无法编译不完整的类型;循环依赖
您好,我在编译一些代码时遇到问题,我遇到了 A 依赖而 B 依赖 A 的情况。我已经提出了声明,但我不断遇到问题。
In file included from src/MemoWriteContext.h:7:0,
from src/MemoWriteContext.cpp:1:
src/MemoContext.h:29:20: error: field ‘memoWriteContext’ has incomplete type
MemoContext.h
#ifndef MEMOCONTEXT_H_
#define MEMOCONTEXT_H_
#include "sqlite/SqliteDb.h"
#include "Context.h"
#include "MemoWriteContext.h"
#include <string>
#include <memory>
#include <map>
namespace bbs
{
class MemoWriteContext;
class MemoContext : public Context
{
public:
//'structors
MemoContext(const std::map<std::string, std::shared_ptr<Context> > &_contexts,
sqlitecpp::SqliteDb &_sqliteDb);
~MemoContext();
protected:
//when called write the data back to the user
void performAction(const std::string &data, std::shared_ptr<UserAgent> agent);
private:
MemoWriteContext memoWriteContext;
}; //class memocontext
}
#endif // MEMOCONTEXT_H_
MemoWriteContext.h
#ifndef MEMOWRITECONTEXT_H_
#define MEMOWRITECONTEXT_H_
#include "Context.h"
#include "sqlite/SqliteDb.h"
#include "sqlite/PreparedStmt.h"
#include "MemoContext.h"
#include <string>
#include <memory>
#include <map>
namespace bbs
{
class MemoContext; //forward decl
class MemoWriteContext : public Context
{
public:
//'structors
MemoWriteContext(const std::map<std::string, std::shared_ptr<Context> > &_contexts,
MemoContext &_memoContext, sqlitecpp::SqliteDb &_sqliteDb);
~MemoWriteContext();
protected:
//when called write the data back to the user
virtual void performAction(const std::string &data, std::shared_ptr<UserAgent> agent);
virtual void onReceiveUserAgent(std::shared_ptr<UserAgent> agent);
private:
MemoContext &memoContext; //parent;
sqlitecpp::SqliteDb &sqliteDb;
sqlitecpp::PreparedStmt writeMemoStmt;
sqlitecpp::PreparedStmt findAgentIdStmt;
};
enum class MemoWriteState : char
{
USERNAME=0,
MESSAGE,
CONFIRM
};
class MemoWriteAgentData : public ContextAgentData
{
public:
MemoWriteState state;
int userId;
std::string message;
}; //class Memo Write Agent data
}
#endif // MEMOWRITECONTEXT_H_
Hi am having problems compiling some code, I have a situation where A depends and B depends on A. I have put forward declarations but I keep getting the problems.
In file included from src/MemoWriteContext.h:7:0,
from src/MemoWriteContext.cpp:1:
src/MemoContext.h:29:20: error: field ‘memoWriteContext’ has incomplete type
MemoContext.h
#ifndef MEMOCONTEXT_H_
#define MEMOCONTEXT_H_
#include "sqlite/SqliteDb.h"
#include "Context.h"
#include "MemoWriteContext.h"
#include <string>
#include <memory>
#include <map>
namespace bbs
{
class MemoWriteContext;
class MemoContext : public Context
{
public:
//'structors
MemoContext(const std::map<std::string, std::shared_ptr<Context> > &_contexts,
sqlitecpp::SqliteDb &_sqliteDb);
~MemoContext();
protected:
//when called write the data back to the user
void performAction(const std::string &data, std::shared_ptr<UserAgent> agent);
private:
MemoWriteContext memoWriteContext;
}; //class memocontext
}
#endif // MEMOCONTEXT_H_
MemoWriteContext.h
#ifndef MEMOWRITECONTEXT_H_
#define MEMOWRITECONTEXT_H_
#include "Context.h"
#include "sqlite/SqliteDb.h"
#include "sqlite/PreparedStmt.h"
#include "MemoContext.h"
#include <string>
#include <memory>
#include <map>
namespace bbs
{
class MemoContext; //forward decl
class MemoWriteContext : public Context
{
public:
//'structors
MemoWriteContext(const std::map<std::string, std::shared_ptr<Context> > &_contexts,
MemoContext &_memoContext, sqlitecpp::SqliteDb &_sqliteDb);
~MemoWriteContext();
protected:
//when called write the data back to the user
virtual void performAction(const std::string &data, std::shared_ptr<UserAgent> agent);
virtual void onReceiveUserAgent(std::shared_ptr<UserAgent> agent);
private:
MemoContext &memoContext; //parent;
sqlitecpp::SqliteDb &sqliteDb;
sqlitecpp::PreparedStmt writeMemoStmt;
sqlitecpp::PreparedStmt findAgentIdStmt;
};
enum class MemoWriteState : char
{
USERNAME=0,
MESSAGE,
CONFIRM
};
class MemoWriteAgentData : public ContextAgentData
{
public:
MemoWriteState state;
int userId;
std::string message;
}; //class Memo Write Agent data
}
#endif // MEMOWRITECONTEXT_H_
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我认为你唯一的问题是
MemoWriteContext.h
有#include "MemoContext.h"
。上下文只需要一个可以使用前向声明的引用。但是,如果您碰巧首先包含MemoWriteContext.h
,它就会在实际声明之前引入MemoContext.h
类 MemoWriteContext
。然后将使用类 MemoWriteContext 的前向声明并失败。您甚至可以在错误消息中看到顺序。只需删除该
#include
或至少颠倒MemoWriteContext.cpp
中包含的顺序(因为每个包含另一个的.h
都有效地颠倒了它们)后退)。I think your only problem is that
MemoWriteContext.h
has#include "MemoContext.h"
. The context only requires a reference which can use the forward declaration. But if you happen to includeMemoWriteContext.h
first it will then bring inMemoContext.h
before it actually declaresclass MemoWriteContext
. That will then use the forward declaration ofclass MemoWriteContext
and fail. You can even see the ordering in your error message.Just remove that
#include
or at least reverse the order of the includes inMemoWriteContext.cpp
(since each.h
including the other effectively reverses them back).这
是一个前向声明。它是“不完整类型”,因此无法实例化。
原因是 C++ 编译器必须知道必须实例化的任何类型的大小。不完整类型没有大小。
顺便说一句,您可以这样做:
因为您实际上声明了一个指针,并且指针具有已知的大小。
如果您想避免动态分配,则必须通过包含
MemoWriteContext.h
并删除前向声明来完全声明类型。This:
Is a forward declaration. It's an "incomplete type", and therefore cannot be instantiated.
The reason is that a C++ compiler must know the size of any type that has to be instantiated. Incomplete types have no size.
By the way, you can do this:
Because you actually declare a pointer, and pointers have a known size.
If you want to avoid dynamic allocations, then you'll have to fully declare the type by including
MemoWriteContext.h
and removing the forward declaration.