Template specialisation with multiple typenames

[Adam Nielsen]

Hi everyone,

I've having some trouble trying to specialise a template. My original
code, with one typename, worked:

template <typename T>
class MyClass
{
private:
T data;

public:
const std::string& toString() const
{
// convert this->data to a string and return
}
};

const std::string& MyClass<std::string>::toString() const
{
return this->data; // don't bother with unnecessary conversion
}

However I then tried to extend the class with a second template
parameter, and it stopped working:

template <class C, typename T>
class MyClass
...

template <class C>
const std::string& MyClass<C, std::string>::toString() const
{
return this->data; // don't bother with unnecessary conversion
}

It would appear to work if I write code for every possible value of C,
but then that's the whole reason for having a template! I'm assuming
I've just gotten the syntax wrong, so if someone would be kind enough to
point out the correct way of doing this it would be much appreciated!

Many thanks,
Adam.

 

[Barry]

Hi everyone,

I've having some trouble trying to specialise a template. My original
code, with one typename, worked:

template <typename T>
class MyClass
{
private:
T data;

public:
const std::string& toString() const
{
// convert this->data to a string and return
}
};

const std::string& MyClass<std::string>::toString() const
{
return this->data; // don't bother with unnecessary conversion
}

However I then tried to extend the class with a second template
parameter, and it stopped working:

template <class C, typename T>
class MyClass

class MyClass<Type1>


for example:

template <class F = void()>
class Func
{
};

template <class Ret, class Arg>
class Func<Ret(Arg)>
^^^^^^^^^^
{
};

without "<Ret(Arg)>", the compiler assumes that you're defining another
*primary class template* with the same name, which violates the ODR.
with "<Ret(Arg)>", it's a partial specialization. Be aware the number if
template parameters in the <> must be same as the primary class template.

You can't have a template class with different template arguments
(default arguments also count). So you can't do it.

 

[Adam Nielsen]

Hi Barry,

Thanks for your reply!

class MyClass<Type1>


for example:

template <class F = void()>
class Func
{
};

template <class Ret, class Arg>
class Func<Ret(Arg)>
^^^^^^^^^^
{
};

without "<Ret(Arg)>", the compiler assumes that you're defining another
*primary class template* with the same name, which violates the ODR.
with "<Ret(Arg)>", it's a partial specialization. Be aware the number if
template parameters in the <> must be same as the primary class template.

Sorry, I don't think I explained myself properly! I'm not after a
second MyClass with different template parameters, I want to remove
MyClass<typename T> and instead have MyClass<class C, typename T>.

This works, but when it comes to defining a function in the class, it
doesn't like my syntax:

// Works with MyClass<typename T> (specialisation when T=std::string)
const std::string& MyClass<std::string>::toString() const
{
return this->data;
}

// Does not work with MyClass<class C, typename T>
template <class C>
const std::string& MyClass<C, std::string>::toString() const
{
return this->data;
}

I would like the second function to be used for all instances where T =
std::string, regardless of what C might be given as when constructing a
MyClass object.

Thanks,
Adam.

 

[xtrigger303]

Hi Barry,

Thanks for your reply!









Sorry, I don't think I explained myself properly! I'm not after a
second MyClass with different template parameters, I want to remove
MyClass<typename T> and instead have MyClass<class C, typename T>.

This works, but when it comes to defining a function in the class, it
doesn't like my syntax:

// Works with MyClass<typename T> (specialisation when T=std::string)
const std::string& MyClass<std::string>::toString() const
{
return this->data;
}

// Does not work with MyClass<class C, typename T>
template <class C>
const std::string& MyClass<C, std::string>::toString() const
{
return this->data;
}

I would like the second function to be used for all instances where T =
std::string, regardless of what C might be given as when constructing a
MyClass object.


Thanks,
Adam.

Hi,
IMVHO in the case you have described you have to partially specialize
the whole class.
Which is something quite boring. You can you something like the
following trick, obviosuly you have to tailor it to your needs...
Bye,
Francesco

#include <iostream>
#include <string>

template< typename T >
class CTypeFromType
{};

template< typename T1, typename T2 >
class A
{
public:

std::string Do( void ) { return Do( CTypeFromType< T2 >() ); }

private:

template< typename T3 >
std::string Do( CTypeFromType< T3 > )
{ return "GENERAL"; }

std::string Do( CTypeFromType< std::string > )
{ return "SPECIALIZED"; }
};




int main()
{
A< int, double > obj1;
std::cout << obj1.Do() << std::endl;

A< int, std::string > obj2;
std::cout << obj2.Do() << std::endl;
}

 

[Adam Nielsen]

IMVHO in the case you have described you have to partially specialize

the whole class.
Which is something quite boring. You can you something like the
following trick, obviosuly you have to tailor it to your needs...

That's quite an interesting trick, I never thought of doing something
like that - very useful! There's probably a bit too much overhead in
that for my intended use, but it's intriguing nonetheless.

Thanks,
Adam.