How to forward declare a class which is in a namespace

Question

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. However, I am unable to forward declare where namespaces are used. See example below. File `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__ ``` File `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; } ``` File `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…

Accepted Answer

To forward declare class type `a` in a namespace `ns1`:

```
namespace ns1
{
    class a;
}
```

To forward declare a type in multiple level of namespaces:

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

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

How to forward declare a class which is in a namespace

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