CODEWITHSUNDEEP
c++templatesc++14variadic-templates
Felix
Felix

Reputation: 2691

Equal number of template and function params

Is there a way to generate a templated function where there is an equal number of template params and function params (of the same type)?

Like the following:

// template function:
template<typename ... Args>
void foo(const std::string& a, const std::string& b, ...) { /*...*/}

// example 1
foo<int>("one argument only");

// example 2
foo<int, double>("first argument", "second argument");

What if there is a default function param (not templated) (or multiple) which always exists? 

template<typename ... Args>
void foo(int i /* as default */, const std::string& a,
                                 const std::string& b, ...)
 { /*...*/}

Would it even be possible to have, let's say the duplicate number of function arguments (calculated by the size of the template params)?

Upvotes: 7

Views: 268

Answers (1)

max66
max66

Reputation: 66200

Is there a way to generate a templated function where there is an equal number of template params and function params (of the same type)?

Yes: it is (Edit: simplified following a Justin's suggestion; thanks!)

#include <string>

template <typename, typename T>
using skip_first = T;

template <typename ... Args>
void foo (skip_first<Args, std::string> ... as)
 { }

int main()
 {
   foo<int>("one argument only");

   foo<int, double>("first argument", "second argument");
 }

What if there is a default function param (or multiple)?

template<typename ... Args>
void foo(int i /* as default */, const std::string& a,
                                 const std::string& b, ...)
 { /*...*/}

If you want one (or more) preceding parameter(s) of different type(s), yes it's possible.

template <typename ... Args>
void foo (int i, long j, skip_first<Args, std::string> ... as)
 { }

If you want a preceding parameter with a default value, no: isn't possible because a variadic list of parameter can't have a default value and because given a parameter with a default value all fallowing parameters have to have a default parameter.

Would it even be possible to have, let's say the duplicate number of function arguments (calculated by the size of the template params)?

I suppose you can multiply (repeat the unpack more times) the unpack part

template <typename ... Args>
void foo (skip_first<Args, std::string> ... as1,
          skip_first<Args, std::string> ... as2)
 { }

Obviously this solution works only if you want multiply the number of template argument for an integer number.

Upvotes: 10

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