In C++, templates provide a powerful mechanism for metaprogramming, allowing us to perform computations and validations at compile-time. One interesting use case of metaprogramming is template compile-time assertions, which can help catch errors or validate assumptions during the compilation process.

To implement template compile-time assertions, we can utilize recursive templates. Recursive templates allow us to perform computations or checks by breaking down a problem into smaller subproblems, and terminating when the base case is reached.

Let’s consider an example of implementing compile-time assertions for the size of a std::array at compile-time. We want to ensure that the size of the std::array is always greater than 0.

template <typename T, std::size_t Size>
struct StaticAssertPositiveSize {
    static_assert(Size > 0, "Size must be greater than 0");
};

template <typename T, std::size_t... Sizes>
struct ArraySizeAssertionHelper;

template <typename T, std::size_t Size, std::size_t... Sizes>
struct ArraySizeAssertionHelper<T, Size, Sizes...>
    : StaticAssertPositiveSize<T, Size>,
      ArraySizeAssertionHelper<T, Sizes...> {};

template <typename T>
struct ArraySizeAssertionHelper<T> {};

template <typename T, std::size_t... Sizes>
struct ArraySizeAssertion : ArraySizeAssertionHelper<T, Sizes...> {};

int main() {
    std::array<int, 5> arr1;
    // ArraySizeAssertion<int, 5> assertion1;

    std::array<int, 0> arr2;  // Compile-time error!
    // ArraySizeAssertion<int, 0> assertion2;  // Compile-time error!

    return 0;
}

In the code above:

• We define a StaticAssertPositiveSize template that takes a type and size and performs a static assertion to ensure that the size is greater than 0.
• The ArraySizeAssertionHelper template is a recursive template that breaks down the list of sizes provided into individual size values, and recursively applies the StaticAssertPositiveSize template for each size.
• The empty specialization of ArraySizeAssertionHelper serves as the base case for the recursion and terminates the recursion when there are no more sizes left.
• Finally, the ArraySizeAssertion template is a convenience template that uses ArraySizeAssertionHelper to perform compile-time assertions for the sizes of std::array.

When we try to compile the code, the compile-time assertions will trigger an error if the size of std::array is 0 or negative, preventing the code from compiling successfully.

Recursive templates for template compile-time assertions provide a flexible and powerful mechanism to validate assumptions about template parameters at compile-time. They can be extended to handle more complex assertions and computations, helping us catch errors during the compilation process and improve code reliability.

#cpp #templates #metaprogramming