Extract first template parameter type from any object-CodePudding

Suppose I had a templated object in C Test<T> and I wanted to find out what the T value is so that when I pass Test<T> as a template argument to TestWrapper<Test<T>>, I can declare a variable called T extradata in the object TestWrapper that is the same as Test's T type.

How can I achieve this by only modifying TestWrapper (i.e. making no changes to Test struct) ? Additionally is it possible to modify it so that it will extract from any object that takes 1 template parameter e.g. foo<T>, bar<T>, rather then just Test<T>>?

Thanks in advance.

template<typename T>
struct Test {
     T value;
};


//template -> extract first type parameter from any object as T
struct TestWrapper {
     T extradata;
};

int main() {

     TestWrapper<Test<int>> temp;
     temp.extradata = 123; // temp.extradata is a int, deduced from Test<int>
}

CodePudding user response：

You can use partial template specialization to do that.

template<typename T>
struct Test {
     T value;
};

template <typename T>
struct TestWrapper;

template <template <typename> typename Outer, typename T>
struct TestWrapper<Outer<T>> {
     T extradata;
};

int main() {
     TestWrapper<Test<int>> temp;
     temp.extradata = 123; // temp.extradata is a int, deduced from Test<int>
}

In this example we have not defined any default definition for TestWrapper, so it will fail to compile if you give something that does not match the specialization.

CodePudding user response：

You can

// primary template
template <typename T>
struct TestWrapper {
    T extradata; // declare extradata with type T
                 // how to declare it depends on your intent
};

// partial specialization
template <template <typename...> class C, typename T, typename... Args>
struct TestWrapper<C<T, Args...>> {
     T extradata; // T is the 1st template parameter of C here
};

CodePudding user response：

You can create a helper class to extract the first parameter of the instantiated template, for example:

template<class>
struct Extract;

template<template<class...> class V, class First, class... Rest>
struct Extract<V<First, Rest...>> {
  using type = First;
};

template<class T>
struct TestWrapper {
  typename Extract<T>::type extradata;
};

Demo.