Error - cannot call member function without object in C++

Question

I get the compile error

cannot call member function ‘bool GMLwriter::write(const char*, MyList<User*>&, std::vector<std::basic_string<char> >)’ without object

when I try to compile

 class GMLwriter{
    public:
    bool write(const char *fn, MyList<User*>& my_vec, vector<string>edges);
};

the function is defined later and called in main with

GMLwriter::write(argv[3], Users, edges);

Users is declared before with MyList<User*> Users; (MyList is a List ADT and I have a User class) and edges is declared with vector<string>edges

to what object is this error referring?

How do you call this function? You need to show that part of the code to get an correct answer instead of guesses. — Alok Save
– Alok Save, Commented Feb 11, 2013 at 4:28
Well then that is not a definition, In C++ to be able to call a non-static member function you need a class object. something like: GMLwriter obj; obj.write(...); — Alok Save
– Alok Save, Commented Feb 11, 2013 at 4:30

billz · Accepted Answer · 2013-02-11 04:35:47Z

19

GMLwriter::write is not static function of GMLwriter, you need to call it through object. For example:

GMLwriter gml_writer;   
gml_writer.write(argv[3], Users, edges);

If GMLwriter::write doesn't depend on any GMLwriter state(access any member of GMLwriter), you can make it a static member function. Then you could call it directly without object:

class GMLwriter
{
public:
   static bool write(const char *fn, MyList<User*>& my_vec, vector<string>edges);
   ^^^^
};

then you could call:

GMLwriter::write(argv[3], Users, edges);

edited Feb 11, 2013 at 4:35

answered Feb 11, 2013 at 4:28

billz

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Pubby · Accepted Answer · 2013-02-11 04:30:01Z

1

GMLwriter is not an object, it's a class type.

Calling member functions requires an object instance, ie:

GMLwriter foo;   
foo.write(argv[3], Users, edges);

Although there's a good chance you intended the function to be free or static:

class GMLwriter{
    public:
    // static member functions don't use an object of the class,
    // they are just free functions inside the class scope
    static bool write(const char *fn, MyList<User*>& my_vec, vector<string>edges);
};

// ...
GMLwriter::write(argv[3], Users, edges);

or

bool write(const char *fn, MyList<User*>& my_vec, vector<string>edges);
// ...
write(argv[3], Users, edges);

answered Feb 11, 2013 at 4:30

Pubby

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Comments

Ronaldo Nazarea · Accepted Answer · 2013-02-11 04:34:10Z

0

You might be trying to call / create a static method, to be

In this case, you might want to precede your declaration with a 'static' modifier.

http://www.functionx.com/cppcli/classes/Lesson12b.htm

answered Feb 11, 2013 at 4:34

Ronaldo Nazarea

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