Variadic template pack expansion

Question

I am trying to learn variadic templates and functions. I can't understand why this code doesn't compile:

template<typename T> static void bar(T t) {}  template<typename... Args> static void foo2(Args... args) {     (bar(args)...); }  int main() {     foo2(1, 2, 3, "3");     return 0;     } 

When I compile it fails with the error:

Error C3520: 'args': parameter pack must be expanded in this context

(in function foo2).

Answer 1

One of the places where a pack expansion can occur is inside a braced-init-list. You can take advantage of this by putting the expansion inside the initializer list of a dummy array:

template<typename... Args> static void foo2(Args &&... args) {     int dummy[] = { 0, ( (void) bar(std::forward<Args>(args)), 0) ... }; } 

To explain the content of the initializer in more detail:

{ 0, ( (void) bar(std::forward<Args>(args)), 0) ... };   |       |       |                        |     |   |       |       |                        |     --- pack expand the whole thing    |       |       |                        |      |       |       --perfect forwarding     --- comma operator   |       |   |       -- cast to void to ensure that regardless of bar()'s return type   |          the built-in comma operator is used rather than an overloaded one   |   ---ensure that the array has at least one element so that we don't try to make an      illegal 0-length array when args is empty 

Demo.

An important advantage of expanding in {} is that it guarantees left-to-right evaluation.

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With C++17 fold expressions, you can just write

((void) bar(std::forward<Args>(args)), ...);