mapnik/mapnik — pull request #1232 by lightmare (2012-05-29 21:19:04+00:00)
Title: fix/prevent to_expression_string misuse #985

A bit late I realized I could've split this in 2 commits...
1) include/mapnik/rule.hpp and src/formatting/expression.cpp : fixed simple typos
2) include/mapnik/expression_string.hpp : added to_expression_string overloads that, when instantiated, yield compilation error; and bindings/python/mapnik_expression.cpp consequently needed a small change

for example, with the new overloads and un-fixed include/mapnik/rule.hpp, I get this error from gcc:
```
include/mapnik/expression_string.hpp: In instantiation of ‘std::string mapnik::to_expression_string(const boost::shared_ptr&) [with T = boost::variant<...>; std::string = std::basic_string]’:
include/mapnik/rule.hpp +209 :73: required from ‘void mapnik::rule::deepcopy_symbolizer::copy_height_ptr(T&) const [with T = mapnik::building_symbolizer]’
include/mapnik/rule.hpp +183 :32: required from here
include/mapnik/expression_string.hpp +59 :59: error: cannot convert ‘mapnik::expr_node_ptr_’ to ‘mapnik::expr_node_ref_’ in initialization
include/mapnik/expression_string.hpp +59 :20: warning: unused variable ‘invalid_argument_type’ [-Wunused-variable]
```
couldn't get it any better than "cannot convert _ptr_ to _ref_" :)