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c++ object constructor question

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# 1  
Old 06-26-2010
c++ object constructor question
I have the following code

Code:
class Param{
    public:
    Param(int aa, int bb){
        a=aa;
        b=bb;
    }
int a,b;
};

void function(Param);

Code:
int main(){

   function(2,3);
    return 0;
}

Code:
void function(Param par){
    cout<<par.a;
}

, and when I run it, I get the following message error:

Code:
.../main.cpp|21|error: conversion from ‘int’ to non-scalar type ‘Param’ requested|

I know the problem is that function should have been called with an object from Param, but I was expecting that the constructor Param(int , int) would do the job.
What is wrong with my assumption, and how could I fix this problem?

thank you in advance!!
santiagorf
Last edited by santiagorf; 06-26-2010 at 04:45 PM..
# 2  
Old 06-26-2010
Quote:
Originally Posted by santiagorf
I know the problem is that function should have been called with an object from Param, but I was expecting that the constructor Param(int , int) would do the job.
It might be if you'd actually called it. You're just feeding it 2,3 not Param(2,3).
# 3  
Old 06-26-2010
Thanks Corona688 for your prompt replay, and in fact if I call as
Code:
function( Param(2,3))

I will have the desired result. However, what I'm trying to generate is the behavior of the code below taken from "Teach yourself in 21 days".

Though the code is a little long there are few steps that are important (in black).

In the main function there is a call to a method SetFirstName from the class Employee
Code:
 Edie.SetFirstName("Edythe");

that pases as parameter a constant character string, but SetFirstName only requires a constant string reference.
This problem is solved by the String constructor
Code:
String(const char *const);

that takes a constant character string and makes a string.

So, in my code I was expecting to have the constructor
Code:
Param(int aa, int bb)

the same behavior as
Code:
String(const char *const);

, but there is something I don't see.



Code:
   class String
   {
      public:
         // constructors
         String();
          String(const char *const);
          String(const String &);
         ~String();

         // overloaded operators
         char & operator[](int offset);
         char operator[](int offset) const;
         String operator+(const String&);
         void operator+=(const String&);
         String & operator= (const String &);

         // General accessors
         int GetLen()const { return itsLen; }
         const char * GetString() const { return itsString; }
         // static int ConstructorCount;

      private:
         String (int);         // private constructor
         char * itsString;
         unsigned short itsLen;

   };

   // default constructor creates string of 0 bytes
   String::String()
   {
      itsString = new char[1];
      itsString[0] = '\0';
      itsLen=0;
      // cout << "\tDefault string constructor\n";
      // ConstructorCount++;
   }

   // private (helper) constructor, used only by
   // class methods for creating a new string of
   // required size.  Null filled.
   String::String(int len)
   {
      itsString = new char[len+1];
      for (int i = 0; i<=len; i++)
         itsString[i] = '\0';
      itsLen=len;
      // cout << "\tString(int) constructor\n";
      // ConstructorCount++;
   }

   // Converts a character array to a String
   String::String(const char * const cString)
   {
      itsLen = strlen(cString);
      itsString = new char[itsLen+1];
      for (int i = 0; i<itsLen; i++)
         itsString[i] = cString[i];
      itsString[itsLen]='\0';
      // cout << "\tString(char*) constructor\n";
      // ConstructorCount++;
   }

   // copy constructor
   String::String (const String & rhs)
   {
      itsLen=rhs.GetLen();
      itsString = new char[itsLen+1];
      for (int i = 0; i<itsLen;i++)
         itsString[i] = rhs[i];
      itsString[itsLen] = '\0';
      // cout << "\tString(String&) constructor\n";
      // ConstructorCount++;
   }

   // destructor, frees allocated memory
   String::~String ()
   {
      delete [] itsString;
      itsLen = 0;
      // cout << "\tString destructor\n";
   }

   // operator equals, frees existing memory
   // then copies string and size
   String& String::operator=(const String & rhs)
   {
      if (this == &rhs)
         return *this;
      delete [] itsString;
      itsLen=rhs.GetLen();
      itsString = new char[itsLen+1];
      for (int i = 0; i<itsLen;i++)
         itsString[i] = rhs[i];
      itsString[itsLen] = '\0';
      return *this;
      // cout << "\tString operator=\n";
   }

   //non constant offset operator, returns
   // reference to character so it can be
   // changed!
   char & String::operator[](int offset)
   {
      if (offset > itsLen)
         return itsString[itsLen-1];
      else
         return itsString[offset];
   }

   // constant offset operator for use
   // on const objects (see copy constructor!)
   char String::operator[](int offset) const
   {
      if (offset > itsLen)
         return itsString[itsLen-1];
      else
         return itsString[offset];
   }

   // creates a new string by adding current
   // string to rhs
   String String::operator+(const String& rhs)
   {
      int  totalLen = itsLen + rhs.GetLen();
      String temp(totalLen);
      int i, j;
      for (i = 0; i<itsLen; i++)
         temp[i] = itsString[i];
      for (j = 0; j<rhs.GetLen(); j++, i++)
         temp[i] = rhs[j];
      temp[totalLen]='\0';
      return temp;
   }

