Iterate through template parameter

Question

I have a function that receives a template parameter.

template<class Container>
void function(const Container& object)
{
     //here i want to iterate through object and print them
}

int main()
{
     function(std::vector<int>{1,3,6,7});
     function(std::vector<std::vector<int>>{{1,2,3},{2,5,7}});
}

Is it possible to do this in one function? Suppose the container argument will be integer.

score 7 · Answer 1 · answered Feb 19 '20 at 10:50

7

One example:

template<class T>
void print(T const& object) {
    std::cout << object;
}

template<class... Args>
void print(std::vector<Args...> const& container) {
    for(auto const& element : container) {
        print(element);
        std::cout << ' ';
    }
    std::cout << '\n';
}

int main() {
     print(std::vector<int>{1,3,6,7});
     print(std::vector<std::vector<int>>{{1,2,3},{2,5,7}});
}

answered Feb 19 '20 at 10:50

Maxim Egorushkin

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I'd name it `std::ostream & operator<<(std::ostream &, std::vector const&)` rather than `void print(std::vector const&)` – Caleth Feb 19 '20 at 11:10
@Caleth That may cause issues with ADL and ODR because `operator<<` cannot be put into `std::` but its arguments are from `std::`. So that `operator<<` must be in global namespace, which may clash with other smart users defining the same operator. – Maxim Egorushkin Feb 19 '20 at 11:12
*"in one function"*. nit picking ;-) – Jarod42 Feb 19 '20 at 12:51

theWiseBro · Answer 2 · 2020-02-19T11:40:05.307

1

This should work for your case. Note that I'm using a trait as implemented here in this amazing solution by @Jarod42 https://stackoverflow.com/a/29634934/8192043.

template<template<typename ...> typename C, typename D, typename ... Others>
void function(const C<D, Others...> &object)
{
    if constexpr(is_iterable<D>::value)
    {
       for(const auto& v : object)
       {
           for (const auto& w : v)
           {...}
       }
    }
    else
    {
       for (const auto& w : object)
       {...}
    }
}

edited Feb 19 '20 at 11:40

answered Feb 19 '20 at 10:55

theWiseBro

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`std::vector` has 3 template arguments, other containers have more or less. Your code can only work when the rest of container template arguments are the defaults. – Maxim Egorushkin Feb 19 '20 at 10:56
1

We can overcome by replacing declaration with this `template – calynr Feb 19 '20 at 11:05
@arnes It should be just `template void function(T const& object)`. – Maxim Egorushkin Feb 19 '20 at 11:10
@MaximEgorushkin I think that that's not a constraint or restriction, just his attempt to print. – calynr Feb 19 '20 at 11:12
1

@MaximEgorushkin Thought about this a lot and yeah, I come to the same solution as yours. Thanks, you have my +1. – theWiseBro Feb 19 '20 at 11:16
Possibly: `template void function(const Container& object) { if constexpr(is_iterable>::value) /*..*/` – Jarod42 Feb 19 '20 at 12:54

score 0 · Answer 3 · answered Feb 20 '20 at 20:59

With is_iterable traits, you might do:

template<typename Container>
void function(const Container& object)
{
    if constexpr(is_iterable<std::decay_t<*object.begin()>>::value)
    {
       for(const auto& v : object)
       {
           function(v); // recursive call
       }
    }
    else
    {
        for (const auto& w : object)
        {
            // ...
        }
    }
}

Iterate through template parameter

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