TITLE: const and template parameters

(Newsgroups: comp.lang.c++.moderated,comp.lang.c++,comp.std.c++, 22 Mar 97)

SILVERMAN: Greg Silverman <greg_silverman@smtp.svl.trw.com>

> template <class T>
> class A {
>   public:
>     void g(const T& t) { }
> };
> 
> int main()
> {
>   A<char*> b;
>   const char* c = new char;
>   b.g(c); //(1)
> }
> 
> The compiler complains about lines (1). It says
>     Formal argument of type char* const & in call to A<char*>::g(char* 
>     const&) is being passed const char*.
>
> Uh? Why does it think the argument type is a "char* const &". I declared
> it as a "const char* &" through the template, but the compiler moved the
> placement of the const type-specifier. Why?


SZONYE: "Bradd W. Szonye" <bradds@concentric.net>

This is one of the reasons I recommend writing 'T const &' and 'T const *'
instead of the equivalent but confusing 'const T &' et al. Now look at

    void g(T const & t);

where T is 'char*' as in your example, then the declaration is

    void g(char * const & t);

which is a reference to a constant pointer, not a reference to pointer to
constant. What you need to do is specialize your template for pointer
types:

    template<class T> A {
    public: void g(T const & tcr);
    };

    template<class T> A<T*> {
    public: void g(T const * & tcpr);
    };