标准向量对象的错误重载运算符[]

Question

I have a polygon class defined like this:

#include <gVector3.h>   // an array containing three floats (x,y,z coordinates)
#include <vector>

class Polygon {

private:

    std::vector <gVector3> vertices;
    std::vector <gVector3> color;

I overloaded the [] operator for this class like so

gVector3 Polygon::operator [](unsigned int i) const {
    return vertices[i];
}

I've written a simple test case:

gVector3 v1(0,0,1), v2(1,1,1), v3(2,0,1);
Polygon *p = new Polygon();
p->push(v1);
p->push(v2);
p->push(v3);
assert(p[0] == v1);   // Assume == for the gVector3 class has been defined correctly

Push is defined like so..

void Polygon::push(gVector3 vec){
    this->vertices.push_back(vec);
}

Bottom line is that this assertion fails, and I'm not sure why. Perhaps I'm misusing the Vector classes indexing method?

Any insights would be helpful!

Answer 1

p is a pointer, so p[0] is the Polygon that it points to. I'm surprised that it compiled; I guess there must be some strange implicit type conversion.

You almost certainly don't want to be messing around with pointers and new :

Polygon p;
p.push(v1);
p.push(v2);
p.push(v3);
assert(p[0] == v1);

but if you do for some reason, then you'll need to dereference the pointer before applying []

assert((*p)[0] == v1);

Answer 2

p[0] is equivalent to *(p + 0) , or just *p , so its just giving you the Polygon object you allocated. You aren't calling operator[] on that object. You need to do (*p)[0] instead, which will first get you the Polygon object and then call operator[] on it.