I'd suggest either going for a tiny little readable helper:
#include <boost/variant.hpp>
#include <map>
using std::map;
struct nmap;
struct nmap: map<std::string, boost::variant<std::string, nmap*>>
{
typedef boost::variant<std::string, nmap*> Variant;
typedef map<std::string, Variant> base;
friend nmap& as_map(Variant& v) { return *boost::get<nmap*>(v); }
friend nmap const& as_map(Variant const& v) { return *boost::get<nmap*>(v); }
friend std::string& as_string(Variant& v) { return boost::get<std::string>(v); }
friend std::string const& as_string(Variant const& v) { return boost::get<std::string>(v); }
};
int main()
{
nmap my_map;
my_map["a"] = "b";
my_map["c"] = new nmap;
as_map(my_map["c"])["d"] = "e";
}
Or go the recursive variant way. Let me sketch:
Recursive Variant Trees
This is IMO more elegant:
#include <boost/variant.hpp>
#include <map>
using nmap = boost::make_recursive_variant<std::string, std::map<std::string, boost::recursive_variant_> >::type;
using map_t = std::map<std::string, nmap>;
#include <iostream>
static std::ostream& operator<<(std::ostream& os, nmap const& map);
int main()
{
nmap my_map = map_t
{
{ "a", "b" },
{ "c", map_t
{
{ "d", "e" },
{ "f", map_t
{
{ "most nested", "leaf node" },
{ "empty", map_t { } },
} },
} },
};
std::cout << my_map;
}
At first glance this may look more complicated, but it actually has a number of important advantages:
- no more inheriting from classes not intended for inheritance
- no more limitation that the 'root' object /must/ be a map (it can be a string too now, so the variant is more consistently honoured)
- no more memory leaks Due to the fact that the variant now actually takes care of allocating the actual
nmap
instances, there's full value-semantics out of the box. allows for idiomatic visiting of the tree, no need for 'ifs and
butsdereferences', e.g. consider how we could do a quick & dirty implementation of thatoperator<<
:static std::ostream& operator<<(std::ostream& os, nmap const& map) { struct print : boost::static_visitor<void> { print(std::ostream& os, int indent = 0) : os(os), indent(indent) { } void operator()(map_t const& map) const { os << "{"; for(auto& item : map) { os << "\n"; do_indent(); os << " " << item.first << ": "; boost::apply_visitor(print(os, indent+4), item.second); } if (!map.empty()) { os << "\n"; do_indent(); }; os << "}"; } void operator()(std::string const& str) const { os << str; } private: std::ostream& os; void do_indent() const { for (int n = indent; n>0; --n) os << ' '; } int indent = 0; }; boost::apply_visitor(print(os), map); return os; }
See it Live On coliru
Output:
# g++ -std=c++11 -Wall -pedantic -Wextra main.cpp && ./a.out
{
a: b
c: {
d: e
f: {
empty: {}
most nested: leaf node
}
}
}