map interface proxy for protobuf

map_proxy_poc.cc

#include "foo.pb.h"
#include <iostream>

/* protocol:
message Foo
{
	message Pair
	{
		required string key   = 1;
		required string value = 2;
	}

	repeated Pair my_map = 1;
}
*/

template <typename T_Pair, typename T_Key, typename T_Value>
class map_proxy
{
	typedef ::google::protobuf::RepeatedPtrField<T_Pair> container_type;
	container_type* m_container;
	T_Key* (T_Pair::*m_get_key)();
	T_Value* (T_Pair::*m_get_value)();
public:
	map_proxy(::google::protobuf::RepeatedPtrField<T_Pair>* container,
		  T_Key* (T_Pair::*get_key)(),
		  T_Value* (T_Pair::*get_value)())
		: m_container(container),
		  m_get_key(get_key),
		  m_get_value(get_value)
	{
	}

	T_Value& operator[](const T_Key& key);
};

template <typename T_Pair, typename T_Key, typename T_Value>
T_Value&
map_proxy<T_Pair, T_Key, T_Value>::operator[](const T_Key& key)
{
	T_Pair* elem = 0;
	for (typename container_type::iterator i=m_container->begin();
	     i != m_container->end();
	     ++i)
	{
		if (*((*i).*m_get_key)() == key)
		{
			elem = &(*i);
			break;
		}
	}
	if (elem == 0)
	{
		elem = m_container->Add();
		*(elem->*m_get_key)() = key;
	}
	return *(elem->*m_get_value)();
}

template <typename T_Pair, typename T_Key, typename T_Value>
map_proxy<T_Pair, T_Key, T_Value>
make_map_proxy(::google::protobuf::RepeatedPtrField<T_Pair>* container,
	T_Key* (T_Pair::*get_key)(),
	T_Value* (T_Pair::*get_value)())
{
	return map_proxy<T_Pair, T_Key, T_Value>(container, get_key, get_value);
}

int main()
{
	Foo f;

	/* "native" way */
	Pair* p = f.mutable_my_map()->Add();
	p->set_key("foo");
	p->set_value("bar");

	/* my way */
	auto mp = make_map_proxy(f.mutable_my_map(),
	                         &Foo::Pair::mutable_key,
	                         &Foo::Pair::mutable_value);

	mp["alice"] = "cat";

	/* debug */
	std::cout << f.DebugString() << std::endl;
}