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POHow can I have multiple parameter packs in a variadic template?
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2012-03-22T22:27:27.713
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Function one() accepts one parameter pack. Function two() accepts two. Each pack is constrained to be wrapped in types A and B. Why is it impossible to instantiate two()? <pre><code>template <typename T> struct A {}; template <typename T> struct B {}; template <typename... Ts> void one(A<Ts> ...as) { } template <typename... Ts, typename... Us> void two(A<Ts> ...as, B<Us> ...bs) { } int main() { auto a = A<int>(); auto b = B<int>(); // Just fine one(); one(a); one(a, a); // All errors two(); two(a); two(a, b); } </code></pre> Tried with gcc and clang. <pre><code>sam@wish:~/x/cpp$ gcc -std=c++0x variadic_templates.cpp variadic_templates.cpp: In function ‘int main()’: variadic_templates.cpp:23:7: error: no matching function for call to ‘two()’ variadic_templates.cpp:23:7: note: candidate is: variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...) variadic_templates.cpp:24:8: error: no matching function for call to ‘two(A<int>&)’ variadic_templates.cpp:24:8: note: candidate is: variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...) variadic_templates.cpp:25:11: error: no matching function for call to ‘two(A<int>&, B<int>&)’ variadic_templates.cpp:25:11: note: candidate is: variadic_templates.cpp:11:6: note: template<class ... Ts, class ... Us> void two(A<Ts>..., B<Us>...) sam@wish:~/x/cpp$ clang -std=c++0x variadic_templates.cpp variadic_templates.cpp:23:3: error: no matching function for call to 'two' two(); ^~~ variadic_templates.cpp:11:6: note: candidate function template not viable: requires at least 1 argument, but 0 were provided void two(A<Ts> ...as, B<Us> ...bs) {} ^ variadic_templates.cpp:24:3: error: no matching function for call to 'two' two(a); ^~~ variadic_templates.cpp:11:6: note: candidate function not viable: requires 0 arguments, but 1 was provided void two(A<Ts> ...as, B<Us> ...bs) {} ^ variadic_templates.cpp:25:3: error: no matching function for call to 'two' two(a, b); ^~~ variadic_templates.cpp:11:6: note: candidate function not viable: requires 0 arguments, but 2 were provided void two(A<Ts> ...as, B<Us> ...bs) {} ^ 3 errors generated. </code></pre>
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<c++><c++11><variadic-templates>
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How can I have multiple parameter packs in a variadic template?
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