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(){}</int></code>
To pass a non-constant variable as the template argument, we might write:
<code class="cpp">for(int i = 0; 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() { std::cout " void modify() { std::cout " void modify() { std::cout " (); } int main() { modify(); }</iostream></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 <:tuple>> struct TupleSize; template <int... args> struct TupleSize<:tuple>> { static const int value = sizeof...(Args); }; template <int n> void callModify(int i) { if constexpr (i (); callModify<n>(i+1); } } int main() { int n = 10; callModify<tuplesize>::value>(0); }</tuplesize></n></int></:tuple></int...></:tuple></tuple></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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