Why Can't I Use Floating-Point Numbers as C Template Parameters?

Why Float Values Are Not Allowed as Template Parameters

In C , attempting to use a float value as a template parameter, as in the following code, results in the following error:

GenericClass<float, 4.6f> gcFloat;
error: `float' is not a valid type for a template constant parameter

Standard Prohibition

According to the C 11 standard, non-type template arguments must be one of the following:

• Converted constant expressions of integral or enumeration types
• Non-type template parameter names
• Expressions designating addresses of static objects or functions
• Null pointers
• Null member pointers
• Pointers to members

Floating point values, however, do not fit into any of these categories.

Reasoning

The primary reason for this restriction is the imprecise nature of floating point computations. Floating point calculations cannot be represented exactly, which could lead to unexpected behavior when using them as template arguments. For example, the following code may not behave as intended due to potential rounding differences:

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

Alternative Solutions

To represent floating point values as template arguments, consider using advanced constant-expressions (constexpr) in C 11 to calculate the numerator and denominator of the floating value at compile time. These can then be passed as separate integer arguments. However, it is crucial to define a threshold to ensure that floating point values close to each other yield the same numerator/denominator ratio.

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