Why Can\'t I Pass a Non-Constant Variable as a Template Argument in C ?

Template Arguments Must Be Constant

In C , template arguments must be constant expressions, which are evaluable during compilation. This ensures that the code generated by the template is fixed and optimized at compile time.

Why Cannot I Pass a Non-Constant Variable as a Template Argument?

The reason you receive an error when trying to pass the non-constant variable i as a template argument is that the compiler cannot evaluate its value at compile time. Since the value of i is only known during execution, it cannot be used as a parameter for template specialization.

Alternative Solution to Iterate Over Template Arguments

To achieve your objective without changing the API interface, one approach is to utilize template specialization. This involves creating multiple overloaded functions specialized for different template arguments. In this case, you would define individual functions modify<1>, modify<2>, ..., modify<10> to handle the different argument values.

Call Template Function with Non-Constant Argument

To call the modify function with a non-constant argument, such as VAR in your code, you can use a preprocessor macro or a helper function that creates and calls the appropriate template specialization dynamically. However, this approach is not standard C and requires caution to avoid compile-time errors.

Example with Template Specialization

Here's an example solution using template specialization:

<code class="cpp">template<>
void modify<1>() { /* ... */ }

template<>
void modify<2>() { /* ... */ }

// ...

template<>
void modify<10>() { /* ... */ }</code>

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