前瞻性声明的目的是什么？ [英] What is the purpose of forward declaration?

问题描述

这是什么的描述或意义：
例如：

what is the description or meaning of this: for example:

class test;

class test
{
  .....
};

推荐答案

C ++一个单程编译器。在编译器需要知道一个符号引用实际定义类之前的类的情况下，转发引用是必要的。这种情况的典型例子是两个类需要包含彼此的指针。即

C++ (like C) was designed to be implementable by a single-pass compiler. Forward references are necessary in cases where the compiler needs to know that a symbol refers to a class before the class is actually defined. The classic example of this is when two classes need to contain pointers to each other. i.e.

class B;

class A {
  B* b;
};

class B {
  A* a;
};

如果没有向前引用B，编译器无法成功解析A的定义， t通过在A之前放置B的定义来修复问题。

Without the forward reference to B, the compiler could not successfully parse the definition for A and you can't fix the problem by putting the definition of B before A.

在像C＃这样需要两遍编译器的语言中，不需要前向引用

In a language like C#, which needs a two-pass compiler, you don't need forward references

class A {
  B b;
}

class B {
  A a;
}

因为编译器的第一遍只是拾取所有符号定义。当编译器第二次通过时，它可以说我知道B是一个类，因为我在第一次通过时看到了定义。

because the compiler's first pass simply picks up all symbol definitions. When the compiler makes its second pass, it can say "I know B is a class because I saw the definition on my first pass".

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