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template function argument deduce

George2 George2 is offline
Senior Member
Join Date: Feb 2008
Posts: 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)?

class Memory { // some Allocator
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
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,
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