英文:
Write an Eigen Matrix to json file using nlohmann json using to_json method
问题
我定义了一个Eigen矩阵 -
```cpp
template <typename T>
using Vec3 = Eigen::Matrix<T, 1, 3, Eigen::DontAlign | Eigen::ColMajor>;
using Vec3d = Vec3<double>;
我尝试将Vec3d写入JSON文件，如下所示 -
template <typename T>
void to_json(json& j, const Vec3<T>& v) {
j = {{"X", v(0)}, {"Y", v(1)}, {"Z", v(2)}};
}
// 用法
Vec3d test(1.f, 2.f, 3.f);
frameJson["Vec3d"] = test;
输出以array的形式存储，而不是如to_json中指定的方式，似乎没有进入to_json？？
"Vec3d": [
1.0,
2.0,
3.0
]
但我想要的是像to_json中指定的方式，如下所示 -
"Vec3d": [
"X": 1.0,
"Y": 2.0,
"Z": 0.0
]
在实现模板化的to_json中是否有问题？如何使其工作。
PS：还有，读取时，模板化的adl_serializer from_json()会是什么样子？
<details>
<summary>英文:</summary>
I have an Eigen matrix defined -
template <typename T>
using Vec3 = Eigen::Matrix<T, 1, 3, Eigen::DontAlign | Eigen::ColMajor>;
using Vec3d = Vec3<double>;
I try to write Vec3d to json file like below -
template <typename T>
void to_json(json& j, const Vec3<T>& v) {
j = {{"X", v(0)}, {"Y", v(1)}, {"Z", v(2)}};
}
// Usage
Vec3d test(1.f, 2.f, 3.f);
frameJson["Vec3d"] = test;
Output is stored as an `array` and not like the way specified in `to_json`, seems like it is not entering the `to_json` ??
"Vec3d": [
1.0,
2.0,
3.0
]
But I want it like the way specified in `to_json` like below -
"Vec3d": [
"X": 1.0,
"Y": 2.0,
"Z": 0.0
]
Is something wrong in the way templated `to_json` is implemented. How do I make this work.
PS : Also how would the templated `adl_serializer from_json()` look like while reading ?
</details>
# 答案1
**得分**: 2
我找不到默认的 `Eigen::Matrix` JSON 序列化器是如何实现的，但要使您的自定义序列化器起作用，只需将其放在 `Eigen` 命名空间内：
```cpp
namespace Eigen
{
template <typename T>
void to_json(json& j, const Vec3<T>& v) {
j = {{"X", v(0)}, {"Y", v(1)}, {"Z", v(2)}};
}
}
或者，如第三方类型序列化文档建议的，您可以编写 adl_serializer 特化版本：
namespace nlohmann {
template <typename T>
struct adl_serializer<Vec3<T>> {
static void to_json(json& j, const Vec3<T>& v) {
j = {{"X", v(0)}, {"Y", v(1)}, {"Z", v(2)}};
}
static void from_json(const json& j, Vec3<T>& v) {
v(0) = j["X"];
v(1) = j["Y"];
v(2) = j["Z"];
}
};
}
通常情况下，第二种选项可能更可取，因为它不会污染您的第三方命名空间。
英文:
I failed to find where the default Eigen::Matrix json serializer is implemented, but to have your custom serializer working, just put it inside Eigen namespace:
namespace Eigen
{
template <typename T>
void to_json(json& j, const Vec3<T>& v) {
j = {{"X", v(0)}, {"Y", v(1)}, {"Z", v(2)}};
}
Alternatively, like suggested in the documentation for third-party types serialization, you can write adl_serializer specialization:
namespace nlohmann {
template <typename T>
struct adl_serializer<Vec3<T>> {
static void to_json(json& j, const Vec3<T>& v) {
j = {{"X", v(0)}, {"Y", v(1)}, {"Z", v(2)}};
}
static void from_json(const json& j, Vec3<T>& v) {
v(0) = j["X"];
v(1) = j["Y"];
v(2) = j["Z"];
}
};
}
Second option might be preferable in general, as it doesn't pollute your third-party namespace.
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