C++: Accessing data of dynamic structures within extern function - c++

Question

I have a problem with passing dynamically allocated structures to a function and accessing it's content.
The program uses mex to pass data from Matlab to C++. I use Visual Studio.
The structure I define in a header in 'InOut.h'
#include <string>
#include <cstdint>
#include <cstdlib>
struct sObjects {
std::string Type;
float *Position;
};
typedef struct sObject sObject;
In the main function I than allocate the structure is in 'MainFcn_Mex.cpp'
#include "MainFcn_Mex.h"
// ...
// get number of Objects from Matlab
int N_Obj = mxGetNumberOfElements(prhs[1]);
sObjects *Objects = new sObjects[N_Obj];
for (int k=0; k<N_Obj; k++) {
// get the pointer pointer map
pMap = mxGetField(prhs[1],k,"Type");
Objects[k].Type = mxArrayToString(pMap);
// get the pointer pointer map
pMap = mxGetField(prhs[1],k,"Position");
// setting pointer to the first Element
Objects[k].Position = (float*)mxGetPr(pMap);
mexPrintf("Objects(%d,1).Type: %s \n", k+1, Objects[k].Type);
}
create_Objects(Objects, N_Obj);
The function create_Objects is in a differente file 'create_Objects.cpp' and included via 'MainFcn_Mex.h':
#include <stdio.h>
#include <direct.h>
#define _USE_MATH_DEFINES
#include "math.h"
#include <cmath>
#include "mex.h"
#include "matrix.h"
#include <cuda.h>
#include <cuda_runtime.h>
#include "device_launch_parameters.h"
#include "InOut.h"
void create_Objects(sObjects *Objects, int N_Obj);
The content of 'create_Objects.cpp' so far is:
#define _USE_MATH_DEFINES
#include "math.h"
#include <cmath>
#include "InOut.h"
#include "mex.h"
void create_Objects(sObjects *Objects, int N_Obj)
{
for (int k=0; k<N_Obj; k++) {
mexPrintf("Objects(%d,1).Type: %s \n", k+1, Objects[k].Type);
}
}
Visual Studio tells me:
"error C2676: binary '[' : 'sObjects' does not define this operator or
a conversion to a type acceptable to the predefined operator"
Why can I access the data in the main function and not in seccondary functions?
How can I access a dynamically allocated structure in other functions, when its size isn't known at compile time?
Thanks a lot for your time!

Answer 1

It looks like you are trying to use struct directly as a typedef. Simply add typedef to your struct definition, will turn it into a type.
Like this:
... #include "mex.h"
typedef struct sObjects {
std::string Type;
float *Position;
};
(Otherwise you should use the full struct keyword as in void create_OpticsObjects(struct sObjects &Objects, int N_Obj).)
Your function prototypes don't need the extern qualifier.
You don't need extern unless you want globals variables. You seem to want simply global structs or types in your example, so extern is not required.
Using extern for globals
What you could be referring to is an instance of your object (or a pointer to your object), and that can be made global by using extern. as in this excerpt from the header file:
... #include "mex.h"
typedef struct sObjects {
std::string Type;
float *Position;
};
extern sObjects *pointerToOnesObjects;
Then in ONE source file, you need to declare the 'real' variable as in (this is good to initialise it here):
sObjects *pointerToOnesObjects = NULL;
With this method your variable pointerToOnesObjects is now available globally (in all your source files that use the same header file).