Why Can\'t Non-Constant Variables Be Used as Template Arguments in C ?

Why Can't a Non-Constant Variable be Passed as a Template Argument?

In C , template arguments must be constant expressions. This means that their values must be known at compile time. The compiler cannot evaluate a non-constant variable in this context.

Consider the code:

<code class="cpp">template <int a>
void modify(){}</code>

To pass a non-constant variable as the template argument, we might write:

<code class="cpp">for(int i = 0; i < 10; i++) {
    modify<i>();
}</code>

However, this triggers an error because the compiler cannot determine the value of i at compile time. The loop body may execute multiple times, changing the value of i.

Achieving the Objective Without Modifying the API

Instead of passing a non-constant variable directly, we can use template specialization to implement an iterative call:

<code class="cpp">#include <iostream>

template<>
void modify<0>() { std::cout << "modify<0>" << std::endl; }

template<>
void modify<1>() { std::cout << "modify<1>" << std::endl; }

// ...

template<int i>
void modify() {
    std::cout << "modify<" << i << ">" << std::endl;
    modify<i+1>();
}

int main() {
    modify<0>();
}</code>

Calling Modify with a Dynamic Value

To call modify with a value that is not known at compile time, we can use a technique called template metaprogramming. Here's a simplified example:

<code class="cpp">#include <tuple>

template <std::tuple<int...>>
struct TupleSize;

template <int... Args>
struct TupleSize<std::tuple<Args...>> {
    static const int value = sizeof...(Args);
};

template <int N>
void callModify(int i) {
    if constexpr (i < N) {
        modify<i>();
        callModify<N>(i+1);
    }
}

int main() {
    int n = 10;
    callModify<TupleSize<std::make_tuple(1,2,3,4,5,6,7,8,9,10)>::value>(0);
}</code>

In this example, callModify takes a parameter N which is the size of a tuple that contains the desired range of values for i. The function uses a recursive metaprogram to generate the calls to modify up to the specified size N.

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