C++ no match for 'operator='

Question

I'm currently working on an assignement concerning mobile robots. I'm developing on Windows with QT-Creator, using CMake and Visual C++ Compiler 10.0.

As the robot's working on Ubuntu, I need to compile the projekt using QT-Creator on Ubuntu with the GNU x86 compiler.

Now here's the problem, concerning the class Box, which is just storing some int-values:

Box.h

public:
Box();                                                                  
Box(int name, int sX, int sY, int width, int heigth);               
Box& operator = (Box &src);                                             
bool operator == (Box &src);                                     

private:
int name;                                                              
int startX, startY, endX, endY;                                       
int cgx, cgy;                                                        
int width, height;

There are also some get-methods in this header (like int getName() ).

The overloaded assignment-operator looks as following:

Box &Box::operator =(Box &src){
    this->cgx = src.getCenterX();
    this->cgy = src.getCenterY();
    this->endX = src.getEndX();
    this->endY = src.getEndY();
    this->startX = src.getStartX();
    this->startY = src.getStartY();

   return *this;} 

I'm using Box objects in another class, stored in a std::list<Box>.

On Windows, everything works fine. But on Ubuntu I get the error-message no match for 'operator=' (operand types are 'Box' and 'const Box') , which occurs in a function of std::list.

I'm quite new to C++ and haven't used const explicit anywhere. So how does this error happen and how do I solve it?

Answer 1

If you want to manually implement the assignment operator=() for your Box class (note that the compiler will implement a default member-wise assignment operator= if you don't provide one), consider following a pattern something like this:

1. use const Box& (not just Box&) as the input source type, since you are not modifying the source;
2. check for self-assignment inside the overloaded operator implementation (if (&src != this)...).

In code:

Box& Box::operator=(const Box& src) 
{
    if (&src != this) 
    {
        this->cgx = src.getCenterX();
        this->cgy = src.getCenterY();
        this->endX = src.getEndX();
        this->endY = src.getEndY();
        this->startX = src.getStartX();
        this->startY = src.getStartY();
    }
    return *this;
} 

Note also that if you manually define a copy assignment operator=(), you may also want to define a copy constructor, probably following the same copy logic implemented in your assignment operator=():

Box(const Box& src) :
    cgx   ( src.getCenterX() ),
    cgy   ( src.getCenterY() ),
    endX  ( src.getEndX()    ),
    endY  ( src.getEndY()    ),
    startX( src.getStartX()  ),
    startY( src.getStartY()  )
{}

---

NOTE

You have a data member name in your Box class that you are not copying. Is this a typo? Are you really sure you don't want to copy it? Maybe this is a bug?