Programming Forums > Application Development > C++

programmingnoob · Aug 25th, 2006, 12:01 AM

I have to accomplish this weird task, no idea how to go about it.

there is a template class named "Scanner", and its constructor is used to open a file.

and there is a class named "Parser", and it's supposed to take the filename from main.cpp and pass it to the class Scanner. Oh also, all the member functions of the Parser class needs to have access to the declaraton involving scanner within the parser class.

I am more confused with the syntax than anything

Trial I have had:

I tried using Scanner <string> Si; inside the public section of the Parser class, then tried updating the "Si" in the Parser constructor so that Si now has the filename, it didnt work :

(Toggle Plain Text)

Parser (string s)
{
Scanner <string> Si(s);
NT = Si.cget();
GlobalRootM = prog();

}

^ string s would be the filename passed to the Parser class by main.cpp, and prog() is a parser member function and it'll call other parser functions, and each of these functions will rely on the Scanner class for the next token. (Typical recursive descent parser!)
I'm obviously doing something wrong with the syntax, but I dont know what!

Oh, dont ask me why i am linking parser and scanner this way to get the filename (I have to!! lol)

grumpy · Aug 25th, 2006, 12:10 AM

If I'm understanding you right, your class declaration looks something like this;

(Toggle Plain Text)

class Parser
{
     public:
          Parser(const std::string &);
     private:
          Scanner<std::string> Si;
};

where your Scanner<std::string> class has a constructor that accepts a std::string.

Assuming this is correct, then one way of implementing your constructor is outside your class declaration;

(Toggle Plain Text)

Parser::Parser(const std::string &s) : Si(s)
{
    NT = Si.get();
     // etc
}

A couple of additional notes;

1) If you have used "using namespace std;" within your class declaration (or in the header file that contains it), good practice says "don't do that".

2) If prog() is a virtual member function of class Parser, or if it calls virtual member functions, then it is usually a good idea to NOT call it from a constructor of class Parser.

Game_Ender · Aug 25th, 2006, 12:29 AM

Virtual functions overridden in a subclass do not get called from the constructor of the base class, only the base class versions get called. This is why its a good idea not to do that.

magnus.therning · Aug 31st, 2006, 11:18 AM

AFAIK that is because the state of the subclass instance is somewhat up in the air while the baseclass constructor is executed.

(I couldn't help butting in :-)

grumpy · Aug 31st, 2006, 6:31 PM

Quote:

Originally Posted by magnus.therning

AFAIK that is because the state of the subclass instance is somewhat up in the air while the baseclass constructor is executed.

I suppose that's one way of putting it. The reason is order of construction. The process of constructing an object of a derived class constructs the base class sub-objects first, and then the parts specific to the derived class. The final resolution of virtual function is among the parts specific to the derived class. This means, the base class constructor has access to no information about the fact a derived class object is being created.

Aug 25th, 2006, 12:01 AM	#1
programmingnoob Hobbyist Programmer Join Date: Feb 2006 Posts: 155 Rep Power: 3	class/constructor I have to accomplish this weird task, no idea how to go about it. there is a template class named "Scanner", and its constructor is used to open a file. and there is a class named "Parser", and it's supposed to take the filename from main.cpp and pass it to the class Scanner. Oh also, all the member functions of the Parser class needs to have access to the declaraton involving scanner within the parser class. I am more confused with the syntax than anything Trial I have had: I tried using Scanner <string> Si; inside the public section of the Parser class, then tried updating the "Si" in the Parser constructor so that Si now has the filename, it didnt work : (Toggle Plain Text) Parser (string s) { Scanner <string> Si(s); NT = Si.cget(); GlobalRootM = prog(); } Parser (string s) { Scanner <string> Si(s); NT = Si.cget(); GlobalRootM = prog(); } ^ string s would be the filename passed to the Parser class by main.cpp, and prog() is a parser member function and it'll call other parser functions, and each of these functions will rely on the Scanner class for the next token. (Typical recursive descent parser!) I'm obviously doing something wrong with the syntax, but I dont know what! Oh, dont ask me why i am linking parser and scanner this way to get the filename (I have to!! lol)

Aug 25th, 2006, 12:10 AM	#2
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,260 Rep Power: 5	If I'm understanding you right, your class declaration looks something like this; (Toggle Plain Text) class Parser { public: Parser(const std::string &); private: Scanner<std::string> Si; }; class Parser { public: Parser(const std::string &); private: Scanner<std::string> Si; }; where your Scanner<std::string> class has a constructor that accepts a std::string. Assuming this is correct, then one way of implementing your constructor is outside your class declaration; (Toggle Plain Text) Parser::Parser(const std::string &s) : Si(s) { NT = Si.get(); // etc } Parser::Parser(const std::string &s) : Si(s) { NT = Si.get(); // etc } A couple of additional notes; 1) If you have used "using namespace std;" within your class declaration (or in the header file that contains it), good practice says "don't do that". 2) If prog() is a virtual member function of class Parser, or if it calls virtual member functions, then it is usually a good idea to NOT call it from a constructor of class Parser.

Aug 31st, 2006, 11:18 AM	#4
magnus.therning Programmer Join Date: May 2006 Location: Cambridge, UK Posts: 85 Rep Power: 3	AFAIK that is because the state of the subclass instance is somewhat up in the air while the baseclass constructor is executed. (I couldn't help butting in :-) __________________ Don't comment bad code - rewrite it. - The Elements of Programming Style (Kernighan & Plaugher)

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Aug 25th, 2006, 12:29 AM	#3
Game_Ender Professional Programmer Join Date: May 2006 Location: Maryland, USA Posts: 306 Rep Power: 3	Virtual functions overridden in a subclass do not get called from the constructor of the base class, only the base class versions get called. This is why its a good idea not to do that.

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Programming Forums > Application Development > C++

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