Deducing Parameter Pack Brace-Enclosed Initializer List: Convenient Approach
In C++, parameter packs are a useful feature that allows us to handle a variable number of template arguments. Combining this feature with brace-enclosed initializer lists can lead to more concise and convenient code. In this article, we will explore how to deduce the length of a parameter pack using a brace-enclosed initializer list.
Prerequisites
Before diving into the concept of deducing the length of a parameter pack using a brace-enclosed initializer list, it is essential to have a good understanding of parameter packs, template parameter deduction, and fold expressions.
Parameter Packs
A parameter pack is a template parameter that can accept a variable number of arguments.
It is denoted by an ellipsis (...) and is used in the context of function or class templates.
Since C++17, parameter packs can also appear as function parameters, allowing for more readable syntax.
Template Parameter Deduction
Template parameter deduction is the process by which the compiler determines the template arguments based on the function or class call. When a function or class template is called with actual arguments, the compiler tries to find the best match for the template parameters based on the given arguments.
Fold Expressions
Fold expressions, introduced in C++17, are a convenient way to apply an operation to a parameter pack.
A fold expression has the following syntax:
( pack operation ... ) or ( operation ... pack ), where pack represents a parameter pack and operation an arbitrary binary or unary expression.
Deducing Parameter Pack Length
To deduce the length of a parameter pack using a brace-enclosed initializer list, we can use a combination of template parameter deduction and fold expressions.
The idea is to create a template function with a parameter pack and a non-type template parameter that represents the size of the parameter pack.
By using a parameter pack expansion and a fold expression in the function definition,
we can initialize a constexpr array and compute its size using std::size():
template
constexpr size_t packSize(Ts... args) {
constexpr std::array arr{args...};
return std::size(arr);
}
In the example above, the template function packSize() takes a parameter pack Ts... and a non-type template parameter N.
With the help of parameter pack expansion args...,
we initialize a constexpr array arr of size N with the elements of the parameter pack args....
Finally, we use std::size() to compute the size of the array, which gives us the length of the parameter pack.
Example: FooImp Class Template
Let's illustrate the use of the packSize() function in a class template implementation.
Consider the following class template FooImp that takes a parameter pack Aux:
template
class FooImp {
// ...
};
We can add a helper static data member
template
class FooImp {
public:
static constexpr size_t pack_length = packSize(Aux{}...);
// ...
};
By calling the `packSize()` function and passing an empty brace-enclosed initializer list Aux{}... for each argument of the parameter pack,
we deduce the size of the parameter pack and initialize the `pack_length` static member.
This member can then be used inside the class implementation to handle the parameter pack accordingly.
Deducing the length of a parameter pack using a brace-enclosed initializer list and the `packSize()` function can lead to more concise and convenient code in C++. By using a combination of template parameter deduction and fold expressions, we can determine the size of a parameter pack at compile-time and make use of it throughout our class or function implementation.