Question

So I’ve been writing a small math library in C++ and when dealing with a scalar multiplied by a vector I get some issues when I try to perform this operation

Vect V2;
Vect V3;
float S;

Vect V1 = V2 + S * (V2 - V3);

The Vect Value I receive in the overloaded operator * is a new Vect object and not the outcome of the (V2 – V3) part of the operation. Here’s the other relevant part of the code. If I follow the operation with the debugging the two overloaded operators work correctly by themselves but not one after the other.

Vect.h

Vect &operator - (const Vect &inVect);
friend const Vect operator*(const float scale, const Vect &inVect);

Vect.cpp

Vect &Vect::operator - (const Vect &inVect)
{
    return Vect (this->x - inVect.x,this->y - inVect.y,this->z - inVect.z, 1);
}

const Vect operator*(const float scale, const Vect &inVect)
{
    Vect result;

    result.x = InVect.x * scale;
    result.z = InVect.y * scale;
    result.y = InVect.z * scale;
    result.w = 1;

    return result;
}

I also overloaded the + and = operator and they work as expected the only problem I encounter is the problem above.

Answer 1

In your operator-, you create a temporary Vect and return a reference to that temporary. The temporary is destroyed at the end of the return statement and the returned reference is left dangling. Doing anything with this reference will result in undefined behaviour.

Instead, your operator- should return a Vect by value:

Vect Vect::operator - (const Vect &inVect)
{
  return Vect (this->x - inVect.x,this->y - inVect.y,this->z - inVect.z, 1);
}