Greetings,
I'm attempting to write my first *real* template function that also deals with
a map of strings to member function pointers that is making the syntax a little
tricky to get right.
The function in question:
36: template <typename Container,
37: typename OutputIterator,
38: typename UnaryOp>
39: void
40: transform_field s_into_matches(
41: typename Container::cons t_iterator first,
42: typename Container::cons t_iterator last,
43: OutputIterator result,
44: const fields_type& fields,
45: const std::map<std::s tring,
46: const std::string& (Container::val ue_type::*)(voi d) const > & fm,
47: UnaryOp op)
48: {
49: typedef const std::string& (Container::val ue_type::*mfp)( void) const;
50:
51: util::Regex criteria;
52: const int cflags(options: :eregex() ?
53: util::Regex::ex tended|util::Re gex::icase : util::Regex::ic ase);
54:
55: for (; first != last ; ++first)
56: {
57: fields_type::co nst_iterator f;
58: for (f = fields.begin() ; f != fields.end() ; ++f)
59: {
60: /* check if field is valid */
61: std::map<std::s tring, mfp>::const_ite rator i = fm.find(f->first);
62: if (i == fm.end())
63: throw InvalidField(f->first);
64:
65: /* it's valid, so compile regex */
66: criteria.assign (f->second, cflags);
67:
68: /* compare criteria against the return value of the
69: * Container::valu e_type member function mapped to
70: * this field. */
71: const typename Container::valu e_type& v(*first);
72: if (criteria != (v.*(i->second))())
73: break;
74:
75: /* we're on the last field, meaning all fields that came before
76: * it also matched, so save it finally. */
77: if ((f+1) == fields.end())
78: *result++ = op(*first);
79: }
80: }
81: }
For some reason I can't figure out, the compile keeps bailing on line 61 with:
"error: expected ';' before i".
I'm thinking maybe it has something to do with the lack of 'typename' when
using Container::valu e_type in the function pointer, but adding that seems to
cause another problem (maybe I'm not putting it in the right place?)
using:
typedef const std::string& (typename Container::valu e_type::*mfp)(v oid) const;
causes:
error: expected unqualified-id before ‘typename’
error: expected `)' before ‘typename’
error: expected initializer before ‘typename’
Any pointers in the right direction?
Much appreciated,
Aaron 4 2064
Aaron Walker wrote: Greetings,
I'm attempting to write my first *real* template function that also deals with a map of strings to member function pointers that is making the syntax a little tricky to get right.
The function in question:
36: template <typename Container, 37: typename OutputIterator, 38: typename UnaryOp> 39: void 40: transform_field s_into_matches( 41: typename Container::cons t_iterator first, 42: typename Container::cons t_iterator last, 43: OutputIterator result, 44: const fields_type& fields, 45: const std::map<std::s tring, 46: const std::string& (Container::val ue_type::*)(voi d) const > & fm, 47: UnaryOp op) 48: {
[snip] Any pointers in the right direction?
well I'm not sure of your actual question because that is wildly complex
syntax you have. But I can see that you are heading in the wrong direction.
Look at this simple code
#include <vector>
template <typename Container>
void f(typename Container::iter ator i)
{
typename Container::valu e_type v;
}
int main()
{
std::vector<int > i;
f(i.begin());
}
It fails to compile. The reason is that the compiler cannot work out
what Container is. The rules of C++ prevent the compiler from deducing
the template argument when the function argument type is of the form
typename T::m.
If you ever got your code to compile you would face this issue and there
isn't a solution (other than specifying the template arguments explcitily).
To pass iterators to a template function you should do the following and
use iterator_traits if you want the value type.
#include <vector>
template <typename I>
void f(I i)
{
std::iterator_t raits<I>::value _type v;
}
int main()
{
std::vector<int > i;
f(i.begin());
}
john
Aaron Walker wrote: Greetings,
I'm attempting to write my first *real* template function that also deals with a map of strings to member function pointers that is making the syntax a little tricky to get right.
The function in question: [...] 61: std::map<std::s tring, mfp>::const_ite rator i = fm.find(f->first); 62: if (i == fm.end())
Please don't post line numbers. Just add a comment to the line you
want to mark.
For some reason I can't figure out, the compile keeps bailing on line 61 with: "error: expected ';' before i".
Add 'typename' at the beginning:
typename std::map<...>:: const_iterator i = ...
I'm thinking maybe it has something to do with the lack of 'typename' when using Container::valu e_type in the function pointer, but adding that seems to cause another problem (maybe I'm not putting it in the right place?)
Probably.
V
John Harrison wrote: Aaron Walker wrote:
Greetings,
I'm attempting to write my first *real* template function that also deals with a map of strings to member function pointers that is making the syntax a little tricky to get right.
The function in question:
36: template <typename Container, 37: typename OutputIterator, 38: typename UnaryOp> 39: void 40: transform_field s_into_matches( 41: typename Container::cons t_iterator first, 42: typename Container::cons t_iterator last, 43: OutputIterator result, 44: const fields_type& fields, 45: const std::map<std::s tring, 46: const std::string& (Container::val ue_type::*)(voi d) const > & fm, 47: UnaryOp op) 48: {
[snip]
Any pointers in the right direction?
well I'm not sure of your actual question because that is wildly complex syntax you have. But I can see that you are heading in the wrong direction.
<snip>
My question was why would this:
49: typedef const std::string& (Container::val ue_type::*mfp)( void) const;
....
61: std::map<std::s tring, mfp>::const_ite rator i = fm.find(f->first);
produce this compile failure:
61: "error: expected ';' before i". It fails to compile. The reason is that the compiler cannot work out what Container is. The rules of C++ prevent the compiler from deducing the template argument when the function argument type is of the form typename T::m.
If you ever got your code to compile you would face this issue and there isn't a solution (other than specifying the template arguments explcitily).
To pass iterators to a template function you should do the following and use iterator_traits if you want the value type.
<snip>
I was wondering if it'd be able to deduce the container type, but hadn't gotten
that far due to the syntax error. I didn't realize I could get the value_type
from iterator_traits .
Thanks for helping with what would probably have been my next problem :)
Aaron
Victor Bazarov wrote: Please don't post line numbers. Just add a comment to the line you want to mark.
Ah, ok apologies. I figured it'd make it easier on whoever was trying to help.
Will keep in mind next time. For some reason I can't figure out, the compile keeps bailing on line 61 with: "error: expected ';' before i".
Add 'typename' at the beginning:
typename std::map<...>:: const_iterator i = ...
Yep, that does it.
Thanks,
Aaron This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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