C++ 不完整类型结构的无效使用，即使有前向声明
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Invalid use of incomplete type struct, even with forward declaration
提问by robev
I'm aware of circular dependencies, but even with forward declarations I get this area. What am I doing wrong?
我知道循环依赖，但即使有前向声明我也得到了这个区域。我究竟做错了什么？
// facility.h
class Area;
class Facility {
public:
Facility();
Area* getAreaThisIn();
void setAreaThisIsIn(Area* area);
private:
Area* __area;
};
// facility.cpp
#include "facility.h"
#include "area.h"
{ ... }
// area.h
class Facility;
class Area {
public:
Area(int ID);
int getId();
private:
std::list<Facility*> _facilities;
};
// area.cpp
#include "area.h"
#include "facility.h"
So this compiles fine, but if I do
所以这编译得很好，但如果我这样做
// foo.h
#include "facility.h"
class Foo { .. };
// foo.cpp
#include "foo.h"
void Foo::function() {
Facility* f = new Facility();
int id = f->getAreaThisIsIn()->getId();
When I get invalid use of incomplete type struct Area
当我得到 invalid use of incomplete type struct Area
采纳答案by ?imon Tóth
For Facility* f = new Facility();you need a full declaration, not just forward declaration.
因为Facility* f = new Facility();您需要一个完整的声明，而不仅仅是前向声明。
回答by Max Lybbert
To clarify: a forward declaration allows you to operate on an object if very limited ways:
澄清：前向声明允许您在非常有限的情况下对对象进行操作：
struct Foo; // forward declaration
int bar(Foo* f); // allowed, makes sense in a header file
Foo* baz(); // allowed
Foo* f = new Foo(); // not allowed, as the compiler doesn't
// know how big a Foo object is
// and therefore can't allocate that much
// memory and return a pointer to it
f->quux(); // also not allowed, as the compiler doesn't know
// what members Foo has
Forward declarations can help in some cases. For instance, if the functions in a header only ever take pointers to objects instead of the objects, then you don't need to #includethe whole class definition for that header. This can improve your compile times. But the implementation for that header is almost guaranteed to need to #includethe relevant definition because you're likely going to want to allocate those objects, call methods on those objects, etc. and you need more than a forward declaration to do that.
在某些情况下，前向声明会有所帮助。例如，如果头文件中的函数只使用指向对象而不是对象的指针，那么您就不需要#include该头文件的整个类定义。这可以改善您的编译时间。但是该标头的实现几乎可以保证需要#include相关定义，因为您可能想要分配这些对象，调用这些对象的方法等，并且您需要的不仅仅是一个前向声明来做到这一点。
回答by Frank Schmitt
Did you #include both area.h and facility.h in foo.cpp (assuming this is the file where you get the error)?
您是否在 foo.cpp 中同时#include了area.h和facility.h（假设这是您收到错误的文件）？
