Template function in class - c++

Question

I don't know what to do. I always get an error by using a simple class and a simple template function. I read all the other solutions but they didn't helped me.
Next to some other classes I have the simple class Data:
class Data{
public:
template <class T>
T dat();
int id;
union{
char c;
int i;
double d;
};
};
and the function dat:
template <class T>
T Data::dat(){
if(id == 1) return i;
if(id == 2) return d;
if(id == 3) return c;
}
As you can see, I want to check the id and return int, double or char.
Now I've tried to print the value in the main function like this:
Data test;
test.id=1;
test.i = 12;
cout<<test.dat();
But I always get this error message:
Error: Could not find a match for Data::dat<Data::T>() needed in main(int, char**).
Where is the problem??
Thank you

Answer 1

To put it precisely, you want the return type of the function to
depend on it's the id field in the object; in other words,
dynamically. Templates are resolved at compile time, so they
cannot help here. You'll have to return something like
boost::variant or boost::any, which supports such dynamic
typing.

Answer 2

Use this:
cout<<test.dat<int>();

Answer 3

dat() has no parameters involving T, so the compiler cannot deduce T from the call and it must be provided explicitly, e.g.:
cout << test.dat<int>();
Also, bear in mind you must implement dat() in the header file.

Answer 4

I don't know what to do. I always get an error by using a simple class and a simple template function. I read all the other solutions but they didn't helped me.
It seems to me that you want to create a discriminated union.
Your implementation won't work, because the return type of a template function is determined at compilation time (i.e. before you set a value in id and try to call the function.
Solution:
class Data
{
enum value_type {
int_val, char_val, double_val
} discriminator; // private (the user doesn't see this)
// this replaces your id
union{
char c;
int i;
double d;
} value;
public:
class wrong_type_error: public std::logic_error
{
public:
wrong_type_error(const std::string& msg): std::logic_error(msg) {}
};
explicit Data(const char c)
: discriminator(Data::char_value)
, value.c(c)
{
}
explicit Data(const int i)
: discriminator(Data::int_value)
, value.i(i)
{
}
explicit Data(const double d)
: discriminator(Data::double_value)
, value.d(d)
{
}
// don't put this here: int id;
// this part can be optimized to simpler (more idiomatic) code
template<typename T> T get() const; // undefined
template<> int get() const {
if(discriminator != Data::int_val)
throw wrong_type_error("Cannot return a int from Data instance");
return value.i;
}
template<> char get() const {
if(discriminator != Data::char_val)
throw wrong_type_error("Cannot return a char from Data instance");
return value.c;
}
template<> double get() const {
if(discriminator != Data::double_val)
throw wrong_type_error("Cannot return a double from Data instance");
return value.d;
}
};
Client code:
Data test(10.5);
cout<<test.get<double>();
All that said, you should consider using a boost::variant or boost::any instance, depending on your needs.

Answer 5

VS2012 says
"error C2783: 'T Data::dat(void)' : could not deduce template argument for 'T'"
You just need to tell the function dat what T is:
cout<<test.dat<int>();
The template type can be deduced if you pass a templated parameter, but it cannmot guess the return type.