Pointer as template argument - how to declare pointer to const inside template

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When a pointer type is passed as argument to template parameter T, how do I declare a pointer to const type? Both const T and T const become const pointer to type, whereas I need to declare a pointer to const type.

template<typename ValueType>
class TestClass {
public:
    void TestMethod(const ValueType x) {
        // When ValueType is int*,
        // type of x is int * const;
        // how do I declare x such that it is const int* ?
        std::cout<<x;
    }
};
templates c++
1 Answers

You can create a trait that does it.

template <typename T>
struct add_inner_const { using type = const T; };

template <typename T>
struct add_inner_const<T*> { using type = const T*; };

template <typename T>
using add_inner_const_t = typename add_inner_const<T>::type;

template<typename ValueType>
class TestClass {
public:
    void TestMethod(add_inner_const_t<ValueType> x) {
        // When ValueType is int*, x is const int*
        std::cout<<x;
    }
};

This will transform int ** into int * const *, if you instead want that to be const int **, then you need a different specialisation

template <typename T>
struct add_inner_const<T*> { using type = add_inner_const_t<T>*; };
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