C++ casting of templated class

Question

Hello,

I have a question about templates. I would like to have a templated class that contains, say, an array of either float's or double's.

I can write a clone() function that duplicates it. No problem. However, I would like to have another function called cast() that does a translation back and forth between double and float. This has been somewhat discussed before but I don't think the problem is the same:

stackoverflow.com/questions/714213/c-template-casting

The problem I have is a compiler error, not a linker error. The error message is:

---

main.cpp: In function `void caster()':
main.cpp:63: error: expected primary-expression before "double"
main.cpp:63: error: expected `;' before "double"
main.cpp:64: error: expected primary-expression before "float"
main.cpp:64: error: expected `;' before "float"
main.cpp:65: error: expected primary-expression before '>' token
main.cpp:65: error: expected primary-expression before ')' token

---

I dumped the code below. The lines 63, 64, and 65 are where I commented with "Error here".

By the way, my compiler is "GNU C++ version 3.4.5 20051201 (Red Hat 3.4.5-2) (x86_64-redhat-linux) compiled by GNU C version 3.4.5 20051201 (Red Hat 3.4.5-2)".

After some Googling, it turns out that someone has already experienced this problem:

gcc.gnu.org/ml/gcc-help/2006-04/msg00022.html

There is a solution here:

gcc.gnu.org/ml/gcc-help/2006-04/msg00023.html

But when the original poster asks as to why it works, the answer is not very clear:

gcc.gnu.org/ml/gcc-help/2006-04/msg00025.html

Unfortunately, the link is dead, and I don't own the third edition of Stroustrup. Now, I have my fix, my code works. But, Stackoverflow, why does it work?

---

#include <stdio.h>

// =================== This would be the header ===================
template <class T>
class foo
{
public:
             foo(const T val) {d_data = new double; *d_data = val;}
    virtual ~foo() {delete d_data;};

    foo* clone() const;

    template<class U>
    foo<U>* cast() const;

private:
    double *d_data;
};

// =================== This would be the implementation of the class ===================
template<class T>
foo<T>* foo<T>::clone() const
{
    return new foo<T>(*d_data);
}

template<class T>
template<class U>
foo<U>* foo<T>::cast() const
{
    return new foo<U>(*d_data);
}

template class foo<float>;
template class foo<double>;

template foo<float>* foo<float>::cast() const;
template foo<float>* foo<double>::cast() const;
template foo<double>* foo<float>::cast() const;
template foo<double>* foo<double>::cast() const;

// =================== Using the class ===================
template <class T>
void caster()
{
    foo<double> *f1 = NULL;
    foo<float>  *f2 = NULL;
    foo<T>      *f3 = NULL;

    // I am looking at something that compiles
    // I don't care about linking for now
    // This will crash at runtime because of
    // NULL, but that's just an example

    f1->cast<double>(); // compiler OK
    f1->cast<float>();  // compiler OK
    f1->cast<T>();      // compiler OK

    f2->cast<double>(); // compiler OK
    f2->cast<float>();  // compiler OK
    f2->cast<T>();      // compiler OK

    f3->cast<double>(); // Error here
    f3->cast<float>();  // Error here
    f3->cast<T>();      // Error here

    f3->foo<T>::template cast<double>(); // It works!
    f3->foo<T>::template cast<float>();  // It works!
    f3->foo<T>::template cast<T>();      // It works!
}

int main(int argc, char **argv)
{
    return 0;
}

Answer 1

f3->cast<double>(); // Error here 

In this line, the compiler does not know if the < after f3->cast is supposed to mean the beginning of template arguments or if it is a less than comparison operator.

To explicitly specify that it stands for beginning of template argument. The correct way is

f3->template cast<double>();

In summary, The .template notation (and similar notations such as ->template) should be used only inside templates and only if they follow something that depends on a template parameter (such as the expression f3 which depends upon the template parameter T)