Greetings,
I cannot figure out why the following code does not compile:
--- BEGIN CODE ---
typedef double ftype(double);
struct A
{
double x;
template<ftype F>
double eval()
{
return F(x);
}
};
template<ftype F>
struct B
{
A a;
double foo()
{
return a.eval<F>();
}
};
--- END CODE ---
It generates the error message:
scratch.cpp: In member function `double B<F>::foo()':
scratch.cpp:23: error: parse error before `;' token
[ line 23 being return a.eval<F>(); ]
Compiler gcc (GCC) 3.3.3 (cygwin special) on Win2K
Any help appreciated,
--
Lionel B 10 1765
"Lionel B" <go****@lionelb .com> schrieb im Newsbeitrag
news:e1******** *************** ***@posting.goo gle.com... Greetings,
I cannot figure out why the following code does not compile:
--- BEGIN CODE ---
typedef double ftype(double);
what you do is:
template <double f> double func(f x);
But: You don't want to give the type of the template argument, since
it's a "template".
So, use
template<class C> double fkt(C argC); struct A { double x;
template<ftype F> double eval() { return F(x); } };
template<ftype F> struct B { A a;
double foo() { return a.eval<F>(); } };
--- END CODE ---
It generates the error message: scratch.cpp: In member function `double B<F>::foo()': scratch.cpp:23: error: parse error before `;' token
[ line 23 being return a.eval<F>(); ]
Compiler gcc (GCC) 3.3.3 (cygwin special) on Win2K
Any help appreciated,
-- Lionel B
Gernot Frisch wrote: "Lionel B" <go****@lionelb .com> schrieb im Newsbeitrag news:e1******** *************** ***@posting.goo gle.com... Greetings,
I cannot figure out why the following code does not compile:
--- BEGIN CODE ---
typedef double ftype(double);
what you do is: template <double f> double func(f x); But: You don't want to give the type of the template argument, since it's a "template". So, use
template<class C> double fkt(C argC);
Apologies if I'm being obtuse, but I don't understand this reply at all
:(
--
Lionel B
Hi
Lionel B wrote: double foo() { return a.eval<F>(); } };
--- END CODE ---
It generates the error message: scratch.cpp: In member function `double B<F>::foo()': scratch.cpp:23: error: parse error before `;' token
This is one of the rare cases where you need to tell the compiler that the
name eval is a member template (it's a dependent name), otherwise the '<'
is considered "less than".
double foo()
{
return a.template eval<F>();
}
Markus
<go****@lionelb .com> schrieb im Newsbeitrag
news:10******** **************@ k26g2000oda.goo glegroups.com.. . Gernot Frisch wrote: "Lionel B" <go****@lionelb .com> schrieb im Newsbeitrag news:e1******** *************** ***@posting.goo gle.com... > Greetings, > > I cannot figure out why the following code does not compile: > > --- BEGIN CODE --- > > typedef double ftype(double);
what you do is: template <double f> double func(f x); But: You don't want to give the type of the template argument, since it's a "template". So, use
template<class C> double fkt(C argC);
Apologies if I'm being obtuse, but I don't understand this reply at all :(
er... do you know what templates are and what you use them for?
What are you trying to do? I don't get a clue from your source code,
that I have in common with your compiler ;)
Markus Moll wrote: Hi
Lionel B wrote:
double foo() { return a.eval<F>(); } };
--- END CODE ---
It generates the error message: scratch.cpp: In member function `double B<F>::foo()': scratch.cpp:23: error: parse error before `;' token This is one of the rare cases where you need to tell the compiler
that the name eval is a member template (it's a dependent name), otherwise the
'<' is considered "less than".
double foo() { return a.template eval<F>(); }
Wow, I wouldn't have got that in a million years... never seen that
syntax before.
Many thanks,
--
Lionel B
Gernot Frisch wrote: <go****@lionelb .com> schrieb im Newsbeitrag news:10******** **************@ k26g2000oda.goo glegroups.com.. . Gernot Frisch wrote: "Lionel B" <go****@lionelb .com> schrieb im Newsbeitrag news:e1******** *************** ***@posting.goo gle.com... > Greetings, > > I cannot figure out why the following code does not compile: > > --- BEGIN CODE --- > > typedef double ftype(double);
what you do is: template <double f> double func(f x); But: You don't want to give the type of the template argument, since it's a "template". So, use
template<class C> double fkt(C argC); Apologies if I'm being obtuse, but I don't understand this reply at
all :(
er... do you know what templates are and what you use them for?
Yes.
What are you trying to do? I don't get a clue from your source code, that I have in common with your compiler ;)
I am implementing Todd Veldhuizen's "Pointer-to-function as a template
parameter" scheme to inline callbacks in extensive loops. See http://osl.iu.edu/~tveldhui/papers/techniques/ (Section 1.4) for the
rationale behind the technique.
My sample code is a "minimalist example" of the real problem which
arose when trying to build a template class (my class B above) with a
function (my function B::foo) that calls the member template function
(my A::eval) for a member object (my B::a). I hope this is clear(ish).
The (rather esoteric) solution I was looking for was supplied by Markus
Moll in a previous post.
Regards,
--
Lionel B
> return a.template eval<F>();
WTF? Never seen C++ like this before. Now, this is totally insane.
Let me get this in my hamster brain:
F is a function of type: double()(double ).
And class A's got a member:
template<ftype F> double eval();
Now, class B has a function that will call a 'eval' of an 'A' object
er... No. I'm too stupid. Honestly, can anyone please give me a like
to what you did here?
"Gernot Frisch" <Me@Privacy.net > wrote in message Now, class B has a function that will call a 'eval' of an 'A' object er... No. I'm too stupid. Honestly, can anyone please give me a like to what you did here?
I think Markus gave the explanation. You may want to read Josuttis and
Vandevoorde's book on Templates - "C++ Templates: The Complete Guide", they
cover many such tips and traps in their book.
Sharad
Gernot Frisch wrote in news:2s******** *****@uni-berlin.de in
comp.lang.c++: return a.template eval<F>();
WTF? Never seen C++ like this before. Now, this is totally insane. Let me get this in my hamster brain: F is a function of type: double()(double ). And class A's got a member: template<ftype F> double eval();
Now, class B has a function that will call a 'eval' of an 'A' object er... No. I'm too stupid. Honestly, can anyone please give me a like to what you did here?
Well the ".template" is only required here because of a compiler error,
with a conforming compiler, you can write:
return a.eval< F >();
However if 'a' was dependant on a template paramiter (it isn't in the
OP's code) the the compiler needs to be told that object a's eval
member is a template member, otherwise the compiler treats the
above as:
return ((a.eval) < F ) > ();
In this case a syntax error ! however:
struct A
{
template < int N >
void eval( int a );
};
struct B
{
int eval;
}
template < typename T >
void example( T t )
{
t.eval < 10 > ( 13 );
};
Substitute A and B for T in example() and you can see that without
the .template both could (but don't) compile with completely
different symantics.
example() is designed to work with B style object's, i.e. with
a simple eval member.
template < typename T >
void another( T t )
{
t.template eval < 10 > ( 13 );
};
another() above is designed to work with A style object's, i.e.
with a template member function eval.
Rob.
-- http://www.victim-prime.dsl.pipex.com/ This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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