Why Can't Floating-Point Values Be Used as C Template Parameters?

Why Can't Float Values Be Used as Template Parameters?

When attempting to instantiate a generic class template with a float value as the default parameter, the compiler may issue errors. While integer values function as expected, floats do not.

Standard Limitations for Floating Point Arguments

According to the C 11 standard (14.3.2/1), non-type template arguments for non-type template parameters must adhere to specific rules. One restriction is that they cannot be floating point values. Instead, they must be either integral or enumeration types, converted constant expressions of a template parameter type, or other specific constructs.

Floating Point Imprecision as a Motivating Factor

This limitation likely stems from the imprecise nature of floating point calculations. Representing floating point values exactly is not always feasible, leading to potential inaccuracies that could cause unexpected behavior. Consider the following example:

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

Though intended to call the same function twice, the floating point representations of 1/3 and 2/6 are not guaranteed to be identical due to potential rounding errors. This could lead to unintended function invocations.

Alternative Approaches

If the goal is to represent floating point values in some way as template arguments, one option is to utilize C 11 constant expressions (constexpr). These allow for the calculation of a floating point value's numerator and denominator at compile time, which can then be passed as separate integer arguments.

However, it's crucial to define a threshold to ensure that floating point values close to each other share the samenumerator/denominator pair. Otherwise, the rationale behind restricting floating point values as non-type template arguments remains valid.

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