CODEWITHSUNDEEP
c++c++17type-alias
Verwirrt
Verwirrt

Reputation: 413

Why can't I access and use a member type aliases from an object?

Is it impossible to access a type alias (or typedef) of a templated class, from an object of that class? For example, why isn't the following possible:

template <typename TKey, typename TData>
class MyClass
{
public:
    using key_t = TKey;
    using data_t = TData;
    TKey key;
    TData data;
    MyClass(TKey _key, TData _data) : key(_key), data(_data) {  }
};

int main() {
    MyClass<int, float> mc{1, 1.0f};
    using DataType = typename mc.data_t; //error: expected ';' after alias declaration
    mc.data_t newData = 2.0f; //error: Cannot refer to type member in 'MyClass<int,float> with '.'

    return 0;
}

Is there another way to do something like this?

Upvotes: 0

Views: 286

Answers (1)

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122133

You got the syntax wrong. Change it to:

int main() {
    MyClass<int, float> mc{1, 1.0f};
    using DataType = typename MyClass<int,float>::data_t;
    MyClass<int,float>::data_t newData = 2.0f;
    return 0;
}

If you want to get the aliases from mc directly, you can using DataType = decltype(mc)::data_t;. You can actually avoid any repetition of MyClass<int,float> :

int main() {
    MyClass<int, float> mc{1, 1.0f};  
    
    // suppose we cannot or do not want to spell out MyClass<int,float> again...  
    using my_class_t = decltype(mc);
    using data_t = my_class_t::data_t;
    data_t newData = 2.0f;
}

Upvotes: 6

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