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

It's invalid to put things into `namespace std`. AFAIK, the standard explicitly forbids it. — lethal-guitar, May 12 '14 at 14:01
to be more specific abaout lethal-guitar: http://stackoverflow.com/questions/320798/adding-types-to-the-std-namespace — okaerin, May 12 '14 at 14:04
I used std in the example to simplify, but i am really interesed in forward decalaring templatized boost log class (which has nested namespaces) — Sammy, May 12 '14 at 14:10
Regarding your 2nd question: [this](http://stackoverflow.com/questions/3879162/how-to-forward-declare-a-template-class) might help — lethal-guitar, May 12 '14 at 14:44
Also: Why didn't you ask about boost from the start? The answer is very different for non-std classes — lethal-guitar, May 12 '14 at 14:46

score 2 · Accepted Answer · answered May 12 '14 at 14:11

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.

score 1 · Answer 2 · answered May 12 '14 at 14:02

1

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;

answered May 12 '14 at 14:02

okaerin

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Forward declaration of tempate class inside a namespace

2 Answers2