   // changes current string, returns nothing
   void String::operator+=(const String& rhs)
   {
      unsigned short rhsLen = rhs.GetLen();
      unsigned short totalLen = itsLen + rhsLen;
      String  temp(totalLen);
      for (int i = 0; i<itsLen; i++)
         temp[i] = itsString[i];
      for (int j = 0; j<rhs.GetLen(); j++, i++)
         temp[i] = rhs[i-itsLen];
      temp[totalLen]='\0';
      *this = temp;
   }

 // int String::ConstructorCount = 0;

Code:
   class Employee
   {

   public:
      Employee();
      Employee(char *, char *, char *, long);
      ~Employee();
      Employee(const Employee&);
      Employee & operator= (const Employee &);

      const String & GetFirstName() const
         { return itsFirstName; }
      const String & GetLastName() const { return itsLastName; }
      const String & GetAddress() const { return itsAddress; }
      long GetSalary() const { return itsSalary; }

      void SetFirstName(const String & fName)
          { itsFirstName = fName; }
      void SetLastName(const String & lName)
         { itsLastName = lName; }
      void SetAddress(const String & address)
           { itsAddress = address; }
      void SetSalary(long salary) { itsSalary = salary; }
   private:
      String    itsFirstName;
      String    itsLastName;
      String    itsAddress;
      long      itsSalary;
   };

   Employee::Employee():
      itsFirstName(""),
      itsLastName(""),
      itsAddress(""),
      itsSalary(0)
   {}

   Employee::Employee(char * firstName, char * lastName,
      char * address, long salary):
      itsFirstName(firstName),
      itsLastName(lastName),
      itsAddress(address),
      itsSalary(salary)
   {}

   Employee::Employee(const Employee & rhs):
      itsFirstName(rhs.GetFirstName()),
      itsLastName(rhs.GetLastName()),
      itsAddress(rhs.GetAddress()),
      itsSalary(rhs.GetSalary())
   {}

   Employee::~Employee() {}

   Employee & Employee::operator= (const Employee & rhs)
   {
      if (this == &rhs)
         return *this;

      itsFirstName = rhs.GetFirstName();
      itsLastName = rhs.GetLastName();
      itsAddress = rhs.GetAddress();
      itsSalary = rhs.GetSalary();

      return *this;
   }

Code:
   int main()
   {
      Employee Edie("Jane","Doe","1461 Shore Parkway", 20000);

      Edie.SetFirstName("Edythe");


     cout << Edie.GetFirstName().GetString();

    return 0;
}

# 4  
Old 06-27-2010
I don't see any similarity in design or inheritance between what you're trying to do and and the example you quote... I don't think the constructor is what lets it do that, are you sure that's the complete code? No external operator functions? Also: You can't cast one value into two values.
Last edited by Corona688; 06-27-2010 at 12:45 AM..
# 5  
Old 06-28-2010
Quote:
Originally Posted by Corona688
I don't see any similarity in design or inheritance between what you're trying to do and and the example you quote... I don't think the constructor is what lets it do that, are you sure that's the complete code? No external operator functions? Also: You can't cast one value into two values.
The code is taken from:
PHP Code:
http://newdata.box.sk/bx/c/htm/ch15.htm 
see listing 15.1 and 15.2. Most of the code is exactly the same as the one I posted with the exception of the main function. According to the book -and I debugged it in order to verify it- this behavior is due to the constructor from String.
# 6  
Old 06-29-2010
I stand corrected, then. You still can't cast one parameter into two parameters, though. It could take a single class that had two members.

A better way to handle this case would be to overload function itself -- make another function of the same name that does in fact take two integers.
# 7  
Old 06-30-2010
Quote:
Originally Posted by santiagorf
I have the following code

Code:
class Param{
    public:
    Param(int aa, int bb){
        a=aa;
        b=bb;
    }
int a,b;
};

void function(Param);

Code:
int main(){

   function(2,3);
    return 0;
}

Code:
void function(Param par){
    cout<<par.a;
}

, and when I run it, I get the following message error:

Code:
.../main.cpp|21|error: conversion from ‘int' to non-scalar type ‘Param' requested|

I know the problem is that function should have been called with an object from Param, but I was expecting that the constructor Param(int , int) would do the job.
What is wrong with my assumption, and how could I fix this problem?

thank you in advance!!
santiagorf
This is known as implicit conversion and for constructors it works if it has a single argument only.

If you have multiple arguments and want to make use of the implicit conversion, then you need a constructor having default values.

Something like...

Code:
class Param{
public:
    Param(int a, int b = 0) : _a(a), _b(b) {}
int _a,_b;
};
void function(Param par)
{
   cout<<par.a;
}
int main(){

   function(2);
   return 0;
}

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