Re: STL Template question

Stephan Rose <>
Sat, 20 Oct 2007 17:05:21 -0500
On Sat, 20 Oct 2007 22:54:31 +0100, Marcin Kalicinski wrote:

So long story short, is there any way to declare variable C in such a
way as to where I don't need to know ahead of time what unit type it's
going to be? I suspect no as I currently can't imagine a way that would
be possible...but since I'm still new to STL I figured I'd ask.

No there isn't. Type of a variable must be known at compile time. If I
understand your problem correctly, the best you can do is the following
(assuming that user also entered unit information as a string):

string userUnit = ...
if (userUnit == "Mil")
    // Compute and store Scalar<Mil>
else if (userUnit == "Inch")
    // Compute and store Scalar<Inch>
else if (...)
     // ...etc...

To store the value in a single variable you can use boost::any:

boost::any scalar;
if (userUnit == "Mil")
    scalar = Scalar<Mil>(3);
else if (userUnit == "Inch")
    scalar = Scalar<Inch>(5);

Note how boost::any gracefully accomodates value of any type. But you
still need the if statements to create scalar of appropriate type. It's
also possible to replace ugly ifs with a more general "factory" for
scalars (untested code):

// Function to create scalar from double typedef boost::any

// Template to generate such functions for all unit types you may have
template<class Unit> boost::any CreateScalar(double v) {
    return boost::any(Scalar<Unit>(v));

// Register all unit types you have in the map map<string,
ScalarFactory> factoriesByUnit; factoriesByUnit.insert(make_pair("Mil",
&CreateScalar<Mil>)); factoriesByUnit.insert(make_pair("Inch",
&CreateScalar<Inch>)); ...etc for all supported units

// Now use the factory to create scalar with appropriate unit double
userValue = ...; // Contains value entered by user string userUnit =
...; // Contains unit entered by user boost::any scalar =
factoriesByUnit[userUnit](userValue); // Now scalar contains value with
appropriate unit. // Use any_cast to extract the value (but again you
must know the unit :-), // so another map may be necessary)


Ahh thank you for the info Marcin.

I'll definitely need to take a closer look at boost then!

Much appreciated,

2003 Yamaha R6


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