Why Must Non-Type Template Parameters Be Constant Integral Expressions?

Non-Type Template Parameters and Constant Integral Expressions

Non-type template parameters are a valuable tool for customizing templates but why is it crucial that they be constant integral expressions?

Consider the following code:

template <std::string temp>
void foo() {
     // ...
}

Compiling this code yields the error:

error C2993: 'std::string' : illegal type for non-type template parameter 'temp'.

Non-type template parameters must be constant integral expressions as they are evaluated during compilation. This allows for direct code generation without runtime evaluation. Types like std::string, which can change at runtime, cannot be substituted during compile-time.

The standard dictates that non-type template parameters can only be the following types:

• Integral or enumeration types
• Pointers to objects or functions
• Lvalue references to objects or functions
• Pointers to members
• std::nullptr_t

By restricting non-type template parameters to compile-time constant values, the compiler can generate efficient code at compile-time, ensuring that the program's behavior can be fully determined and optimized ahead of runtime.

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