CODEWITHSUNDEEP
c++templatestype-traits
lazug
lazug

Reputation: 53

How to write a type trait method

I defined a templated class (DataArray<T>) and I want to define a min() function to compute the minimum value of integral type array (double, float, int, ...) or complex type array (std::complex<double>, std::complex<float>, ...).

I am attempting to use type traits to select the correct function. Despite of the excellent discussion here, my code does not compile:

DataArray<double> and DataArray<std::complex<double>>: no matching overloaded function

What is the problem? Here is the minimal part of my code:

#include <iostream>
#include <vector>
#include <complex>
#include <type_traits>

template <typename T>
class DataArray {
public:
    DataArray(T * data) : m_data(data) {}

    template<typename T>
    using isComplex = std::is_same<T, std::complex<typename T::value_type>>;

    template <typename T>
    typename std::enable_if<isComplex<T>::value>::type min() {
        std::cout << "Min for complex" << std::endl;
    }

    template <typename T>
    typename std::enable_if<std::is_arithmetic<T>::value>::type min() {
        std::cout << "Min for arithmetic values." << std::endl;
    }

private:
    T * m_data;
};



int main()
{
    double v1[2] ={ 1., 2.};
    DataArray<double> d1(v1);

    d1.min();  // min function for double : **compilation error** 

    std::complex<double> v2[2] = { { 1, 2 },{ 3, 4 } };
    DataArray<std::complex<double>> d2(v2);

    d2.min();  // min function for complex<double> : compilation error 
}

Upvotes: 5

Views: 335

Answers (1)

songyuanyao
songyuanyao

Reputation: 173014

The template parameter T of member templates shadows the template parameter T of class template. Give them another name; and specify default value for template parameter of min(), otherwise they can't be deduced. e.g.

template<typename X>
using isComplex = std::is_same<X, std::complex<typename X::value_type>>;

template <typename X = T>
typename std::enable_if<isComplex<X>::value>::type min() {
    std::cout << "Min for complex" << std::endl;
}

template <typename X = T>
typename std::enable_if<std::is_arithmetic<X>::value>::type min() {
    std::cout << "Min for arithmetic values." << std::endl;
}

LIVE

Upvotes: 4

Related Questions