When Would You Use Non-Type Parameters in C Templates?

Template with Non-Type Parameter

Template parameters are commonly used to specify type requirements, but it's also possible to declare templates with non-type parameters, such as unsigned integers.

Syntax and Meaning

The syntax for a template with a non-type parameter is:

template <unsigned int n></unsigned>

where N represents the non-type parameter. This parameter can then be used within the template as a compile-time constant of type unsigned int.

Use Cases

Non-type parameters are useful in various scenarios:

• Array Sizing: N can be used to define the size of arrays or other fixed-size data structures.
• Constant Values: It can serve as a constant expression, providing a fixed value that doesn't change at runtime.
• Loop Bounds: N can be employed to specify the bounds of loops or iterations.
• Conditional Compilation: Different code paths can be chosen based on the value of N.

Type Parameters vs. Non-Type Parameters

It's important to note the difference between type parameters and non-type parameters. Type parameters represent types (e.g., class T or template T), while non-type parameters represent constants (e.g., unsigned int N).

Example

Consider the following template:

<code class="cpp">template <unsigned int n>
struct Vector {
    unsigned char bytes[N];
};</unsigned></code>

Here, N is a non-type parameter that determines the size of the bytes array.

<code class="cpp">Vector v; // Vector with 3 bytes</code>

Default Values

It's possible to specify default values for non-type parameters, allowing the template to be used without explicitly providing the parameter. For instance:

<code class="cpp">template <unsigned int size="3">
struct Vector {
    unsigned char buffer[SIZE];
};</unsigned></code>

The above template defaults SIZE to 3, so the following is valid:

<code class="cpp">Vector v; // Equivalent to Vector</code>

Conclusion

Non-type parameters in templates offer flexibility and allow for code reuse with fixed or constant values. They enable a range of use cases, from array sizing to conditional compilation.

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