Forward declaration of tempate class inside a namespace

Question

Why doesn't this work ?

//file1.hpp

#include <vector>
namespace std
{ 
   typedef vector<int> IntVec;
}

//file2.hpp
//forward declare IntVec

namespace std {
   class IntVec;
}

class MyClass {
    std::IntVec* myVec;
public:
     MyClass();
};

#include "file1.hpp"
//file2.cpp
MyClass::MyClass()
{
   myVec = new std::IntVec;
}

Visual studio errors with 'std::IntVec' redefinition, different basic types; no appropriate default constructor.

What I am interested in is forward declaring Boost severitly logger

//i want to do this so that I don't need to include boost log headers in my headers 
typedef boost::log::sources::severity_logger<SeverityLevel> MyLogger

Answer 1

You cannot forward declare a typedef. Furthermore, you cannot forward declare std classes - you just have to include the relevant files in these cases (<iosfwd> is a notable exception, though).

So I don't think what you're trying to achieve here is possible. Just put your typedef in a header, and include that instead of a forward-declaration.

Btw.: Do not heap-allocate vector class members. A vector already takes care of heap-allocation internally. So having this:

class MyClass {
    IntVec myVec; // No pointer
};

Makes the constructor obsolete, since myVec is automatically default-constructed when you initialize a MyClass in this case.

Answer 2

because your typedef and the class definition "collide" with each other. Besides you did not specify any default constructor for your class "IntVec" and you're trying to instantiate it with:

 myVec = new std::IntVec;