CODEWITHSUNDEEP
c++pass-by-const-reference
user829209
user829209

Reputation: 219

c++ passing by const reference

In the following program body cosists of a vector of pointers. Points is a struct of x,y,z coordinates and a point_id. I believe as body is passed by const reference, the following step should produce an error. BUt the program is running without any problem. Can you please explain me why is this. 

void readOutFile(const Body& body, int n){

    ....

    body.bp[0]->points.push_back(Point_id(p,i));
}

Upvotes: 20

Views: 2085

Answers (4)

Eight
Eight

Reputation: 4284

This is one of best example which shows why data members should not be public.

here, body is constant hence its data members must not be changed, but in body.bp[0]->points point is being changed which is not the member of Body.Hence no const violation.

Upvotes: 16

ecatmur
ecatmur

Reputation: 157334

Here's the issue:

body.bp[0]->points.push_back(Point_id(p,i));
          ^^

Indirecting through a pointer removes any constness; rather, the constness of the result is dependent on the type of the pointer.

T *t;              // pointer to T: can modify t and (*t)
const T *t;        // pointer to const-T: can modify t but not (*t)
T *const t;        // const-pointer to T: can modify (*t) but not t
const T *const t;  // const-pointer to const-T: can't modify either t or (*t)

Upvotes: 39

DevSolar
DevSolar

Reputation: 70253

Yes, body is constant. That means that no non-const member functions may be called, and no member variables be modified.

Neither is being done. The only member of body used is body.bp[0], which is not changed either, but merely used to get at points, which might or might not be constant...

Corrolary: Don't make data members public.

Upvotes: 6

Rohit Vipin Mathews
Rohit Vipin Mathews

Reputation: 11787

Only body is constant.

body.bp[0]->points is no way affected by the constantness of body

Upvotes: 6

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