Why Are Floating-Point Values Not Allowed as C Template Parameters?

Why Can't You Use Float Values as Template Parameters?

The Rationale

The C standard explicitly forbids the use of floating-point values as non-type template parameters. This restriction is outlined in section 14.3.2/1 of the C 11 standard, which states that non-type template arguments must adhere to specific types, including converted constant expressions of integral or enumeration types.

The Reason

This restriction stems from the inherent impreciseness of floating-point calculations. Unlike integers, floating-point values cannot be represented exactly, leading to potential inaccuracies when performing operations or making comparisons.

Implications

Consider the following code snippet:

func<1/3.f>();
func<2/6.f>();

Although the intention is to call the same function twice, the floating-point representation of these values may not be identical. This could result in erroneous or unexpected behavior, as the function invocations would not be considered equal.

Alternative Approaches

To represent floating-point values as template arguments, consider the following approach:

1. Use constant expressions (constexpr): With C 11, you can construct advanced compile-time constant expressions that calculate the numerator and denominator of a floating-point value. These integer values can then be passed as separate arguments to the template.
2. Define a threshold: Establish a tolerance threshold so that floating-point values close to each other result in the same numerator/denominator pair. This ensures consistency in template argument evaluation.

Remember, the main reason for disallowing floating-point template arguments is to prevent potential errors arising from the impreciseness of floating-point calculations. By employing alternative approaches, you can overcome this limitation while maintaining accuracy and predictability.

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