Programming Forums > Application Development > C++

rwm · Jul 11th, 2007, 3:46 AM

this compiles fine with GCC, but not with visual studio 2005...

im trying to pass a default value to a template class

i.e.

myclass<float,1.0f>

(Toggle Plain Text)

#include <iostream>
using namespace std;

template<typename T,const T TUNIT>
class TClass {
	public:
		TClass() : data(TUNIT) {}
		TClass(const T &t) : data(t) {}
		T get_data() {
			return data;
		}
	private:
		T data;
};

int main() {
	//
	TClass<int,1> tc;
	cout << tc.get_data() << endl;

	return 0;
}

any idea why? i should have copied the error message from home

rwm · Jul 11th, 2007, 3:56 AM

i dont understand!

this works

(Toggle Plain Text)

TClass<int,1> tc;

but this doesnt

(Toggle Plain Text)

TClass<float,3.142f> tc;

heres the error:

(Toggle Plain Text)

test.cpp:18: error: �float� is not a valid type for a template constant parameter
test.cpp:18: error: invalid type in declaration before �;� tok

pretty much the same error i get with visual studio...

Eoin · Jul 11th, 2007, 7:27 AM

Just as the error states you can't use a float as a non-type template parameter unfortunately, see http://publib.boulder.ibm.com/infoce...parameters.htm for example.

rwm · Jul 11th, 2007, 8:10 AM

arrrrrrrrrrgghhhhhhhh why?

im trying to pass values to the template so i can initialize properly..

i.e.

(Toggle Plain Text)

template<typename T,T TZERO,T TUNIT>
class TClass {
    public:
        TClass() : zero(TZERO),unit(TUNIT) {}
        ...
    private:
        //data
        T zero;
        T unit;
};

is there any way to do this?

thanks for the reply!

Oxyd · Jul 11th, 2007, 10:38 AM

Hm, no. Template parameter can only be a type or an integral constant -- floats are simply not allowed. You could pass a default value to the constructor, though:

(Toggle Plain Text)

template <typename T>
class TClass {
  public:
    TClass(T zero, T unit) : zero(zero), unit(unit) { }

  private:
    T const zero;
    T const unit;
};

Eoin · Jul 11th, 2007, 10:42 AM

To be honest off hand I'm not sure. You might be able to support floats as a fraction of two integers but that may not be allowed.

rwm · Jul 11th, 2007, 10:45 AM

ok thanks for the help! bit of a strange restriction right?

EDIT: only saw your post Eoin, ah but thats tricky buggery and id like to avoid it...

would be nice to put in

(Toggle Plain Text)

TClass<float,1.0f> tc1;
TClass<double,1.0> tc2;

etc

francoissoft · Jul 11th, 2007, 10:54 AM

Quote:

Originally Posted by rwm

ok thanks for the help! bit of a strange restriction right?

EDIT: only saw your post Eoin, ah but thats tricky buggery and id like to avoid it...

would be nice to put in

(Toggle Plain Text)

TClass<float,1.0f> tc1;
TClass<double,1.0> tc2;

etc

Not really, templates are macros based on specified data types, therefore a template can only take parameters that are data types.

Check out http://www.cplusplus.com for more details.

rwm · Jul 12th, 2007, 2:39 AM

well i found a way, its feels pretty much like a hack but works

(Toggle Plain Text)

template<typename T>
class TClass {
        public:
                TClass() : data(TUNIT) {} //init data
                ...
        private:
                //static members - internal use only
                static const T TUNIT;
                //data
                T data;
};

template<> const float TClass<float>::TUNIT(1.0f);

works fine, so should be content then!

Jul 11th, 2007, 3:46 AM	#1
rwm Professional Programmer Join Date: Jan 2007 Location: Cape Town Posts: 291 Rep Power: 2	template doesnt compile this compiles fine with GCC, but not with visual studio 2005... im trying to pass a default value to a template class i.e. myclass<float,1.0f> (Toggle Plain Text) #include <iostream> using namespace std; template<typename T,const T TUNIT> class TClass { public: TClass() : data(TUNIT) {} TClass(const T &t) : data(t) {} T get_data() { return data; } private: T data; }; int main() { // TClass<int,1> tc; cout << tc.get_data() << endl; return 0; } #include <iostream> using namespace std; template<typename T,const T TUNIT> class TClass { public: TClass() : data(TUNIT) {} TClass(const T &t) : data(t) {} T get_data() { return data; } private: T data; }; int main() { // TClass<int,1> tc; cout << tc.get_data() << endl; return 0; } any idea why? i should have copied the error message from home

