Cannot iterate a flat map - boost

Question

I am working with a boost flat_map and trying to iterate through it, however, I cannot figure out how to create the iterator.
my_map = mySeg.find<tlsSHMMap>("temp_map").first; //fetch a pointer to the map
tlsShmemAllocator alloc_inst (mySeg.get_segment_manager());
for (boost::container::flat_map<int, tlsStorage, std::less<int>() ,alloc_inst >::const_iterator row = my_map->begin();
row != my_map->end();
++row)
{
//do stuff
}
The 'tlsStorage' is a struct that I use to store data from a database. The boost flat map is declared as follows somewhere else in the code:
boost::container::flat_map tls_temp_map = mySeg.construct<tlsSHMMap>("temp_map") (std::less<int>() ,alloc_inst); //object name
The code that I have above does not work. Here are the errors, any ideas?
src/dbm/dbm_shm_server.cc: In member function 'int redcom::dbm::ShmServer::StartServer()':
src/dbm/dbm_shm_server.cc:353:24: warning: unused variable 'tls_main_map' [-Wunused-variable]
tlsSHMMap* tls_main_map;
^
src/dbm/dbm_shm_server.cc:354:24: warning: unused variable 'tls_temp_map' [-Wunused-variable]
tlsSHMMap* tls_temp_map;
^
src/dbm/dbm_shm_server.cc: In member function 'void redcom::dbm::ShmServer::fake_notify()':
src/dbm/dbm_shm_server.cc:2023:84: error: the value of 'alloc_inst' is not usable in a constant expression
for (boost::container::flat_map<int, tlsStorage, std::less<int>() ,alloc_inst >::const_iterator row = my_map->begin();
^
src/dbm/dbm_shm_server.cc:2021:40: note: 'alloc_inst' was not declared 'constexpr'
const tlsShmemAllocator alloc_inst (mySeg.get_segment_manager());
^
src/dbm/dbm_shm_server.cc:2023:95: error: type/value mismatch at argument 4 in template parameter list for 'template<class Key, class T, class Compare, class Allocator> class boost::container::flat_map'
for (boost::container::flat_map<int, tlsStorage, std::less<int>() ,alloc_inst >::const_iterator row = my_map->begin();
^
src/dbm/dbm_shm_server.cc:2023:95: error: expected a type, got 'alloc_inst'
src/dbm/dbm_shm_server.cc:2023:113: error: invalid type in declaration before 'row'
for (boost::container::flat_map<int, tlsStorage, std::less<int>() ,alloc_inst >::const_iterator row = my_map->begin();
^
src/dbm/dbm_shm_server.cc:2023:113: error: expected ';' before 'row'
src/dbm/dbm_shm_server.cc:2023:113: error: 'row' was not declared in this scope
src/dbm/dbm_shm_server.cc:2024:37: error: expected ')' before ';' token
row != my_map->end();
^
src/dbm/dbm_shm_server.cc:2023:98: warning: unused variable 'const_iterator' [-Wunused-variable]
for (boost::container::flat_map<int, tlsStorage, std::less<int>() ,alloc_inst >::const_iterator row = my_map->begin();
^
src/dbm/dbm_shm_server.cc:2025:19: error: 'row' was not declared in this scope
++row)
^
src/dbm/dbm_shm_server.cc:2025:22: error: expected ';' before ')' token
++row)
^
distcc[31606] ERROR: compile src/dbm/dbm_shm_server.cc on localhost failed
scons: *** [debug/build/x86_64-unknown-freebsd9.2/dbm/dbm_shm_server.o] Error 1
scons: building terminated because of errors.

Answer 1

Your code appears to be thoroughly confused. And the errors only slightly related to the code shown...[1]
Never mind, I have figured out that in your actual code (ShmServer::fake_notify) you are declaring allocInst, nearly like you showed, but const:
const tlsShmemAllocator alloc_inst (mySeg.get_segment_manager());
Which also handsomely explains why your loop control variable has an invalid type:
error: the value of 'alloc_inst' is not usable in a constant expression
error: expected a type, got 'alloc_inst'
I mean, really, the compiler couldn't be much more explicit about it. In case this wasn't clear enough it added pretty ascii art:
for(flat_map<int, tlsStorage, std::less<int>() ,alloc_inst >::const_iterator row = my_map->begin();
So yeah... you're trying to pass an allocator as an allocator type argument? Instead, use types as template arguments. Note the use of typedefs to reduce code clutter:
typedef boost::container::flat_map<
int, tlsStorage, std::less<int>, tlsShmemAllocator> ShmTable;
typedef ShmTable::const_iterator ShmTableIt;
for(ShmTableIt rowit=my_map->begin(); rowit!=my_map->end(); ++rowit)
{
ShmTableIt::value_type const& row = *rowit;
int id = row.first;
tlsStorage const& rowData = row.second;
}
Now of course, with C++11 you could write that without all those typedefs
for(auto rowit=my_map->begin(); rowit!=my_map->end(); ++rowit)
{
int id = rowit->first;
auto& rowData = rowit->second;
}
Or even more to the point:
for(auto const& row : *my_map)
{
int id = row.first;
auto& rowData = row.second;
}
Try to reduce on cruft so you don't get overwhelmed by the code :)
[1] some points in case:
boost::container::flat_map is a template, so your declaration cannot possibly be correct. I suspect you actually have
tlsSHMMap* tls_temp_map;
Why would you give us false code? That's unrelated?
In fact, is it my_map in your actual code? or tls_temp_map? Or tls_main_map (which you don't show, but is declared and never used...)?