在C ++结构中打印所有字段的值 [英] Printing values of all fields in a C++ structure

问题描述

考虑一个简单的结构：

  struct abc 
 {
 int a; 
 char b; 
} 
  

我在一个定义为其结构的变量中得到了一些值，现在我想以打印以下内容。

  * a = [某些值] 
b = [某字符] * 
  

对于任意结构，无需写入转储... ）函数为我遇到的每个结构？

解决方案

看来你已经找到了解决方案，

你所要求的是 Reflection ，即一个对象描述自身的能力。 p>

大多数语言通过元数据实现反射，在python中，例如对象的函数和属性存储在字典元素中。

C ++没有任何本机反射系统，不像C＃或Java，这防止（例如）这种自动打印/序列化或反序列化。

C ++有非常强大的元编程支持，允许我们（通过使用模板）模拟反射（在编译时）。这通常使用 Boost.Fusion 完成，该库

正如您的链接中显示的示例， BOOST_FUSION_ADAPT_STRUCT 宏允许您采用标准 struct ，并赋予它所需的接口作为Fusion.Sequence。

另一个例子是使用 Fusion.Vector 或 Fusion.Map 存储类的属性，然后将此Sequence显示为自动打印/序列化/反序列化方法。

但是，这个系统有一个限制：元编程与OO编程不能很好地结合。

  struct Base {char一个; }; // Adapt 
 struct Derived：Base {char b; }; // Adapt 
 
 void print（Base const& b）{boost :: fusion :: for_each< Base>（b，Print } 
  

将只打印 Base （这里 a ）。当使用多态性时，您需要在某一点或另一点使用 virtual 方法：）

Consider a simple structure:

struct abc  
{  
    int a;  
    char b;  
}  

I got some value in a variable defined as its structure, and now I want to print the below.

*a = [some value]  
b = [some character]*

What is the best way to achieve this for an arbitrary structure without having to write a dump...(...) function for each of the structure I encounter?

解决方案

It seems you already found the solution, but I'll expand a bit.

What you are calling for is called Reflection, ie the ability for an object to describes itself.

Most languages implement reflection thanks to metadata, in python for example the functions and attributes of an object are stored in a dictionary element.

C++ does not have any native reflection system unlike C# or Java, which prevents (for example) this kind of automatic printing / serialization or deserialization.

However, C++ has very powerful metaprogramming support which allows us (through the use of templates) to emulate reflection (at compile-time). This is usually done using Boost.Fusion, this library being meant for crossing over from compile-time to run-time.

As the example demonstrated in your link, the BOOST_FUSION_ADAPT_STRUCT macro allows you to take a standard struct and give it the required interface to be treated as a Fusion.Sequence.

Another example would be to use Fusion.Vector or Fusion.Map to store the attributes of the class and then expose this Sequence to automatic print/serialization/deserialization methods.

There is a limitation to this system however: Metaprogramming does not mesh well with OO-programming.

struct Base { char a; };            // Adapt
struct Derived: Base { char b; };   // Adapt

void print(Base const& b) { boost::fusion::for_each<Base>(b, Print()); }

will only print the member of Base (here a). When using polymorphism, you need to use virtual methods at one point or another :)

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