Determine constructor from template

[Kibiz0r]

Basically, I want a templated method to do one thing if the type T has
a constructor that takes a std::istringstream, and another thing if it
doesn't.

The problem is that if I put the T(std::istringstream) call in there,
every type that is used for that method has to have that constructor
or it errors.

Can someone please point me in the right direction?
Here's (a slightly simplified version of) the code, if you want a look
at it.

template<typename T>
T getData() const
{
if (m_data->isSerialized())
{
const EventData<std::istringstream>* data =
dynamic_cast<const
EventData<std::istringstream>*>(m_data);
return T(data->getData());
}
else
{
const EventData<T>* data =
dynamic_cast<const EventData<T>*>(m_data);
return data->getData();
}
}

Thank you!

[Kai-Uwe Bux]

Kibiz0r wrote:

Basically, I want a templated method to do one thing if the type T has
a constructor that takes a std::istringstream, and another thing if it
doesn't.

The problem is that if I put the T(std::istringstream) call in there,
every type that is used for that method has to have that constructor
or it errors.

Can someone please point me in the right direction?

Such a constructor would make std::istringstream convertible to T. I think,
Boost has an

is_convertible< From, To >

template that would assign to

is_convertible< std::istringstream, T >::value

the value you are interested in. Then, you could use that value for partial
specialization

template < typename T,
bool convertible_from_istringstream =
is_convertible< std::istringstream, T >::value >
class some_action;

template < typename T >
class some_action< T, true {...};

template < typename T >
class some_action< T, false {...};

Here's (a slightly simplified version of) the code, if you want a look
at it.

template<typename T>
T getData() const
{
if (m_data->isSerialized())
{
const EventData<std::istringstream>* data =
dynamic_cast<const
EventData<std::istringstream>*>(m_data);
return T(data->getData());
}
else
{
const EventData<T>* data =
dynamic_cast<const EventData<T>*>(m_data);
return data->getData();
}
}

Thank you!

Best

Kai-Uwe Bux

[Kibiz0r]

On Nov 11, 7:45 pm, Kai-Uwe Bux <[e-mail]:

Kibiz0r wrote:

Basically, I want a templated method to do one thing if the type T has
a constructor that takes a std::istringstream, and another thing if it
doesn't.

The problem is that if I put the T(std::istringstream) call in there,
every type that is used for that method has to have that constructor
or it errors.

Can someone please point me in the right direction?

Such a constructor would make std::istringstream convertible to T. I think,
Boost has an

is_convertible< From, To >

template that would assign to

is_convertible< std::istringstream, T >::value

the value you are interested in. Then, you could use that value for partial
specialization

template < typename T,
bool convertible_from_istringstream =
is_convertible< std::istringstream, T >::value >
class some_action;

template < typename T >
class some_action< T, true {...};

template < typename T >
class some_action< T, false {...};

Here's (a slightly simplified version of) the code, if you want a look
at it.

template<typename T>
T getData() const
{
if (m_data->isSerialized())
{
const EventData<std::istringstream>* data =
dynamic_cast<const
EventData<std::istringstream>*>(m_data);
return T(data->getData());
}
else
{
const EventData<T>* data =
dynamic_cast<const EventData<T>*>(m_data);
return data->getData();
}
}

Thank you!

Best

Kai-Uwe Bux

Awesome. Thanks for the help.

I noticed you can't specify default template parameters for functions.
I got on fine without it, but I'm not entirely confident in my
solution, because I've never done this type of TMP before.

Here's what I came up with: http://pastebin.com/m263b2f9