Function templates are implemented for the most part. The compiler can correctly determine template parameter values, and will delay instantiation of a function that uses templates until the requisite type information is available.
The following limitations remain:
Narrowed specification: function declarations should not prevent template expansion. When you declare a function, gnu C++ interprets the declaration as an indication that you will provide a definition for that function. Therefore, gnu C++ does not use a template expansion if there is also an applicable declaration. gnu C++ only expands the template when there is no such declaration.
The specification in Bjarne Stroustrup's The C++ Programming Language, Second Edition is narrower, and the gnu C++ implementation is now clearly incorrect. With this new specification, a declaration that corresponds to an instantiation of a function template only affects whether conversions are needed to use that version of the function. It should no longer prevent expansion of the template definition.
For example, this code fragment must be treated differently:
template <class X> X min (X& x1, X& x2) { ... }
int min (int, int);
...
int i; short s;
min (i, s); // should call min(int,int)
// derived from template
...
The compiler does not yet understand function signatures where types are nested within template parameters. For example, a function like the following produces a syntax error on the closing `)' of the definition of the function f :
template <class T> class A { public: T x; class Y {}; };
template <class X> int f (A<X>::Y y) { ... }
If you declare an inline function using templates, the compiler can only inline the code after the first time you use that function with whatever particular type signature the template was instantiated.
Removing this limitation is akin to supporting nested function definitions in gnu C++; the limitation will probably remain until the more general problem of nested functions is solved.
All the method templates (templates for member functions) for a class must be visible to the compiler when the class template is instantiated.