C ++，＃ifdef问题

Question

Not coding in C++ right now but a question came up when I have a question in C#. Hope experts here can easily give a anwser.

Class A{
  #ifdef AFlag
  public void methodA(){...}
  #endif
}

Class B{
...
  A a;
  a.methodA();
...
}

Class C {
...
  A a;
  a.methodA();
...
}

If the AFlag is NOT defined anywhere, what will happen? A compile error or no error but the methodA AND those statements calling that method will not be compiled? thanks

Answer 1

将会出现编译错误。

Answer 2

Preprocessing happens before compilation. By the time your code goes to the compiler, the definition of method A in class A will be removed. Effectively its as if as you never wrote it. So this will result in compilation error.

Answer 3

一类将不具有methodA所以编译类B或C将失败。

Answer 4

You will have a complier error, as the function methodA is not declared anywhere. You could use this syntax instead:

Class A{ 

  public void methodA()
  {
#ifdef AFlag 
    ...
#endif 
  } 

} 

Which will allow methodA to be declared / defined, but it will be optimized away if you turn optimizations on.

Answer 5

Hard to say for certain, since code in the "..." could affect the answer, or mean that I've misunderstood the question. The statement a.methodA(); has to be in the body of a function.

You'll get compile errors at the lines a.methodA(); (or perhaps linker errors if the code is split across multiple translation units with inconsistent definitions of class A). Calling a function means it has to be there. The call is not "ignored" if the function doesn't exist.

Answer 6

如果AFlag ，则类A将没有成员函数methodA() ，因此在类B和C对其的调用将是错误的。

Answer 7

您将看到编译错误，因为未在类A上定义methodA方法。