Jul 11th, 2007, 3:56 AM	#2
rwm Professional Programmer Join Date: Jan 2007 Location: Cape Town Posts: 291 Rep Power: 2	i dont understand! this works (Toggle Plain Text) TClass<int,1> tc; TClass<int,1> tc; but this doesnt (Toggle Plain Text) TClass<float,3.142f> tc; TClass<float,3.142f> tc; heres the error: (Toggle Plain Text) test.cpp:18: error: �float� is not a valid type for a template constant parameter test.cpp:18: error: invalid type in declaration before �;� tok test.cpp:18: error: �float� is not a valid type for a template constant parameter test.cpp:18: error: invalid type in declaration before �;� tok pretty much the same error i get with visual studio...

Jul 11th, 2007, 7:27 AM	#3
Eoin Hobbyist Programmer Join Date: Jun 2006 Location: Ireland Posts: 152 Rep Power: 3	Just as the error states you can't use a float as a non-type template parameter unfortunately, see http://publib.boulder.ibm.com/infoce...parameters.htm for example. __________________ Visit my website BinaryNotions.

Jul 11th, 2007, 8:10 AM	#4
rwm Professional Programmer Join Date: Jan 2007 Location: Cape Town Posts: 291 Rep Power: 2	arrrrrrrrrrgghhhhhhhh why? im trying to pass values to the template so i can initialize properly.. i.e. (Toggle Plain Text) template<typename T,T TZERO,T TUNIT> class TClass { public: TClass() : zero(TZERO),unit(TUNIT) {} ... private: //data T zero; T unit; }; template<typename T,T TZERO,T TUNIT> class TClass { public: TClass() : zero(TZERO),unit(TUNIT) {} ... private: //data T zero; T unit; }; is there any way to do this? thanks for the reply!

Jul 11th, 2007, 10:38 AM	#5
Oxyd Newbie Join Date: Dec 2004 Location: Czech Republic Posts: 1 Rep Power: 0	Hm, no. Template parameter can only be a type or an integral constant -- floats are simply not allowed. You could pass a default value to the constructor, though: (Toggle Plain Text) template <typename T> class TClass { public: TClass(T zero, T unit) : zero(zero), unit(unit) { } private: T const zero; T const unit; }; template <typename T> class TClass { public: TClass(T zero, T unit) : zero(zero), unit(unit) { } private: T const zero; T const unit; };

Jul 11th, 2007, 10:42 AM	#6
Eoin Hobbyist Programmer Join Date: Jun 2006 Location: Ireland Posts: 152 Rep Power: 3	To be honest off hand I'm not sure. You might be able to support floats as a fraction of two integers but that may not be allowed. __________________ Visit my website BinaryNotions.

Jul 11th, 2007, 10:45 AM	#7
rwm Professional Programmer Join Date: Jan 2007 Location: Cape Town Posts: 291 Rep Power: 2	ok thanks for the help! bit of a strange restriction right? EDIT: only saw your post Eoin, ah but thats tricky buggery and id like to avoid it... would be nice to put in (Toggle Plain Text) TClass<float,1.0f> tc1; TClass<double,1.0> tc2; TClass<float,1.0f> tc1; TClass<double,1.0> tc2; etc

Jul 12th, 2007, 2:39 AM	#9
rwm Professional Programmer Join Date: Jan 2007 Location: Cape Town Posts: 291 Rep Power: 2	well i found a way, its feels pretty much like a hack but works (Toggle Plain Text) template<typename T> class TClass { public: TClass() : data(TUNIT) {} //init data ... private: //static members - internal use only static const T TUNIT; //data T data; }; template<> const float TClass<float>::TUNIT(1.0f); template<typename T> class TClass { public: TClass() : data(TUNIT) {} //init data ... private: //static members - internal use only static const T TUNIT; //data T data; }; template<> const float TClass<float>::TUNIT(1.0f); works fine, so should be content then!

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Programming Forums > Application Development > C++

template doesnt compile