[英]Custom get method in c++ boost json ptree
I'm trying to access missing values in boost json ptree.我正在尝试访问 boost json ptree 中的缺失值。 When the key was not matched, a method should be called, in which I can define the returned value.
当键不匹配时,应该调用一个方法,我可以在其中定义返回值。 For calculation of returned value I need to access the ptree.
为了计算返回值,我需要访问 ptree。 Example: the ptree is:
{ "1": 10, "3": 30 }
I would like to interpolate the returned value, ie when I query for "2"
i want 20
to be returned.示例: ptree 是:
{ "1": 10, "3": 30 }
我想插入返回值,即当我查询"2"
时,我希望返回20
。 Is it possible?可能吗?
Everything is possible in a Turing-complete programming language (hint: that's all of them).在图灵完备的编程语言中一切皆有可能(提示:仅此而已)。
On actual property trees you could use get_optional
and then value_or_eval
:在实际的属性树上,您可以使用
get_optional
然后value_or_eval
:
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#define BOOST_BIND_GLOBAL_PLACEHOLDERS
#include <boost/property_tree/json_parser.hpp>
#include <iostream>
int main() {
boost::property_tree::ptree pt;
pt.add("1", 10);
pt.add("3", 30);
for (auto probe : { "1", "2", "3" }) {
auto lookup = pt.get_optional<double>(probe).value_or_eval([&pt] {
write_json(std::cout << "(interpolate from ", pt);
std::cout << ")\n";
return 42;
});
std::cout << "probe " << probe << " -> " << lookup << "\n";
}
}
Printing印刷
probe 1 -> 10
(interpolate from {
"1": "10",
"3": "30"
}
)
probe 2 -> 42
probe 3 -> 30
The problem with this is that it's neither JSON nor strongly typed.问题在于它既不是 JSON 也不是强类型。
Here's a random take (literally) and it uses a Json library as opposed to Property Tree:这是一个随机的(字面意思),它使用 Json 库而不是属性树:
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#include <boost/json.hpp>
#include <boost/json/src.hpp> // header-only
#include <boost/lexical_cast.hpp>
#include <map>
#include <iostream>
#include <iomanip>
#include <random>
using boost::conversion::try_lexical_convert;
namespace json = boost::json;
template <typename Key = double, typename Value = double>
struct MyLookup {
MyLookup(std::string const& text) : data(parse(text))
{ }
using Table = std::map<Key, Value>;
static Table parse(std::string const& text) {
auto raw = json::value_to<std::map<std::string, Value>>(
json::parse(text));
Table converted;
auto key_convert = [](auto &raw_pair) -> std::pair<Key, Value> {
return {boost::lexical_cast<Key>(raw_pair.first),
raw_pair.second};
};
std::transform(raw.begin(), raw.end(), inserter(converted, converted.end()), key_convert);
return converted;
}
Value operator[](Key key) const {
auto [low,up] = data.equal_range(key);
if (low == data.end())
throw std::range_error("out of bounds");
if (up == std::next(low)) {
return low->second;
} else {
if (low == data.begin() || up == data.end())
throw std::range_error("out of bounds");
low = std::prev(low);
auto dx = up->first - low->first;
auto dy = up->second - low->second;
auto slope = dy/dx;
return low->second + (key - (low->first)) * slope;
}
}
auto begin() const { return data.begin(); }
auto end() const { return data.end(); }
private:
Table data;
};
int main() {
MyLookup data(R"({ "1": 10, "3": 30 })");
for (auto [k,v] : data) {
std::cout << "table: " << k << " -> " << v << "\n";
}
std::mt19937 prng;
std::uniform_real_distribution<double> probe(0,5);
for (int i = 10; --i;) {
auto key = probe(prng);
std::cout << "Linear interpolation: " << std::setw(7) << key << " -> ";
try {
std::cout << data[key] << '\n';
} catch (std::exception const &e) {
std::cout << e.what() << '\n';
}
}
}
Prints eg打印例如
table: 1 -> 10
table: 3 -> 30
Linear interpolation: 0.677385 -> out of bounds
Linear interpolation: 4.17504 -> out of bounds
Linear interpolation: 4.84434 -> out of bounds
Linear interpolation: 1.10517 -> 11.0517
Linear interpolation: 1.54084 -> 15.4084
Linear interpolation: 2.7361 -> 27.361
Linear interpolation: 0.94191 -> out of bounds
Linear interpolation: 4.96441 -> out of bounds
Linear interpolation: 4.98231 -> out of bounds
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