Construct object in another class after initializing variables - c++

Question

I have something like the following:
QuadMesh.h:
#include "StructureIpsim.h" // this is a struct
class QuadMesh {
public:
QuadMesh(StructureIpsim s) {//do stuff}
};
SEMPotential.h:
#include "QuadMesh.h"
#include "SpecialFuncs.h"
class SEMPotential {
public:
StructureIpsim SI;
QuadMesh mesh;
SEMPotential( //args);
};
SEMPotential::SEMPotential( //args) {
// init structure in here, need to call functions from SpecialFuncs.h to do so
// How to construct the QuadMesh object mesh? Can't put in initialization list.
}
As you can see, the QuadMesh object takes in a StructureIpsim sruct, but this struct must be initialized using several functions in the constructor of SEMPotential before being passed to the QuadMesh constructor. What's the standard way around this?

Answer 1

You CAN use the initializer-list. Use a helper function, which should probably be a private static member function, that accepts the relevant arguments, does the calculations, and returns a QuadMesh object. Use the initialization list to initialize the member from the return value of that helper.
class SEMPotential
{
static QuadMesh mesh_creator( /* args */ );
public:
QuadMesh mesh;
SEMPotential(/* args */) : mesh(mesh_creator(args)) {}
};
QuadMesh SEMPotential::mesh_creator( /*args*/ )
{
StructureIpsum s;
// init structure in here, calling functions from SpecialFuncs.h to do so
return QuadMesh(s);
}
As Matt McNabb points out in his comment, that helper function could be a constructor of StructureIpsum. But I present a solution that doesn't require modifying StructureIpsum or QuadMesh class definitions.
If you want to preserve the StructureIpsum instance, use the helper method trick to initialize it, and then simply use it to initialize the mesh:
SEMPotential(/* args */) : SI(ipsum_creator(args)), mesh(SI) {}
Initialization order of members is guaranteed (it's the order they appear in the class, the ordering in the initialization list has no effect).