How to assign a variable length array in a structure?

[zhshqzyc]

Suppose a structure


struc data{
double *aa;
int b;};
aa is an array.


double *abc
struct data *d1;
abc = d1->aa

can I assign abc as the above

[DragForce]

Technically yes, practically any code like this is scary and dangerous.

Use std::vector instead of vector and smartpointers instead of pointers. You could also consider make datamembers of the structure private and define setters/getters.

[treuss]

Suppose a structure


struc data{
double *aa;
int b;};
aa is an array.

aa is a pointer.

double *abc
struct data *d1;
abc = d1->aa

can I assign abc as the above

In your example d1 points to undefined memory. So doing the assignment will likely cause a coredump. What you are trying is anyways a very dangerous thing to do. Example:

struc data {
  double *aa;
  int b;
};

data mydata; // create struct on stack
double *abc = mydata.aa; // abc points to same place as aa
mydata.aa = new double[256]; // create the memory for the array. Allocates memory.
double x = abc[0]; // Big problem. abc does not point to the same place as aa any more.

Use std::vector instead:

struc data {
  vector<double> aa;
  int b;
};

data mydata; // create struct on stack
vector<double>& abc = mydata.aa; // abc is a reference to mydata.aa
mydata.aa.resize(256); // create the memory for the array.
double x = abc[0]; // No problem

[zhshqzyc]

if aa is a two dimension array,can I follow the same principle?

[XZminX]

yes.
all you have to do is to make a vector of vectors