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私は他のすべてが C で書かれている会社のために C++ でいくつかの作業を行っています (C を使用することは私にとってオプションではありません :( )。それらは非常に類似した多くのデータ構造を持っています (つまり、それらはすべてフィールドを持っています)。 「名前」、「住所」など. しかし、何らかの理由で、他のすべての基礎となる共通の構造がありません (何かを地獄にする. とにかく、私はシステム全体を行う必要があります.メモリ内にあるこれらの構造体の分析, そしてそれをすべてテーブルに. それほど悪くはありませんが, テーブルにはすべての変数のすべてのフィールドのエントリが含まれている必要があります.にはフィールド "latency" がありますが、構造体 a にはありません。テーブルでは、a の各インスタンスのエントリには、"latency" の空のエントリが必要です。

私の質問は、テンプレート関数に渡された構造体に特定のフィールドがあるかどうかを実行時に判断する方法はありますか? または、それを行う黒魔術マクロを作成する必要がありますか? （問題は基本的にテンプレート特化が使えないことです）

ありがとう！ご不明な点がございましたら、お気軽にお問い合わせください。

ここで私が考えていたことの断片です...

struct A
{
  char name[256];
  int index;
  float percision;
};

struct B
{
  int index;
  char name[256];
  int latency;
};

/* More annoying similar structs... note that all of the above are defined in files that were compiled as C - not C++ */

struct Entry
{
  char name[256];
  int index;
  float percision;
  int latency;
  /* more fields that are specific to only 1 or more structure */
};

template<typename T> struct Entry gatherFrom( T *ptr )
{
  Entry entry;

  strcpy( entry.name, ptr->name, strlen( ptr->name ) );
  entry.index = ptr->index;
  /* Something like this perhaps? */
  entry.percision = type_contains_field( "percision" ) ? ptr->percision : -1;
}

int main()
{
  struct A a;
  struct B b;

  /* initialization.. */

  Entry e  = gatherFrom( a );
  Entry e2 = gatherFrom ( b );

  return 0;
}
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3 に答える 3

2

everything else written in C (using C isn't an option for me :( ).

First I'd like to quote what Linus Torvalds had to say about this issue:

From: Linus Torvalds <torvalds <at> linux-foundation.org>
Subject: Re: [RFC] Convert builin-mailinfo.c to use The Better String Library.
Newsgroups: gmane.comp.version-control.git
Date: 2007-09-06 17:50:28 GMT (2 years, 14 weeks, 16 hours and 36 minutes ago)

C++ is a horrible language. It's made more horrible by the fact that a lot 
of substandard programmers use it, to the point where it's much much 
easier to generate total and utter crap with it. Quite frankly, even if 
the choice of C were to do *nothing* but keep the C++ programmers out, 
that in itself would be a huge reason to use C.

http://harmful.cat-v.org/software/c++/linus

They have a number of data structures that are VERY similar (i.e., they all have fields such as "name", "address", etc. But, for whatever reason there isn't a common structure that they used to base everything else off of (makes doing anything hell).

They may have had very sound reasons for this. Putting common fields into a single base structure (class) may sound like a great idea. But it makes things really difficult if you want to apply major changes to one of the structures (replace some fields, change types, etc.) while leaving the rest intact. OOP is certainly not the one true way to do things.

So, my question is, is there a way to determine at runtime if a structure that has been passed into a template function has a specific field?

No this is not possible. Neither in C nor in C++, because all information about types gets discarded when the binary is created. There's neither reflection nor introspection in C or C++. Well, technically the debug information the compiler emits does provide this information, but there's no language builtin feature to access this. Also this sort of debug information relies on analysis performed at compile time, not at runtime. C++ has RTTI, but this is only a very coarse system to identify which class an instance is off. It does not help with class or struct members.

But why do you care to do this at runtime anyway?

Anywho, I need to do a system-wide analysis of these structs that are in memory, and through it all into a table.

You should be actually happy that you have to analyse C and not C++. Because C is really, really easy to parse (unlike C++ which is tremendously difficult to parse, mostly because of those darn templates). Especially structs. I'd just write a small and simple script, that extracts all the struct definitions from the C sources. However since structs are of constant size, they often contain pointers to dynamically allocated data. And unless you want to patch your allocator, I think the most easy way to analyse this, is by hooking into a debugger and record the memory usage of every unique object whose pointer is assigned to a struct member.

于 2012-07-05T22:23:18.160 に答える
1

はい、これはまったく難しいことではありません。anAと an の両方Entryを 1 つのオブジェクトに入れて、Entry二流市民にします。

void setDefaultValues(Entry*); // You should be able to provide these.
struct Entry {
  int x;
  int y;
};
struct Indirect : public Entry { };
template<typename T> struct EntryOr : public T, Indirect
{
  setDefaultValues(this);
};

// From C code
struct A {
  int x;
}

int main()
{
  EntryOr<A> foo;
  foo.x = 5; // A::x
  std::cout << foo.x << foo.y; // Prints A::x and Entry::y
}

（リンク）

于 2012-07-06T14:13:36.620 に答える