simple(?) question about template

[dgront]

Maybe my question is too simple, but I've spent some time on it and
still don't know..

I need a template function, converting from string to other data types:
template <typename T> converter(std::string s);

For T = double, int, etc. it works fine. But how to do it for a type
which already is a template, like

template<typename T> class Point { T x, y,z; };
template<typename T> class AnotherClass { T data; };

How to make converter working both for Point<T>, AnotherClacc<T>,
double, int, etc...?

Dominik

 

[Victor Bazarov]

Maybe my question is too simple, but I've spent some time on it and
still don't know..

I need a template function, converting from string to other data types:
template <typename T> converter(std::string s);

You mean,

template<typename T> T converter(std::string const& s);

, don't you?

For T = double, int, etc. it works fine. But how to do it for a type
which already is a template,

There is no such thing. It's either a type or a template.

like

template<typename T> class Point { T x, y,z; };
template<typename T> class AnotherClass { T data; };

How to make converter working both for Point<T>, AnotherClacc<T>,
double, int, etc...?

How do you plan on using it?

V

 

[rod]

A way I got this to work is as follows:

template< template<typename U> class T >
void converter(string & sName)
{
T<typename> nVar;
sName = nVar.toString(); \\ or whatever string conversion function
}

 

[Victor Bazarov]

A way I got this to work is as follows:

template< template<typename U> class T >
void converter(string & sName)
'void'?

{
T<typename> nVar;
sName = nVar.toString(); \\ or whatever string conversion function
}

So, how does the value get back? And how do you see it used?

 

[rod]

Sorry I was working the other way around. Try this:

template< template<typename U> class T >
T<typename> converter(const string & sData)
{
T<typename> nVar;
nVar.fromString(sData);
return nVar;
}

 

[rod]

Sorry I was converting the other way around. Try this:

template< template<typename U> class T >
T<typename> converter(const string & sData)
{
T<typename> nVar;
nVar.fromString(sData);
return nVar;
}

 

[Victor Bazarov]

Sorry I was working the other way around. Try this:

template< template<typename U> class T >
T<typename> converter(const string & sData)
{
T<typename> nVar;
nVar.fromString(sData);
return nVar;
}

Uh... Doesn't seem to work. Here's a complete program, please make
it compile with Comeau online:

#include <string>
#include <sstream>

template<class T> class Point {
public:
T x,y,z;
void fromString(std::string const& s) {
std::istringstream is(s);
is >> x >> y >> z;
}
};

template< template<typename U> class T >
T<typename> converter(const std::string & sData)
{
T<typename> nVar;
nVar.fromString(sData);
return nVar;
}

int main()
{
Point<double> pd;
pd = converter<Point>("1.0 2.0 3.0"); // I am guessing here ???

return 0;
}

 

[Daniel T.]

Maybe my question is too simple, but I've spent some time on it and
still don't know..

I need a template function, converting from string to other data types:
template <typename T> converter(std::string s);

For T = double, int, etc. it works fine. But how to do it for a type
which already is a template, like

template<typename T> class Point { T x, y,z; };
template<typename T> class AnotherClass { T data; };

How to make converter working both for Point<T>, AnotherClacc<T>,
double, int, etc...?

I suggest you use boost's lexical_cast.

Point p = lexical_cast<Point>( myStringRep );

Below is a rough version, the boost version is much more robust.

template < typename T, typename U >
T lexical_cast( const U& arg ) {
std::stringstream ss;
T result;

if ( !( ss << arg && ss >> result ) )
throw std::exception();
return result;
}

 

[rod]

That code worked fine for me on VC7.1 but did not on Comeau. The only
way I've got this to work is as follows (which is not necessarily the
way you wanted to use it):

#include <string>
#include <sstream>

template<class T>
class Point
{
public:
T x,y,z;

void fromString(const std::string & s)
{
std::istringstream is(s);
is >> x >> y >> z;
}

void outputString()
{
std::cout << x << " " << y << " " << z << std::endl;
}
};

template< class T >
T converter(const std::string & sData)
{
T nVar;
nVar.fromString(sData);
return nVar;
}

int main()
{
Point<double> pd;
pd = converter< Point<double> >("1.0 2.0 3.0"); // I am guessing here
???
pd.outputString();
return 0;
}

 

[Victor Bazarov]

That code worked fine for me on VC7.1 but did not on Comeau. The only
way I've got this to work is as follows (which is not necessarily the
way you wanted to use it):

[...]
template< class T >
T converter(const std::string & sData)
{
T nVar;
nVar.fromString(sData);

Sure. But going back to the OP's question, how do you make it work with
built-in types now? They don't have 'fromString' member functions, do
they?

return nVar;
}

V

 

[Victor Bazarov]

I suggest you use boost's lexical_cast.

Point p = lexical_cast<Point>( myStringRep );

'Point' is not a class. It's a template.

Below is a rough version, the boost version is much more robust.

template < typename T, typename U >
T lexical_cast( const U& arg ) {
std::stringstream ss;
T result;

if ( !( ss << arg && ss >> result ) )
throw std::exception();
return result;
}

V

 

[rod]

One way would be to use template specialization...one for each built-in
type you want to convert a string to:

template<>
double converter(const std::string & sData)
{
// Code that converts a string to a double
}

.....

 

[rod]

One way would be to use template specialization for each built-in type
you wanted to convert to from a string, i.e:

template<>
double converter(const std::string & sData)
{
// Code that converts to a double
}

 

[Daniel T.]

'Point' is not a class. It's a template.
Fine:



V