如何转发声明位于命名空间中的类

2022-01-14 00:00:00 namespaces c++

我正在尝试在头文件中使用前向声明来减少使用的 #include 的数量，从而减少用户包含我的头文件时的依赖关系.

I am trying to use forward declarations in header files to reduce the number of #include used and hence reduce dependencies when users include my header file.

但是，我无法转发声明使用命名空间的位置.请参见下面的示例.

However, I am unable to forward declare where namespaces are used. See example below.

文件a.hpp:

#ifndef __A_HPP__
#define __A_HPP__

namespace ns1 {

   class a {
   public:
      a(const char* const msg);

      void talk() const;

   private:
      const char* const msg_;
   };
}

#endif //__A_HPP__

文件a.cpp:

#include <iostream>

#include "a.hpp"

using namespace ns1;

a::a(const char* const msg) : msg_(msg) {}

void a::talk() const { 
   std::cout << msg_ << std::endl; 
}

文件consumer.hpp:

#ifndef __CONSUMER_HPP__
#define __CONSUMER_HPP__

// How can I forward declare a class which uses a namespace
//doing this below results in error C2653: 'ns1' : is not a class or namespace name
// Works with no namespace or if I use using namespace ns1 in header file
// but I am trying to reduce any dependencies in this header file
class ns1::a;

class consumer
{
public:
   consumer(const char* const text) : a_(text) {}
   void chat() const;

private:
   a& a_;
};

#endif // __CONSUMER_HPP__

实现文件consumer.cpp:

#include "consumer.hpp"
#include "a.hpp"

consumer::consumer(const char* const text) : a_(text) {}

void consumer::chat() const {
   a_.talk();
}

测试文件main.cpp:

#include "consumer.hpp"

int main() {
   consumer c("My message");
   c.chat();
   return 0;
}

更新:

这是我非常人为的工作代码，使用下面的答案.

Here is my very contrived working code using the answer below.

文件a.hpp:

#ifndef A_HPP__
#define A_HPP__

#include <string>

namespace ns1 {

   class a {
   public:
      void set_message(const std::string& msg);
      void talk() const;

   private:
      std::string msg_;
   };

} //namespace

#endif //A_HPP__

文件a.cpp:

#include <iostream>
#include "a.hpp"

void ns1::a::set_message(const std::string& msg) {
    msg_ = msg;
}
void ns1::a::talk() const { 
   std::cout << msg_ << std::endl; 
}

文件consumer.hpp:

#ifndef CONSUMER_HPP__
#define CONSUMER_HPP__

namespace ns1
{
   class a;
}

class consumer
{
public:
   consumer(const char* text);
   ~consumer();
   void chat() const;

private:
   ns1::a* a_;
};

#endif // CONSUMER_HPP__

文件consumer.cpp:

#include "a.hpp"
#include "consumer.hpp"

consumer::consumer(const char* text) {
   a_ = new ns1::a;
   a_->set_message(text);
}
consumer::~consumer() {
   delete a_;
}
void consumer::chat() const {
   a_->talk();
}

文件main.cpp:

#include "consumer.hpp"

int main() {
   consumer c("My message");
   c.chat();
   return 0;
}

推荐答案

在命名空间ns1中转发声明类类型a:

To forward declare class type a in a namespace ns1:

namespace ns1
{
    class a;
}

在多级命名空间中转发声明一个类型:

To forward declare a type in multiple level of namespaces:

namespace ns1
{
  namespace ns2
  {
    //....
     namespace nsN
     {
        class a;
     }
    //....    
  }
}

您正在使用 a consumer 的成员，这意味着它需要具体类型，您的前向声明不适用于这种情况.

Your are using a a member of consumer which means it needs concrete type, your forward declaration won't work for this case.

相关文章