Question

I would like to have a vector z, and be able to access it as z[i] or *pz[i], where *pz[i] points to z[i]. so there is one set of values but two ways to access it.

This is the code that I had:

std::vector<double> z;
std::vector<double*> pz;

for (int i = 0; i < 5; i++) {
  z.push_back(7+0.01*i);
  std::cout << i << "  z = " << z << std::endl;
  pz.push_back(&z.back());
  std::cout << i << "  pz = " << pz << std::endl;
  std::cout << i << "  *pz = " ;
  for (int j = 0; j < pz.size(); j++) {
    std::cout << "  " << *pz[j];
  }
  std::cout << std::endl;
}

z[1]=17.3;
std::cout << "z[1] = " << z[1] << std::endl;
std::cout << "*pz[1] = " << *pz[1] << std::endl;

*pz[2]=34.1;
std::cout << "z[2] = " << z[2] << std::endl;
std::cout << "*pz[2] = " << *pz[2] << std::endl;

the output:

0  z = vector(1) [ 7 ]
0  pz = vector(1) [ 0x1d00b80 ]
0  *pz =   7
1  z = vector(2) [ 7, 7.01 ]
1  pz = vector(2) [ 0x1d00b80, 0x1d00bc8 ]
1  *pz =   1.50254e-316  7.01
2  z = vector(3) [ 7, 7.01, 7.02 ]
2  pz = vector(3) [ 0x1d00b80, 0x1d00bc8, 0x1d00bf0 ]
2  *pz =   1.50254e-316  7.01  7.02
3  z = vector(4) [ 7, 7.01, 7.02, 7.03 ]
3  pz = vector(4) [ 0x1d00b80, 0x1d00bc8, 0x1d00bf0, 0x1d00bf8 ]
3  *pz =   1.50254e-316  7.01  7.02  7.03
4  z = vector(5) [ 7, 7.01, 7.02, 7.03, 7.04 ]
4  pz = vector(5) [ 0x1d00b80, 0x1d00bc8, 0x1d00bf0, 0x1d00bf8, 0x1d00c60 ]
4  *pz =   1.50254e-316  7.01  7.02  7.03  7.04
z[1] = 17.3
*pz[1] = 7.01
z[2] = 7.02
*pz[2] = 34.1

There are 3 problems:

1. The first element of *pz gets undefined after second push.
2. When I assign z[1], *pz[1] is not z[1] anymore.
3. When I assign *pz[2], it’s not assigning to z[2] anymore.

(In my actual program I need to have a vector to access even elements and a vector to access odd elements but I don’t want to make a clone and use more memory.)

Answer 1

Two things:

1. When you do a push_back() or any operation that might cause the vector to be reallocated, you invalidate all pointers to elements of that vector.
2. In 64-bit, a pointer will probably be the same size as a double so you won’t save any memory that way.

If you really want to create a second vector with pointers to elements of the first vector, you need to finish building the first vector first. Otherwise, the push_back() operations will invalidate the pointers.