How does the \'ellipsis\' (...) notation work in variadic templates and what are its different roles in packing and unpacking arguments?

Variadic Templates: Unveiling the Mysteries of the Ellipsis

Variadic templates, introduced in C 11, expand the capabilities of templates by allowing functions and classes to accept and process a variable number of arguments. However, certain aspects of their syntax can evoke curiosity. One such enigma is the enigmatic "ellipsis" (...) notation, which triggers questions about its purpose and usage.

In the context of variadic templates, the ellipsis (...) serves as a critical tool for either packing or unpacking arguments and parameters. Its role is determined by its relative position within the template definition or call.

Unpacking: Expanding Parameters into Arguments

When placed to the right of an expression, the ellipsis acts as an unpacker. It enables the expansion of template parameters into a sequence of actual arguments. For instance, in the following function template:

<code class="cpp">template&lt;class T, class... Args&gt;
unique_ptr&lt;T&gt; make_unique(Args&amp;&amp;... args) {
    return unique_ptr&lt;T&gt;(new T(std::forward&lt;Args&gt;(args)...));
}</code>

The ellipsis in std::forward<Args>(args)... unpacks the Args template parameter into the args function argument list, enabling the construction of a new unique_ptr with the provided arguments.

Packing: Combining Arguments into a Parameter

Conversely, when placed to the left of a parameter, the ellipsis functions as a packer. It packs a sequence of arguments into a single parameter. This packing occurs during both template argument deduction and function call invocation.

For illustration, consider the following function call:

<code class="cpp">make_unique&lt;int&gt;(1, 2, 3);</code>

The ellipsis in make_unique<int>(... ) packs the arguments 1, 2, and 3 into a single parameter pack, matching the Args` template parameter.

The example template function f demonstrates the packing and unpacking capability:

<code class="cpp">template &lt;typename ...T&gt;
void f(T ...args) {
    g(args...);  // Unpacking: Pattern = args
    h(x(args)...);  // Unpacking: Pattern = x(args)
    m(y(args...)...);  // Packing: Pattern = args (arg to y())
}</code>

When calling f with T as {int, char, short}, each function call expands as:

<code class="cpp">g(arg0, arg1, arg2);           
h(x(arg0), x(arg1), x(arg2));
m(y(arg0, arg1, arg2));</code>

Placement Inconsistencies

You may notice an apparent inconsistency in the placement of the ellipsis between the template argument list and the function parameter list. In the template argument, it appears in the middle of the parameter pack, while in the function parameter, it appears at the end. This difference arises from the fact that in the template argument, the ellipsis is a syntactic sugar representing the unpacking of a parameter pack, while in the function parameter, it is a part of the syntax that defines the type of the packed argument.

In summary, the ellipsis (...) in variadic templates plays a pivotal role in expanding parameters into arguments and combining arguments into parameters, enabling the creation of flexible and extensible code that can handle a varying number of arguments.

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