class, which use the template argument?

For example with this class as the parent:

template<typename T>
class Parent {
public:
    virtual T someFunction(const T& data);
};

and I try to inherit like this:

class CharVersion : Parent<char> {
public:
    T someFunction(const T& t) override {
        return 'c'
    }
};

How can I get the function in the child to act like:

char someFunction(const char& t) override {
    return 'c'
}

The compiler obviously doesn't know T in the subclass, so how can I express this?

Related: https://stackoverflow.com/q/57546714/56541 – David Z Feb 01 '21 at 01:18 — David Z, Feb 01 '21 at 01:18

score 1 · Accepted Answer · answered Feb 01 '21 at 01:21

When you write Parent<char>, the compiler effectively generates a class definition as if you had substituted char into the template text. That is, behind the scenes it's as if the parent class of CharVersion is the following template instantiation:

class Parent_char {
public:
    virtual char someFunction(const char& data);
};

You can write your subclass just as if you were extending that class. (Of course, the name of the generated template instantiation is not Parent_char, but that's unlikely to matter.)

Thanks, always appreciate some an explanation over just 'do this'. — AK_is_curious, Feb 01 '21 at 12:08

score 1 · Answer 2 · answered Feb 01 '21 at 02:05

1

class CharVersion : Parent<char> {
public:
    T someFunction(const T& t) override {
        return 'c'
    }
};

should be

class CharVersion : Parent<char> {
public:
    char someFunction(const char& t) override {
        return 'c'
    }
};

answered Feb 01 '21 at 02:05

Doga Oruc

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Specify template argument by subclassing

2 Answers2