从 Boost 属性树中提取 STL 映射 [英] Extracting STL Map from Boost Property Tree

问题描述

我知道 boost::property_tree::ptree 可能包含键值对，其中值"可以是任何类型.现在我的问题是，现在我的值"是 std::map，当我尝试 pt.get<std::map<std::string, std::string>> 其中 pt 是属性树，我收到编译时错误.因此，我的问题是:

I understand that boost::property_tree::ptree may contain key-value pairs where the 'value' may be of any type. Now my question is that, now my 'value' is a std::map, and when I attempt to pt.get<std::map<std::string, std::string>> where pt is a property tree, I get a compile time error. Hence, my question is:

如何从 boost 属性树中提取 STL 映射值?如果不行，有没有办法在STL map和boost属性树之间进行转换(这样属性树的每个节点都不是STL容器而是一个简单的数据类型).

推荐答案

很简单:

生活在 Coliru

ptree pt;

std::map<std::string, ptree> m(pt.begin(), pt.end());

但是，请注意键不必是唯一的:

However, note that keys need not be unique:

std::multimap<std::string, ptree> mm(pt.begin(), pt.end());

如果您想将所有值转换为 std::string(假设它们都有一个字符串值):

And if you are want to transform all values to std::string (assuming they all have a string value):

std::map<std::string, std::string> dict;
for (auto& [k,v]: pt) {
    dict.emplace(k, v.get_value<std::string>());
}

并排比较

在编译器资源管理器上直播

#include <boost/property_tree/ptree.hpp>
#include <map>
#include <fmt/ranges.h>
using boost::property_tree::ptree;

template<> struct fmt::formatter<ptree> : fmt::formatter<std::string>
{
    template<typename Ctx>
    auto format(ptree const& pt, Ctx& ctx) {
        return format_to(ctx.out(), "'{}'", pt.get_value<std::string>());
    }
};

int main() {
    ptree pt;
    pt.put("keyA", "valueA-1");
    pt.put("keyB", "valueB");
    pt.put("keyC", "valueC");

    pt.add("keyA", "valueA-2"); // not replacing same key
    
    std::map<std::string, ptree> m(pt.begin(), pt.end());
    std::multimap<std::string, ptree> mm(pt.begin(), pt.end());

    std::map<std::string, std::string> dict;
    for (auto& [k,v]: pt) {
        dict.emplace(k, v.get_value<std::string>());
    }

    fmt::print(
        "map:\n\t{}\n"
        "multimap:\n\t{}\n"
        "dict:\n\t{}\n",
        fmt::join(m, "\n\t"),
        fmt::join(mm, "\n\t"),
        fmt::join(dict, "\n\t")
    );
}

印刷品

map: 
        ("keyA", 'valueA-1')
        ("keyB", 'valueB')
        ("keyC", 'valueC')
multimap:
        ("keyA", 'valueA-1')
        ("keyA", 'valueA-2')
        ("keyB", 'valueB')
        ("keyC", 'valueC')
dict:
        ("keyA", "valueA-1")
        ("keyB", "valueB")
        ("keyC", "valueC")

奖励:来自字符串字典的 Ptree

添加ptree_from_map 功能

template <typename MapLike>
ptree ptree_from_map(MapLike const& m) {
    ptree pt;
    for (auto const& [k,v]: m) {
        pt.add(k, v);
    }
    return pt;
}

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