template function argument deduce

Discussion in 'C Programming' started by George2, Mar 11, 2008.

  1. George2

    George2

    Joined:
    Feb 12, 2008
    Messages:
    108
    Hello everyone,


    Why in the following code segment (along with Bjarne's comments), function get_new can not deduce template parameter type to be int by its return value assignment to int * p1, and deduce T* is int*, and so T is int (e.g. as we did for function release in the same sample below)?

    Code:
    class Memory { // some Allocator
    public:
    template <class T> T * get _new ();
    template <class T > void release (T&);
    / / ...
    };
    
    template <class Allocator> void f (Allocator &m )
    {
    int * p1 = m.get _ new <int>(); // syntax error: int after lessthan
    operator
    int * p2 = m.template get _ new <int>(); // explicit qualification
    / / ...
    m.release (p1); // template argument deduced: no explicit qualification needed
    m.release (p2 );
    }
    
    Explicit qualification of get _new() is necessary because its template parameter cannot be deduced. In this case, the t e m p l a t e prefix must be used to inform the compiler (and the human reader) that get _new is a member template so that explicit qualification with the desired type of element is possible. Without the qualification with template , we would get a syntax error because the < would be assumed to be a less than operator. The need for qualification with template is rare because most template parameters are deduced.


    thanks in advance,
    George
     
    George2, Mar 11, 2008
    #1
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