Given such a gperf file with User-supplied struct:
%define class-name Table
%define lookup-function-name m
%struct-type
%language=C++
%{
#include <cstring>
#include <cstdio>
// defination of LookupTableElement is put to a header file in the real project and included in
namespace bar {
struct LookupTableElement {
const char *name;
void (*func) ();
};
}
// a handler
void ack() {
puts("you said hello");
}
// namespace bar {
%}
struct bar::LookupTableElement;//gperf needs the declaration
%%
######
hello, ack
######
%%
// } // end namespace bar
int main() {
auto p = Table::m("hello", sizeof("hello") - 1);
if (!p) return 1;
p->func();
return 0;
}
To compile:
$ gperf foo.gperf > foo.cc && g++ -std=c++0x foo.cc
makes g++ (gcc 4.7.3 and 4.8.2 tested) warn:
foo.gperf:26:13: warning: declaration ‘struct bar::LookupTableElement’ does not declare anything [enabled by default]
struct bar::LookupTableElement;//declare look up table's element
^
If namespace bar
is removed, there will be no warning anymore.
What's the best way to avoid the warning?
// namespace bar {
and // } // end namespace bar
and change struct bar::LookupTableElement
to struct LookupTableElement
. But in this way we will drag a lot of stuff into the namespace (have a look at the generated foo.cc you'd know that).There is an option to gperf:
-T, --omit-struct-type
Prevents the transfer of the type declaration to the output file. Use
this option if the type is already defined elsewhere.
So, without any namespaces tricks,
struct bar::LookupTableElement;
and this option it generates completely acceptable code (e.g. gperf -T foo.gperf > foo.gperf.cpp
).