std::initializer_list as constructor argument in template class

Question

I'm trying to use std::initializer_list in a constructor to accept variable number of arguments. With non-template class it works fine, but with template class it gives compiling errors. Could anyone possiblly point me to the right direction? My g++ is capable of C++17.

#include <iostream>
#include <vector>
#include <type_traits>
#include <initializer_list>

struct Nontemplate
{
    // std::initializer_list for a function taking variable number of arguments
    template <class Iterator>
    Nontemplate(std::initializer_list<Iterator> list) {
        std::cout << "size: " << list.size() << std::endl;
        for(auto it : list) {
            std::cout << *it << std::endl;
        }
    }
};

template <class T, class TIterator>
struct Template
{
    std::vector<T> vec;

    Template(std::initializer_list<TIterator> list)
    {
        if(!std::is_same<typename std::iterator_traits<TIterator>::value_type, T>::value)
            throw std::runtime_error("Iterator value type is not the expected.");

        std::cout << "size: " << list.size() << std::endl;

        vec.clear();
        vec.reserve(list.size());
        for(T val : list)
            vec.push_back(val);
    }
};

int main()
{
    double vec[] = {0,1,2,3,4,5};

    Nontemplate cls0({vec, vec+2, vec+3, vec+4}); // no problem
    //Template cls1({vec, vec+2, vec+3, vec+4});  // compiling error
    return 0;
}

Thanks in advance.

@1201ProgramAlarm: The error message is: main.cpp:42:45: error: class template argument deduction failed: 42 | Template cls1({vec, vec+2, vec+3, vec+4}); // compiling error — sofname
– sofname, Commented Dec 13, 2020 at 3:38
You need to pass Template Argumnets in Template<?,?> cls1.... — user10057478
– user10057478, Commented Dec 13, 2020 at 4:14
Elements of list are of type TIterator, not T. TIterator may or may not be the same as T* — Igor Tandetnik
– Igor Tandetnik, Commented Dec 13, 2020 at 4:23
@jianz Template<double, double*> should fix your compiler error, but there's no template argument deduction here. You could just have one template parameter: TIterator and base your vector on that: std::vector<typename std::iterator_traits<TIterator>::value_type> vec; Then you don't have to supply the types yourself to the template. Plus it becomes a compile time check. — jignatius
– jignatius, Commented Dec 13, 2020 at 5:58

Jarod42 · Accepted Answer · 2020-12-14 09:03:31Z

1

Template cls1({vec, vec+2, vec+3, vec+4}) has no ways to deduce T.

You might use

Template<double, double*> cls1({vec, vec+2, vec+3, vec+4});

or provide custom deduction guide:

template <typename It>
Template(std::initializer_list<It>)
-> Template<typename std::iterator_traits<It>::value_type, It>;

Or simplify your class to remove that T

template <class TIterator>
struct Template
{
    using T = typename std::iterator_traits<TIterator>::value_type;
    std::vector<T> vec;

    Template(std::initializer_list<TIterator> list)
    {
        std::cout << "size: " << list.size() << std::endl;

        vec.clear();
        vec.reserve(list.size());
        for(T val : list)
            vec.push_back(val);
    }
};

edited Dec 14, 2020 at 9:03

answered Dec 13, 2020 at 9:58

Jarod42

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2 Comments

sofname Over a year ago

This is wonderful. Before your post, I have been using the "Template<double, double*>" method. What I was looking for are the other two alternatives you provided here. Thanks a lot. Could you possible correct typos - I have already accepted this as the solution. 1) custom deduction guide method: "-> Template<typename std::iterator_traits<It>::value_type, It>;" 2) Removing T method: "using T = typename std::iterator_traits<TIterator>::value_type;"

Jarod42 Over a year ago

@jianz: typos fixed. (feel free to edit if it remains some).

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