c++ - 我如何能够将不完整的类本身用作参数？

Question

struct test{
 void call(test t1){
   //
 }
};

我问了一个类似的问题，并找到了一些其他链接，例如:

How am I able to use a class as a function parameter in itself?

Incomplete types in member function definitions

但他们都没有回答这个问题:

编译器需要为 t1 发出代码到堆栈上分配的空间，但在那点 test 不完整，它怎么知道它需要多少空间？

Answer 1

struct test 类不是不完整类型。

来自 cppreference.com :

Incomplete type

• the type void (possibly cv-qualified);
• incompletely-defined object types
  • class type that has been declared (e.g. by forward declaration) but not defined;
  • array of unknown bound;
  • array of elements of incomplete type;
  • enumeration type from the point of declaration until its underlying type is determined.

All other types are complete.

自己测试一下，在你的 call 函数中做一个测试，比如:

void call(test t1)
{
    std::cout << sizeof t1 << std::endl;
}