C++ template with float arguments

Question

Possible Duplicate:
C++: Why can’t I use float value as a template parameter?

I have this class:

template<typename ValueType, ValueType DefaultValue>
class SomeClass
{
    public:
        SomeClass() : m_value(DefaultValue){}

        ValueType m_value;
};

I want to use it like this:

SomeClass<int, 1> intObj; //ok
SomeClass<float, 1.f> floatObj; //error: 'float' : illegal type for non-type template parameter 'DefaultValue'

Can you please explain me why I get this error when using float?

I want to use something similar to represent RGBA colors and to initialize the channels with default value for different color representations(White for example).

The short answer is simply that the standard doesn't allow it. — Jerry Coffin, Jan 10 '13 at 20:11
@Felics: I've already voted to close it, waiting for 4 more votes... — K-ballo, Jan 10 '13 at 20:15
That duplicate is very handy as to the why, as well as a workaround. — chris, Jan 10 '13 at 20:16

score 3 · Accepted Answer · edited May 23 '17 at 12:34

3

The language does not allow the use of floating-point types as non-type template arguments. For an extensive discussion, see Why can't I use float value as a template parameter?

edited May 23 '17 at 12:34

Community

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answered Jan 10 '13 at 20:11

NPE

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score 3 · Answer 2 · answered Jan 10 '13 at 20:12

§ 14.1/7 (C++11 N3485) explicitly forbids this:

A non-type template-parameter shall not be declared to have ﬂoating point, class, or void type. [ Example:

template<double d> class X; // error  
template<double* pd> class Y; // OK  
template<double& rd> class Z; // OK

score 0 · Answer 3 · answered Jan 10 '13 at 20:11

0

C++ doesn't support floating-point non-type template parameters unfortunately.

answered Jan 10 '13 at 20:11

Stuart Golodetz

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score 0 · Answer 4 · answered Jan 10 '13 at 20:11

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You get that error because non-type template parameters cannot be of type float. They may only be integrals, enumerations, member-pointers or addresses.

answered Jan 10 '13 at 20:11

K-ballo

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score 0 · Answer 5 · answered Jan 10 '13 at 20:12

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That's simple: nontype template parameters have to be compiletime constants of integral type or pointer type, i.e. bool, enums, pointers, pointer-to-members, long, int, short, char. Floating point parameters are not allowed in the current standard.

answered Jan 10 '13 at 20:12

Arne Mertz

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They can be _enumerations_ as well. – K-ballo Jan 10 '13 at 20:13
added that just at the time you posted your comment ;) – Arne Mertz Jan 10 '13 at 20:13

C++ template with float arguments

5 Answers5