Accessing a protocol buffer extension repeated field from C++ - c++

Question

In the following protocol buffer, how does one access the repeated fields in the extension from C++?
base.proto
message Base {
optional int32 id = 1;
repeated int32 ids = 2;
optional string name = 3;
repeated string names = 4;
extensions 1000 to 1999;
}
ext.proto
import "base.proto";
extend Base {
repeated string names = 1000;
optional int32 number = 1001;
repeated int32 numbers = 1002;
optional string name = 1003;
}
The following doesn't compile (in VS2010)
#include "base.pb.h"
#include "ext.pb.h"
using namespace ::google::protobuf;
int main(int argc, char *argv[])
{
Base b;
RepeatedPtrField<std::string> base_names = b.names(); // OK
RepeatedField<int> base_ids = b.ids(); // OK
int ext_number = b.GetExtension(number); // OK
std::string ext_name = b.GetExtension(name); // OK
assert( b.HasExtension(numbers) ); // OK
assert( b.HasExtension(names) ); // OK
int32 i = b.GetExtension(numbers); // ? Compiles but doesn't make sense.
RepeatedField<int32> ext_numbers = b.GetExtension(numbers); // compilation fails:
RepeatedPtrField<std::string> ext_names = b.GetExtension(names); // compilation fails:
return 0;
}
Compilation errors
1>test_proto.cpp(17): error C2440: 'initializing' : cannot convert from 'int' to 'google::protobuf::RepeatedField<Element>'
1> with
1> [
1> Element=google::protobuf::int32
1> ]
1> No constructor could take the source type, or constructor overload resolution was ambiguous
1>\test_proto.cpp(18): error C2440: 'initializing' : cannot convert from 'const std::string' to 'google::protobuf::RepeatedPtrField<Element>'
1> with
1> [
1> Element=std::string
1> ]
1> No constructor could take the source type, or constructor overload resolution was ambiguous

Answer 1

Thanks to Feng Xiao on the protobuf mailing list, use ExtensionSize(id) and GetExtension(id, index):
Base b;
int index = 0;
if (index < b.ExtensionSize(names))
{
std::string s_value = b.GetExtension(names, index);
}