Question

I am studying C++ concepts – Template specialization and Partial template specialization. Have a code as below, which I want to understand so I get these concepts correctly.

I have few questions regarding that, which are asked inline below:

template <typename T, int nSize>
class Buffer
{
private:   
    T m_atBuffer[nSize];

public:
    T* GetBuffer()
    {
        return m_atBuffer;
    }

    T& operator[](int nIndex)
    {
        return m_atBuffer[nIndex];
    }
};


template <typename T, int nSize>
void PrintBufferString(Buffer<T, nSize> &rcBuf)
{
    std::cout << rcBuf.GetBuffer() << std::endl;
}


void PrintBufferString(Buffer<char, 10> &rcBuf)
{
    std::cout << rcBuf.GetBuffer() << std::endl;
}


int main()
{
    // declare a char buffer
    Buffer<char, 13> cChar10Buffer;

    // copy a value into the buffer
    strcpy(cChar10Buffer.GetBuffer(), "Ten");

    PrintBufferString(cChar10Buffer); //This prints "Ten"

    // declare an int buffer
    Buffer<int, 10> cInt10Buffer;

    // copy values into the buffer
    for (int nCount=0; nCount < 10; nCount++)
        cInt10Buffer[nCount] = nCount;

    PrintBufferString(cInt10Buffer); // This prints address of the buffer- m_atBuffer for object cInt10Buffer

    return 0;
}

So if we pass any type other than char to the templated function PrintBufferString() it cout prints the address of the buffer rather than the string, which is a problem.

So to solve this problem, it said that we define a template specialization as shown below, to ensure that only arrays of type char can be passed to PrintBufferString()

void PrintBufferString(Buffer<char, 10> &rcBuf)
{
    std::cout << rcBuf.GetBuffer() << std::endl;

}

Question 1: So adding this template specialization for function PrintBufferString (), I thought that it should have given a compilation error when I tried to call

PrintBufferString(cInt10Buffer) by passing a Buffer object with templated type parameter int, but it compiled fine? How is that? Then what is the use of adding this template specialization for type char?

I thought adding a template specialization for type char, we cannot call it for any other type

Question 2: Then I added another function call as below in main:

Buffer<char, 11> cChar11Buffer;
strcpy(cChar11Buffer.GetBuffer(), "Eleven");

PrintBufferString(cChar11Buffer); //

It said this would give compilation error, but it compiled fine in MS -Visual C++ 2010 Express.IT even executed fine and printed “Ten” “some address” “Eleven”.

Why did it compile and execute fine? Because I had understanding that type a Class Buffer is different than Class Buffer, and function
PrintBufferString() accepts object of class type Buffer, and both of these cannot be intermixed?

Question 3: Then it went on to define a partial template specialization as below to handle the case when a object buffer of type char but any size can be passed Class object type, which is then passed to the function PrintBufferString();

template<int nSize>
void PrintBufferString(Buffer<char, nSize> &rcBuf)
{
    std::cout << rcBuf.GetBuffer() << std::endl;
}

Now also it printed “Ten” “Eleven” just as before. So what special did this partial template specialization achieve? Is this not a good example of partial template specialization? The concept is not very clear to me, from this example.

Answer 1

C++ doesn’t have/support partial specialization of function templates, only of class templates. As such, none of the code you’re looking at is related to what you (at least say you) want to study/understand at all.

A partially specialized class template would be something like this:

template <class T, class U>  // the "base" (unspecialized) template
class X { 
};

template <class T>            // partial specialization for `X<whatever, int>`
class X<int> {
};

Note that (as here) the un-specialized template must precede the partially specialized template.