template class with a single method specialized in C++

问题

I only have an hpp file for a school assignment in C++ (I am not allowed to add a cpp file, declaration and implementation should be both written in the file).

I wrote this code inside it:

template<class T>
class Matrix
{
   void foo()
   {
       //do something for a T variable.
   }
};

I would like to add another foo method, but this foo() will be specialized for only an <int>. I have read in some places that I need to declare a new specialization class for this to work. But what I want is that the specialized foo will lie just beneath the original foo, so it will look like this:

template<class T>
class Matrix
{
   void foo(T x)
   {
       //do something for a T variable.
   }
   template<> void foo<int>(int x)
   {
       //do something for an int variable.
   }
};

Why am I getting an error for this syntax ("expected unqualified-id before '<' token")?
Why isn't this possible?
How can I fix this without declaring a new specialized class?

Thanks

回答1:

foo isn't a template. It's a member function of a template. Thus foo<int> is meaningless. (Also, explicit specializations must be declared at namespace scope.)

You can explicitly specialize a member function of a particular implicit instantiation of a class template:

template<class T>
class Matrix
{
   void foo(T x)
   {
       //do something for a T variable.
   }
};

// must mark this inline to avoid ODR violations
// when it's defined in a header
template<> inline void Matrix<int>::foo(int x)
{
     //do something for an int variable.
}

回答2:

You need to define the original foo method as a template, and actually there is no need for your class to be a template, only the method:

class Matrix
{
    template<typename T> void foo(T x)
    {
        //do something for a T variable.
    }
    template<> void foo<int>(int x)
    {
        //do something for an int variable.
    }
};

UPDATE: The code works only in Visual Studio. Here is a code that should work elsewhere too:

class Matrix
{
    template<typename T> void foo(T x)
    {
        //do something for a T variable.
    }
};

template<> void Matrix::foo<int>(int x)
{
    //do something for an int variable.
}

来源：https://stackoverflow.com/questions/25724360/template-class-with-a-single-method-specialized-in-c

标签

c++

templates

template-specialization