Mixing variadic template values and variadic deduced types

Question

Is the following perfectly defined by the standard ?

#include <iostream>

template <unsigned int... Values, class... Types>
void f(Types&&... values)
{
    std::cout<<sizeof...(Values)<<" "<<sizeof...(Types)<<std::endl;
}

int main()
{
    f<7, 5>(3);
    return 0;
}

It compiles well under g++ 4.8 but I wonder if it is normal.

Well I am not good at throwing around standard paragraphs, but why would you not expect that to work? Integrals are valid template parameter values, and type deduction from the arguments is there also for a very long time. — Vinzenz
– Vinzenz, Commented Jan 13, 2014 at 21:12
As your Types are deduced from your function parameters, and your Values are specified explicitly, and since 5 is most-definitely not a type, I have a hard time seeing how this doesn't compile exactly as you have specified (and expected) it should. What would you expect may be a concern of non-standard-compliance? — WhozCraig
– WhozCraig, Commented Jan 13, 2014 at 21:15
[temp.param]/11 "A template parameter pack of a function template shall not be followed by another template parameter unless that template parameter can be deduced from the parameter-type-list of the function template or has a default argument" So I'd say it's legal unless Types only appears in non-deduced contexts. — dyp
– dyp, Commented Jan 13, 2014 at 21:27

Salvatore Avanzo · Accepted Answer · 2014-03-02 09:44:22Z

2

From ISO C++ standard's current working draft 14.1 (11):

A template parameter pack of a function template shall not be followed by another template >parameter unless that template parameter can be deduced from the parameter-type-list of >the function template or has a default argument

In your case 'Types' is a function parameter pack and 'Values', that is a template parameter pack, can be always followed by a function parameter pack. Also this code works for the same reason:

#include <iostream>

template <class... Values, class... Types>
void f(Types&&... values)
{
    std::cout<<sizeof...(Values)<<" "<<sizeof...(Types)<<std::endl;
}

int main()
{
    f<int, float>(-3, 5);
    return 0;
}

answered Mar 2, 2014 at 9:44

Salvatore Avanzo

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