使用NlohmannJson写JSON保留插入顺序
1. 正文
nlohmann/json是一个c++的读写json的组件,号称使用现代c++范式写的。简单看了一下,这个组件确实包含了很多cpp11以上的特性,在vs2015及一下的版本甚至没办法正常编译。要正常使用需要vs2017及以上版本才行。
在使用过程中,遇到了一个问题是没办法保持插入的顺序,每个插入的键值对会按照字符串的顺序排列的,因为其内部用到了std:map。查看了github的主页说明,是这么说的:
by default, the library does not preserve the insertion order of object elements. this is standards-compliant, as the json standard defines objects as "an unordered collection of zero or more name/value pairs". if you do want to preserve the insertion order, you can specialize the object type with containers like tsl::ordered_map (integration) or nlohmann::fifo_map (integration).
这段话的意思是json标准的定义是零个或多个键值对对的无序集合,如果要保证插入顺序,可以使用tsl::ordered_map(integration)或nlohmann::fifo_map(integration)等容器专门化对象类型。nlohmann::fifo_map同样在github上找到,“专门化对象类型”的意思是nlohmann/json组件内部用到了很多std容器,只需要将其替换成可以保存插入顺序的容器就可以了,也就是nlohmann::fifo_map。
重新找了一些英文资料,最终找到的解决方案如下:
#include "json.hpp" #include "fifo_map.hpp" #include <iostream> using namespace nlohmann; // a workaround to give to use fifo_map as map, we are just ignoring the 'less' compare template<class k, class v, class dummy_compare, class a> using my_workaround_fifo_map = fifo_map<k, v, fifo_map_compare<k>, a>; using my_json = basic_json<my_workaround_fifo_map>; int main() { my_json j; j["f"] = 5; j["a"] = 2; my_json j2 = { {"pi", 3.141}, {"happy", true}, {"name", "niels"}, {"nothing", nullptr}, {"answer", { {"everything", 42} }}, {"list", {1, 0, 2}}, {"object", { {"currency", "usd"}, {"value", 42.99} }} }; std::cout << j.dump(4) << std::endl; std::cout << j2.dump(4) << std::endl; return 0; }
运行结果如下所示,可以看到输出的json不再是字符串顺序而是插入顺序:
2. 参考
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