question regarding consrtuctor calls

This is a discussion on question regarding consrtuctor calls within the C++ Programming forums, part of the General Programming Boards category; I am working through a practice exam for a programming final and I have a question about the way a ...

  1. #1
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    question regarding constructor calls

    I am working through a practice exam for a programming final and I have a question about the way a program calls class constructors. How am I able to tell how many times the constructor is called in a specific program? For example...

    Code:
    int main(){
      String A, B("Examination");
      cout << "Enter a string: ";
      cin >> A;
    
      cout << "The string you entered was: " << A << endl;
    
      if(A == "Midterm" || A == "Final"){
        cout << A << " " << B << endl;
      }
    }
    This is main() is based off a String class interface provided on the sample exam by the instructor. There is a question that asks how many calls to the constructor there are. When I look at this, I see 2 for sure and maybe 1. Where String A and B are declared and I believe where the user inputs data into A. Now, according to the sample exam the answer is 4 and I am not really sure why. Is it perhaps that they are called when they are declared and the when they are outputted? That would be 4 but doesn't really seem right to me.
    Last edited by alt234; 12-11-2005 at 02:17 PM.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Well I'd say the == also cause temporary String objects to be created out of the "strings", so that the equality overloaded function can be called.

  3. #3
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    There are difference answers depending on how the String class and various operators are implemented.

    For example, does operator<< accept a reference to String or a String by value? Does the String class support an operator==() that accepts a const char *, or does the comparison rely on creating a temporary String [as suggested by Salem]?

    The working of operator || (i.e. it shortcuts) can also reduce the number of comparisons done and (if the comparison creates a temporary String) will reduce the number of times a constructor is called. Consider what happens if A == "Midterm" returns true.

    These is also an issue of compiler optimisation: a compiler is allowed (by the C++ standard) to avoid creating temporaries if the only way of detecting those temporaries is to monitor calls to constructors and destructors.

  4. #4
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    Hmm, ok thanks. Here is the interface, we aren't given an implementation.

    Code:
    /******************************************************************************
    Figure 1 -- Interface for type "String" (in file "String.h")
    ******************************************************************************/
    #ifndef STRING_
    #define STRING_
    #include <iostream>
    using namespace std;
    class String
    {
    public:
    // Capacity of a string
    //
    static const int MAX = 64;
    // Construct empty string
    //
    String() { Length = 0; }
    // Construct string using array of characters
    //
    String( const char Source[] );
    // Return string length
    //
    unsigned length() const { return Length; }
    // Return reference to element I
    //
    char& operator[]( unsigned I ) { return Mem[I]; }
    const char& operator[]( unsigned I ) const { return Mem[I]; }
    // Append string Source to the current string
    //
    String& operator+=( const String& Source );
    // I/O operations
    //
    friend istream& operator>>( istream& Out, String& One );
    friend ostream& operator<<( ostream& Out, const String& One );
    private:
    char Mem[MAX]; // Memory to hold characters in string
    unsigned Length; // Number of characters in string
    };
    // Return string which is the concatenation of strings One and Two
    //
    String operator+( const String& One, const String& Two );
    // Compare two strings (equality operator)
    //
    bool operator==( const String& One, const String& Two );
    #endif

  5. #5
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    Given the answer from the sample exam, the interface could be assumed. The one posted matches what would be expected.

    To see when and where a constructor is called, you must also look through the sample program and look at all potential function calls. This includes the operator>>, operator<<, and operator== functions.

    If a String is passed by reference to one of these functions, then no constructor is called because a pass by reference doesn't create a new object, it just passes a reference to an existing one.

    If a String is passed by value to one of these functions, then a copy is created using the copy constructor, and so a constructor is called. Also, if a String is returned by value, then again a copy is made, and a constructor is called (think about what constructor would be called if the sample program used operator+).

    Finally, if a function parameter takes a String reference, but something else is passed (like a character array), then a conversion must take place. A conversion from one type to the String type would use a constructor to create a temporary instance of the String type which could then be passed to the function that takes a String reference. This is where the last two constructor calls occur. See if you can see and understand where that is happening.

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