Using libpq++

I want to use libpq++
I compile like this :

CC -I/usr/local/pgsql/include -o essai essai.cpp -L/usr/local/pgsql/lib
-lpq++

I get a link error:

CC -I/usr/local/pgsql/include -o essai essai.cpp -L/usr/local/pgsql/lib
-lpq++
Undefined first referenced
symbol in file
PgConnection::PgConnection #Nvariant 1(const char*) essai.o
PgConnection::~PgConnection #Nvariant 1() essai.o
__builtin_delete /usr/local/pgsql/lib/libpq++.so
__length_error(const char*)
/usr/local/pgsql/lib/libpq++.so
__rtti_user /usr/local/pgsql/lib/libpq++.so
terminate(void) /usr/local/pgsql/lib/libpq++.so
endl(ostream&) /usr/local/pgsql/lib/libpq++.so
int PgConnection::ConnectionBad() essai.o
cerr /usr/local/pgsql/lib/libpq++.so
__ls__7ostreamPCc /usr/local/pgsql/lib/libpq++.so
__rtti_si /usr/local/pgsql/lib/libpq++.so
__throw /usr/local/pgsql/lib/libpq++.so
const char*PgConnection::ErrorMessage() essai.o
__out_of_range(const char*)
/usr/local/pgsql/lib/libpq++.so
ld: fatal: Symbol referencing errors. No output written to essai
*** Error code 1
make: Fatal error: Command failed for target `essai'

Could someone help me ?
Regards, Renaud THONNART