Define a const static object variable inside the class

Question

I need to create a static object inside a class definition. It is possible in Java, but in C++ I get an error:

../PlaceID.h:9:43: error: invalid use of incomplete type ‘class
PlaceID’ ../PlaceID.h:3:7: error: forward declaration of ‘class
PlaceID’ ../PlaceID.h:9:43: error: invalid in-class initialization of static data

My class looks like this:

#include <string>

class PlaceID {

public:

    inline PlaceID(const std::string placeName):mPlaceName(placeName) {}

    const static PlaceID OUTSIDE = PlaceID("");

private:
    std::string mPlaceName;
};

Is it possible to make an object of a class inside this class? What are prerequisites that it must hold?

Some programmer dude · Accepted Answer · 2012-07-25 10:00:57Z

12

You can't define the member variable because the class isn't fully defined yet. You have to do like this instead:

class PlaceID {

public:

    inline PlaceID(const std::string placeName):mPlaceName(placeName) {}

    const static PlaceID OUTSIDE;

private:
    std::string mPlaceName;
};

const PlaceID PlaceID::OUTSIDE = PlaceID("");

answered Jul 25, 2012 at 10:00

Some programmer dude

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9 Comments

Benjamin Over a year ago

I was thinking about this, but my concern was about memory. But now I realized, that memory used in both options are actually equal. BTW, in your answer should be "const static" but you're right anyway.

Some programmer dude Over a year ago

@Benjamin The static keyword is only needed when declaring the variable inside the class, and not when defining it outside.

MNCODE Over a year ago

Getting a compiler warning for multiple declarations of the static const in classes where I include this class. LNK4006: already defined in x.obj

Some programmer dude Over a year ago

@MNCODE Definitions must happen only once. Move the definition to a single source file.

MNCODE Over a year ago

Thank you I defined the static const in the header file after class definition. It works in the source file.

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