Is this template possible?

[Alex Buell]

Is there anyway I can do something like this? This won't compile. Any
ideas.

#include <iostream>

template <typename T>
class sample
{
public:
sample(T n) { std::cout << sizeof(n) << "\n"; }
~sample();
};

int main(int argc, char *argv[])
{
unsigned char n = 0x55;

sample s(n);
return 0;
}

 

[Kai-Uwe Bux]

Is there anyway I can do something like this? This won't compile. Any
ideas.

#include <iostream>

template <typename T>
class sample
{
public:
sample(T n) { std::cout << sizeof(n) << "\n"; }
~sample();
};

int main(int argc, char *argv[])
{
unsigned char n = 0x55;

sample s(n);
return 0;
}

#include <iostream>

class sample
{
public:

template < typename T >
sample(T const & n) {
std::cout << sizeof(T) << "\n";
}

};

int main(int argc, char *argv[])
{
unsigned char n = 0x55;
sample s(n);
return 0;
}


But *why*?


Best

Kai-Uwe Bux

 

[Luke Meyers]

Is there anyway I can do something like this? This won't compile. Any
ideas.

template <typename T>
class sample
{
public:
sample(T n) { std::cout << sizeof(n) << "\n"; }
~sample();
};

int main(int argc, char *argv[])
{
unsigned char n = 0x55;
sample s(n);
return 0;
}

Hmm, so you want the compiler to infer the class template parameter
from the constructor? Seems you can't, and I guess it's because the
compiler must know which class template to instantiate before it can
look up the constructor. Yeah, I'm pretty sure that's it. But you can
templatize the constructor itself, as already mentioned.

Luke

 

[Ian Collins]

Is there anyway I can do something like this? This won't compile. Any
ideas.

#include <iostream>

template <typename T>
class sample
{
public:
sample(T n) { std::cout << sizeof(n) << "\n"; }
~sample();
};

not very practical, but you can use

template <typename T>

int main(int argc, char *argv[])
{
unsigned char n = 0x55;

sample s(n); maker(n);
return 0;
}

 

[Luke Meyers]

Is there anyway I can do something like this? This won't compile. Any
ideas.

template <typename T>
class sample
{
public:
sample(T n) { std::cout << sizeof(n) << "\n"; }
~sample();
};

int main(int argc, char *argv[])
{
unsigned char n = 0x55;
sample s(n);
return 0;
}

Hmm, so you want the compiler to infer the class template parameter
from the constructor? Seems you can't, and I guess it's because the
compiler must know which class template to instantiate before it can
look up the constructor. Yeah, I'm pretty sure that's it. But you can
templatize the constructor itself, as already mentioned.

Oh, just thought of another option:

// free factory function
template <class T>
sample<T> createSample(T const& t);

Luke

 

[Alex Buell]

class sample
{
public:

template < typename T >
sample(T const & n) {
std::cout << sizeof(T) << "\n";
}

};

Ah, brilliant thanks.

 

[Rolf Magnus]

not very practical, but you can use

template <typename T>
sample<T> maker( const T t ) { return sample<T>( t ); }

Why not practical? The C++ standard library does that, too. Think of
std::make_pair.

int main(int argc, char *argv[])
{
unsigned char n = 0x55;

sample s(n); maker(n);
return 0;
}

 

[Ian Collins]

Ian Collins wrote:


Why not practical? The C++ standard library does that, too. Think of
std::make_pair.

I'd overlooked return value optimisations, so I guess it is practical,
as long as you don't use it as a syntactic crutch for frequently
building complex objects.

This example and std::make_pair both create simple objects.

 

[Salt_Peter]

Is there anyway I can do something like this? This won't compile. Any
ideas.

#include <iostream>

template <typename T>
class sample
{
public:
sample(T n) { std::cout << sizeof(n) << "\n"; }
~sample();

Your destructor is declared but not defined.

};

int main(int argc, char *argv[])
{
unsigned char n = 0x55;

sample s(n);

try...

return 0;
}

This works:

#include <iostream>
#include <ostream>

template< typename T >
class Sample
{
T t;
public:
Sample(T n) : t(n) { }
~Sample() { }
T get() const { return t; }
};

int main()
{
typedef unsigned char UChar;
UChar uc = 0x137;

Sample< UChar > sample(uc);
std::cout << sample.get() << std::endl;

return 0;
}

/*
7
*/