类型不可知的吸气方法 [英] Type agnostic getter methods

问题描述

我正在为系统的客户端编写程序，该系统以随机顺序返回自然类型的值（有些可以是int，有些可以浮点数，有些可以是字符串[好，几乎是自然的]）。问题是，我不知道在编译时值将是什么类型。

I'm writing a client for a system that returns values of natural types in random order (some can be int, others float, others string [well, almost natural]). The problem is, I don't know what type a value will be at compile time.

因为我不知道要返回的值的类型，直到之后已经查询了远程系统，提供统一接口的最佳方法是什么，该接口允许客户端库的用户提取正确类型的值？

Since I don't know the type of the value to be returned until after the remote system has been queried, what is the best way to provide a uniform interface that allows a user of the client library to extract the value in the right type?

如果查询远程系统一次返回一个字符串，我希望我的 get_value（）返回一个字符串。如果是int，则使其返回int。或者，如何让客户端库用正确的类型调用getter？

If querying the remote system once returns a string, I'd like my get_value() to return a string. If an int, make it return an int. Alternatively, how to have the client library call the getter with the right type?

我想带有类型提示的模板将是实现此目标的好方法？

I guess templates with type hinting would be a good way to achieve this?

推荐答案

有两种不同的用例。如果客户端程序可以提前知道所需值的类型，则可以为每种可能的类型使用不同的getter（旧的C语言，例如 getInt ， getDouble ， getString ），或使用模板化的吸气剂（现代C ++方式）：

There are two different use case. If the client program can know in advance the type of the value it wants, you can either use a different getter for each possible type (the good old C way with for example getInt, getDouble, getString), or use templated getters (modern C++ way):

template <class T>
T get(char *byte_array) {
    T value;
    # manage to extract the value
    return T;
}

并明确实例化它们以确保它们可用。

and explictely instanciate them to make sure that they will be available.

在客户端库中，用法为：

In the client library, the usage will be:

int i = get<int>(byte_array);

如果客户端程序可能以编译时不知道的顺序接收数据，则必须找到一个返回 variant 数据类型的方法（旧的Basic程序员请记住这一点）。您可以在boost或C ++ 17中找到实现，但是简单的实现可能是：

If the client program may received data in an order which is unknow at compile time, you must find a way to return a variant data type (old Basic programmers remember that). You can find implementations in boost or C++ 17, but a trivial implementation could be:

struct variant {
    enum Type { INT, DOUBLE, STRING, ... } type;
    union {
        int int_val;
        double d_val;
        std::string str_val;
        ...
    };
};

在这种情况下，客户端程序将使用

In that case the client program will use

variant v = get(byte_array);
switch v.type {
case INT:
    ...
}

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