'Invalid use of void expression' passing arguments of functions - c++

Question

This is a piece of my code that solves systems of differential equations.
Vector dydx(Neq);
void DiffEq(Vector x, Vector &dydx)
{
dydx(0) = x(1);
dydx(1) = -x(0);
}
double MidPoint(int n, Vector x)
{
double h=H/n;
Matrix z(n+1,n+1);
z.fillRow(0,x);
DiffEq(x, dydx);
z.fillRow(1,AddVec(x, dydx*h)); //Error: Invalid use of void expression
for (int j=1; j<n; j++)
{
DiffEq(z.getRow(j), dydx);
z.fillRow(j+1, AddVec(z.getRow(j-1), dydx*h*2)); //error: void value not ignored as it ought to be
}
DiffEq(z.getRow(n), dydx);
return 0.5*AddVec(z.getRow(n), z.getRow(n-1), dydx*h); //Error: Invalid use of void expression
}
The classes Vector and Matrix are custom. This is Vector
class Vector
{
public:
Vector(size_t size): vSize(size), vData(size){}
int getSize(){return vSize;}
double& operator()(size_t i){return vData[i];}
double operator()(size_t i) const {return vData[i];}
void operator+(double d) // These only overload the Vector + int operator
{
for (int i=0; i<vSize; i++) {vData[i]=vData[i]+d;}
}
void operator*(double d)
{
for (int i=0; i<vSize; i++) {vData[i]=vData[i]*d;}
}
size_t vSize;
vector<double> vData;
};
With a function
Vector AddVec(Vector v1, Vector v2)
{
Vector totVec(v1.getSize());
for (int i=0; i<v1.getSize(); i++) {totVec(i) = v1(i) + v2(i);}
return totVec;
}
And the same function for 3 vectors.
Now I realise that the error means that I'm passing void to some function, but I cannot figure out where things go wrong. When I try writing a test program, everything seems to work fine.

Answer 1

Your operators return void
void operator+(double d) // These only overload the Vector + int
{
for (int i=0; i<vSize; i++) {vData[i]=vData[i]+d;}
}
void operator*(double d)
{
for (int i=0; i<vSize; i++) {vData[i]=vData[i]*d;}
}
So when you call AddVec(x, dydx*h) it calls AddVec(x, void)

Answer 2

You've overloaded operators that don't behave like the conventional ones, don't do that!!! Doing that gives C++ a bad name.
void operator*(double d)
{
for (int i=0; i<vSize; i++) {vData[i]=vData[i]*d;}
}
This doesn't return anything, so you can't use it like this:
z.fillRow(1,AddVec(x, dydx*h));
^^^^^^
operator* should return a new object, not modify its left operand. If you want to modify the left operand, use operator*= but you should still return something.
Vector& operator*=(double d)
{
for (int i=0; i<vSize; i++)
vData[i] *= d;
return *this;
}
Vector operator*(double d)
{
Vector v(*this);
v *= d;
return v;
}
The operator* would be better as a non-member, and should take its argument by reference:
Vector operator*(const Vector& v, double d)
{
Vector v2(v);
v2 *= d;
return v2;
}