Compile-Time Evaluation

constexpr and consteval with Variadic Functions

Can I use constexpr or consteval with variadic functions or function templates?

Yes, you can use both constexpr and consteval with variadic functions and function templates. This combination is particularly powerful for creating flexible, compile-time computations. Let's explore this with someÂ examples

Basic Variadic constexpr Function

Here's a simple example of a variadic constexpr function that sums itsÂ arguments:

#include <iostream>

constexpr int sum() { return 0; }

template<typename T, typename... Args>
constexpr int sum(T first, Args... args) {
return first + sum(args...);
}

int main() {
constexpr int result1 = sum(1, 2, 3, 4, 5);
std::cout << "Sum: " << result1 << '\n';

constexpr int result2 = sum(10, 20, 30);
std::cout << "Sum: " << result2 << '\n';
}
Sum: 15
Sum: 60

Variadic consteval Function

Similarly, we can create a variadic constevalÂ function:

#include <iostream>

consteval int product() { return 1; }

template<typename T, typename... Args>
consteval int product(T first, Args... args) {
return first * product(args...);
}

int main() {
constexpr int result1 = product(2, 3, 4);
std::cout << "Product: " << result1 << '\n';

constexpr int result2 = product(5, 10, 2);
std::cout << "Product: " << result2 << '\n';
}
Product: 24
Product: 100

Compile-Time String Operations

Here's a more complex example that performs compile-time stringÂ concatenation:

#include <algorithm>
#include <array>
#include <iostream>
#include <string_view>

template <std::size_t... Lengths>
constexpr auto concat(
const std::array<
std::string_view, sizeof...(Lengths)>& strs) {
constexpr std::size_t total_length = (
Lengths + ... + 0);
std::array<char, total_length + 1> result{};

char* ptr = result.data();
for (auto& str : strs) {
std::copy_n(str.data(), str.length(), ptr);
ptr += str.length();
}
result[total_length] = '\0';

return result;
}

int main() {
constexpr auto result = concat<5, 7, 6>({
"Hello", " World!", " C++20"
});
std::cout << result.data();
}
Hello World! C++20

Variadic Templates with if constexpr

C++17 introduced if constexpr, which works great with variadicÂ templates:

#include <iostream>
#include <type_traits>

template<typename T>
constexpr auto to_string(T value) {
if constexpr (std::is_same_v<T, int>) {
return "int";
} else if constexpr (std::is_same_v<T, double>) {
return "double";
} else {
return "unknown";
}
}

template<typename... Args>
constexpr void print_types(Args... args) {
((std::cout << to_string(args) << ' '), ...);
std::cout << '\n';
}

int main() {
print_types(42, 3.14, 'a');
}
int double unknown

Compile-Time Type Traits

Variadic templates with constexpr are often used in typeÂ traits:

#include <iostream>
#include <type_traits>

template <typename... Ts>
struct all_integral : std::true_type {};

template <typename T, typename... Ts>
struct all_integral<T, Ts...>
: std::bool_constant<std::is_integral_v<T>
&& all_integral<Ts...>::value> {};

template <typename... Ts>
constexpr bool all_integral_v
= all_integral<Ts...>::value;

int main() {
constexpr bool result1
= all_integral_v<int, char, long>;
std::cout << "All integral 1: "
<< std::boolalpha << result1 << '\n';

constexpr bool result2
= all_integral_v<int, double, char>;
std::cout << "All integral 2: "
<< std::boolalpha << result2 << '\n';
}
All integral 1: true
All integral 2: false

These examples demonstrate the power and flexibility of combining constexpr/consteval with variadic functions and templates. They allow for complex compile-time computations, type manipulations, and metaprogrammingÂ techniques.

When using these features, remember to keep your code readable and maintainable, and be aware of potential increases in compilation time for very complex compile-timeÂ computations.

This Question is from the Lesson:

Compile-Time Evaluation

Learn how to implement functionality at compile-time using constexpr and consteval

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This Question is from the Lesson:

Compile-Time Evaluation

Learn how to implement functionality at compile-time using constexpr and consteval

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