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

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this piece of code 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     (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 ought     }        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 , matrix custom. 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 overload vector + int operator     {         (int i=0; i<vsize; i++) {vdata[i]=vdata[i]+d;}     }      void operator*(double d)     {         (int i=0; i<vsize; i++) {vdata[i]=vdata[i]*d;}     }       size_t vsize;     vector<double> vdata; };

with function

vector addvec(vector v1, vector v2) {     vector totvec(v1.getsize());     (int i=0; i<v1.getsize(); i++) {totvec(i) = v1(i) + v2(i);}     return totvec; }

and same function 3 vectors.

now realise error means i'm passing void function, cannot figure out things go wrong. when try writing test program, seems work fine.

your operators return void 

void operator+(double d)                        // these overload vector + int  {     (int i=0; i<vsize; i++) {vdata[i]=vdata[i]+d;} }  void operator*(double d) {     (int i=0; i<vsize; i++) {vdata[i]=vdata[i]*d;} }

so when call addvec(x, dydx*h) calls addvec(x, void)


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