Hi all.
I'm using a thrid-party software library that uses two tag classes (Tag_true
and Tag_false) to flag if certain operations are available in a (template)
class.
Now I think the idea behind them is to be able to perform compile-time
checks, but I don't understand how...
These tag classes are defined as empty structs, i.e.:
struct Tag_true {};
struct Tag_false {};
Now, in a class definition they use the following construction:
class Fixed_precision _nt{
public:
typedef Tag_false Has_gcd;
typedef Tag_true Has_division;
typedef Tag_false Has_sqrt;
//<snip>
};
indicating that this class supports division, but not square root or gcd
calculation. In my application, I want to be able to perform compile-time
tests on these tags, something like this:
#define Fixed_precision _nt scalar
//<snip>
#if scalar::Has_gcd == Tag_true
//...
#endif
What is the proper way of doing this? When I try to compile the above code,
it
complains on the '#if' line with "unexpected tokens following preprocessor
directive - expected a newline". What am I doing wrong?
Thanks,
Leon. 1 1672
Leon wrote: Hi all.
I'm using a thrid-party software library that uses two tag classes (Tag_true and Tag_false) to flag if certain operations are available in a (template) class. Now I think the idea behind them is to be able to perform compile-time checks, but I don't understand how...
These tag classes are defined as empty structs, i.e.: struct Tag_true {}; struct Tag_false {};
Now, in a class definition they use the following construction: class Fixed_precision _nt{ public: typedef Tag_false Has_gcd; typedef Tag_true Has_division; typedef Tag_false Has_sqrt; //<snip> };
indicating that this class supports division, but not square root or gcd calculation. In my application, I want to be able to perform compile-time tests on these tags, something like this:
#define Fixed_precision _nt scalar //<snip> #if scalar::Has_gcd == Tag_true //... #endif
What is the proper way of doing this?
That is not the way to use templates.
Here it is:
You parameterize your classes by a type that will represent Tag_true or
Tag_false:
// declaration.
template<class GCD> class A;
// specialization for Tag_true and Tag_false:
template<>
class A<Tag_true>
{
//implementation of A when template parameter is Tag_true
};
template<>
class A<Tag_false>
{
//implementation of A when template parameter is Tag_false
};
So you can use it:
A<Fixed_precisi on_nt::Has_gcd> myA;
and you will have myA that depends at compile-time of
Fixed_precision _nt::Has_gcd.
Yannick This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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