I'm going to be using this float to default-initialize a construction parameter. For example: struct default_alpha { static const float value = 1.0f; }; class foo { foo( float r, float g, float b, float a = default_alpha::value ); }; I'm not 100% sure if the above is legal C++. I haven't gotten to the point to where I could test it. I suppose I could try it with integrals, but I just haven't gotten around to it. On Dec 10, 2007 12:21 PM, Richard Hadsell wrote:

Robert Dailey wrote:

Is there a way I can define constant floats within class scope like you can with integral values? I was hoping boost could help in this area. For example:

struct default_alpha { static const float value = 1.0f; };

The above is not legal, of course. Anyone? Thanks.

In .h header:

struct default_alpha { static const float value; };

In .cc source file:

const float default_alpha::value = 1.0f;

Is this answer too simple?

-- Dick Hadsell 914-259-6320 Fax: 914-259-6499 Reply-to: hadsell@blueskystudios.com Blue Sky Studios http://www.blueskystudios.com 44 South Broadway, White Plains, NY 10601

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Robert Dailey writes:

I'm going to be using this float to default-initialize a construction parameter. For example: struct default_alpha { static const float value = 1.0f; }; class foo {     foo( float r, float g, float b, float a = default_alpha::value ); };

You can use constant in namespace scope: namespace default_alpha { static const float value = 1.0f; } HTH, Roman Perepelitsa.

Sorry, small corrections to my last code example (I had a few typos): template< typename T > struct default_alpha; template<> struct default_alpha<float> { static const float value = 1.0f; // Note that this is not legal. I'm simply just presenting pseudo code. }; template<> struct default_alpha<unsigned char> { static const unsigned char value = 255; }; template< typename T > class Color { Color( T r, T g, T b, T a = default_alpha<T>::value ); }; On Dec 11, 2007 2:01 PM, Robert Dailey wrote:

Well, the namespace idea won't work either. There's certain details I omitted because I wasn't sure if they would be directly related to the issue. However I'll go ahead and elaborate now.

I plan to specialize the struct containing the float to determine what 'value' I want. For example:

template< typename T > struct default_alpha;

template<> struct default_alpha<float> { static const float alpha = 1.0f; };

template<> struct default_alpha<unsigned char> { static const unsigned char = 255; };

So, if I have a 'color' class that takes a template parameter, like below, I can use the above specialized structures to figure out how to default initialize the 'a' construction parameter.

template< typename T > class Color { Color( T r, T g, T b, T a = default_alpha<T>::value ); };

Again, I wasn't sure if these new details were required from the very beginning. Apologies for this. Thank you for everyone's continued help.

On Dec 11, 2007 1:14 AM, Roman Perepelitsa wrote:

...

Robert Dailey writes:

...

I'm going to be using this float to default-initialize a construction parameter. For example: struct default_alpha { static const float value = 1.0f; }; class foo { foo( float r, float g, float b, float a = default_alpha::value ); };

You can use constant in namespace scope:

namespace default_alpha { static const float value = 1.0f; }

HTH, Roman Perepelitsa.

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Are function calls allowed to default initialize a construction parameter? Does the compiler turn the function into a constant? How does that work? I wasn't aware you could do this... On Dec 11, 2007 8:02 PM, Stjepan Rajko wrote:

On Dec 11, 2007 1:03 PM, Robert Dailey wrote:

...

template< typename T > struct default_alpha;

template<> struct default_alpha<float> { static const float value = 1.0f; // Note that this is not legal. I'm simply just presenting pseudo code.

};

template<> struct default_alpha<unsigned char> { static const unsigned char value = 255; };

template< typename T > class Color { Color( T r, T g, T b, T a = default_alpha<T>::value ); };

So why not use Jeff Flinn's suggestion:

template< typename T > struct default_alpha;

template<> struct default_alpha<float> { static float value() { return 1.0f; } };

template<> struct default_alpha<unsigned char> { static unsigned char value() { return 255; } };

template< typename T > class Color { public: Color( T r, T g, T b, T a = default_alpha<T>::value() ); };

...?

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Well I read the standard, and it really more or less discusses what you *can't* use to initialize default parameters. Amongst that list I didn't see functions. And behold, it worked! I guess I don't need boost after all. All these years I kept thinking that only compile-time constants could be used to initialize default parameters. I never use default parameters anyway, so I can understand. Just goes to show that you learn new things every day, regardless of how obvious things are :) Thanks guys. On Dec 12, 2007 10:08 AM, Stjepan Rajko wrote:

...

Are function calls allowed to default initialize a construction

On Dec 12, 2007 8:07 AM, Robert Dailey wrote: parameter?

...

Does the compiler turn the function into a constant? How does that work? I wasn't aware you could do this...

I was asking the same questions, which is what prompted me to try to find the answers in the standard. If you can get your hands on it (I'm looking at ISO/IEC 14882:2003), section 8.3.6. talks about what can and can't be used for default argument values. Anyway, the compiler will evaluate the expression when the function is called, but not all expressions are valid (the ones I put in the example should be). If this doesn't work for you, you can always have two constructors - one taking 4 arguments and one taking 3, initializing the alpha value to just about anything.

I would suggest a C++ list or channel to get more info, we've strayed a bit from Boost :-) But I found it helpful too.

Cheers,

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