I encountered some really strange c code that I have never seen before, even though I have a little bit of experience. I tryed searching for it but I had no luck. This is super strange and either im really stupid or this is some wizard magic code. My question is how can the GetHelixCenter have a bool return type but still fill the helixcenterpos[] array? The GetHelixCenter function is called from another function like:

Double_t helixcenterpos[2];
GetHelixCenter(pparam,helixcenterpos);

and later on helixcenterpos is accessed:

Double_t xpos = helixcenterpos[0];

but it is untouched in between. The GetHelixCenter function looks like:

Bool_t AliV0ReaderV1::GetHelixCenter(const AliExternalTrackParam *track,Double_t center[2]){

  // Get Center of the helix track parametrization

  Int_t charge=track->Charge();
  Double_t b=fInputEvent->GetMagneticField();

  Double_t  helix[6];
  track->GetHelixParameters(helix,b);

  Double_t xpos =  helix[5];
  Double_t ypos =  helix[0];
  Double_t radius = TMath::Abs(1./helix[4]);
  Double_t phi = helix[2];

  if(phi < 0){
    phi = phi   2*TMath::Pi();
  }

  phi -= TMath::Pi()/2.;
  Double_t xpoint =  radius * TMath::Cos(phi);
  Double_t ypoint =  radius * TMath::Sin(phi);

  if(b<0){
    if(charge > 0){
      xpoint = - xpoint;
      ypoint = - ypoint;
    }
  }
  if(b>0){
    if(charge < 0){
      xpoint = - xpoint;
      ypoint = - ypoint;
    }
  }
  center[0] =  xpos   xpoint;
  center[1] =  ypos   ypoint;

  return 1;
}

CodePudding user response：

The helixcenterpos array is passed as the second argument:

Bool_t AliV0ReaderV1::GetHelixCenter(const AliExternalTrackParam *track,Double_t center[2]){

Here, center decays to a pointer to the first value of your original array. Hence

center[0] =  xpos   xpoint;
center[1] =  ypos   ypoint;

write to that array.

It should be pointed out, that the parameter would be better chosen as

Bool_t AliV0ReaderV1::GetHelixCenter(const AliExternalTrackParam *track,Double_t (&center)[2]){

so that you are sure that GetHelixCenter always receives the correct array.

CodePudding user response：

I'm not sure if I understand your question correctly. Are you unsure why your function is capable of modifying the external array?

That's because in C when passing arrays, you're actually passing a pointer to the start of the array, instead of passing a copy of the array, as is the case with non-array types.

So, for example:

void myFunction(int a) {
    a = 3;
}

int main() {
    int a = 0;
    myFunction(a);
    std::cout << a << std::endl;
}

would still print 0. Because the integer is copied when calling myFunction and that copy is modified.

But when doing:

void myFunction(int a[1]) {
    a[0] = 3;
}

int main() {
    int a[1];
    myFunction(a);
    std::cout << a[0] << std::endl;
}

would print 3, as you're actually modifying the first element of the array pointed to by a.