CODEWITHSUNDEEP
c++c++11templates
Sasidiran Sangamanautram
Sasidiran Sangamanautram

Reputation: 263

How can I specialise a template with a template?

Consider a template class

template<class T>
class Foo
{
};

for which I can write a simple specialisation

template<>
class Foo<int>
{
};

I have a situation where I want to specialise Foo with a template class, in detail with a bool which serves as a compile-time flag:

template<>
class Foo<int, bool> // Clearly not the correct notation.
{

}

Uses would include Foo<1, true> and Foo<1, false>.

What is the correct notation for the class name, where I've marked "Clearly not the correct notation."?

I code to the C++11 standard.

Upvotes: 0

Views: 94

Answers (3)

Yuriy
Yuriy

Reputation: 781

Seems like default value for template argument.

template<class T, bool flag = false>
class Foo
{
};

template<>
class Foo<int>
{
    //"false" specialization (default)
};

template<>
class Foo<int, true>
{
    //"true" specialization
};

Upvotes: 1

NathanOliver
NathanOliver

Reputation: 180575

You need to change the primary template to

template<class T, bool B>
class Foo
{
};

and then you can specialize it like

template<>
class Foo<int, true>
{
};

template<>
class Foo<int, false>
{
};
...

and then you would use it like

Foo<int, true> FooT;
Foo<int, false> FooF;

If you are going to use values for the first parameter like

Foo<1, true>

Then the primary template should be

template<int I, bool B>
class Foo
{
};

and then you can specialize it like

template<>
class Foo<1, true>
{
};

template<>
class Foo<1, false>
{
};
...

Upvotes: 5

SergeyA
SergeyA

Reputation: 62563

This is not directly possible. Your template wants a single parameter, you can't specialize it for two. However, you can (partially) specialize it for some other type which is a template of two parameters.

Example:

template<class T>
class Foo;

template<int, bool> class tag;

template<int>
class Foo<tag<int, true>> { ... };

template<int>
class Foo<tag<int, false>> { ... };

And than you can use it 

Foo<tag<1, true>> foo;

Upvotes: 4

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