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From: gautron@nuri.inria.fr (Philippe Gautron)
Newsgroups: comp.lang.c++
Subject: Assignment operator
Message-ID: <1943@seti.inria.fr>
Date: 19 Feb 91 18:18:27 GMT
Sender: news@seti.inria.fr
Reply-To: gautron@nuri.inria.fr (Philippe Gautron)
Organization: INRIA Rocquencourt, Le Chesnay, France
Lines: 100

The following example shows how difficult it can be to well understand
assignment operator and copy constructor.
This example was a student's answer to an examen about C++. The raw subject
was supposed easy, a matrix 2-2 with operator overloading.
Look at the code:
- a constructor with default arguments
- a copy constructor (no real need here except for completeness)
- an assignment operator: here will be our problem
- a printing function

-------------------------------------------
# include <stream.h>

// classe Matrix 2-2
class Matrix {
    int a1, a2, a3, a4;

  public:
    Matrix (int a1=0, int a2=0, int a3=0, int a4=0);
    Matrix (const Matrix&);

    Matrix operator= (const Matrix&);
    friend ostream& operator<< (ostream&, const Matrix&);
};

Matrix::Matrix (int a1, int a2, int a3, int a4){
    this->a1 = a1;
    this->a2 = a2;
    this->a3 = a3;
    this->a4 = a4;
}

Matrix::Matrix (const Matrix& m){
    this->a1 = m.a1;
    this->a2 = m.a2;
    this->a3 = m.a3;
    this->a4 = m.a4;
}

Matrix Matrix::operator= (const Matrix& m){
    return Matrix(m);		// <===
}

ostream& operator<< (ostream& stream, const Matrix& m){
    stream << "a1= " << m.a1
	   << " a2= " << m.a2
	   << " a3= " << m.a3
	   << " a4= " << m.a4
	   << '\n';
    return stream;
}

void f(){
    Matrix m1(1,1,1,1),
    	    m2(2,2,2,2);

    cout << m1 << m2;
    m1 = m2;
    cout << m1 << m2;
}
-------------------------------------------

In the assignment operator function, the student used a smalltalk-like
style, creating a new matrix in the function block and expecting
a copy as return value.
What cfront (the result is similar with g++) generates is (pseudo-C code):

void operator= (Matrix *this, Matrix *result, Matrix *m){
	ctor (result, m);
}

void f(){
	Matrix m1;
	Matrix m2;

	ctor (&m1, 1, 1, 1, 1);
	ctor (&m2, 2, 2, 2, 2);

	Matrix result;

	// printing
	operator = (&m1, &result, &m2);
	// printing
}

There is no copy of m2 into m1.

Note I'm using
<<AT&T C++ Translator 2.0 06/30/89>>
g++ version 1.37.1 (based on GCC 1.37)

Any comment, please email-me directly, my News access is with some
contortion by these days.

-----------------------
Philippe Gautron,			UUCP:	gautron@rxf.ibp.fr
Rank Xerox France / LITP
Universite de Paris VI
4 place Jussieu, 75252 PARIS CEDEX 05
FRANCE