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:

template 
void modify(){}

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

for(int i = 0; i ();
}

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:

#include 

template
void modify() { std::cout " 
void modify() { std::cout " 
void modify() {
    std::cout " ();
}

int main() {
    modify();
}

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:

#include 

template <:tuple>>
struct TupleSize;

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

template 
void callModify(int i) {
    if constexpr (i ();
        callModify(i 1);
    }
}

int main() {
    int n = 10;
    callModify::value>(0);
}

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.