Pointer (address) with if statement

Question

I had a working code that gave me the address of a mesh (if i'm correct):

MyMesh &mesh = glWidget->mesh();

Now I want if thingie to assign different mesh adresses. One is mesh() first function and another function mesh(int): How is this done?

 MyMesh &mesh;  //error here: need to be initialized

 if(meshNum==0){
mesh = glWidget->mesh();
 }
 else if (meshNum==1){
mesh = glWidget->mesh(0);
 }
 else{
  return;
 }

 //mesh used in functions...
 function(mesh,...);

Answer 1

References must be bound to an object at initialization ... you cannot have a default-initialized or zero-initialized reference. So code like:

MyMesh &mesh;

where mesh is a non-constant l-value reference to a Mesh object, is inherently ill-formed. At the point of declaration, you must bind the non-constant reference to a valid memory-addressable object.

Answer 2

If your case is simple enough that meshNum is constrained, you can use the ?: operator:

MyMesh &mesh = (meshNum == 0) ? glWidget->mesh() : glWidget->mesh(0);

Otherwise, you need a pointer since references must be initializated at the definition point, and cannot be reseated to refer to anything else.

MyMesh *mesh = 0;
if( meshNum == 0 ) {
    mesh = &glWidget->mesh();
} else if ( meshNum == 1 ){
    mesh = &glWidget->mesh(0);
}

function( *mesh, ... );

Answer 3

References are valid at all times in a well behaved program, so no, you cannot do that. However, why not just:

if(meshNum != 0 && meshNum != 1)
    return;
function((meshNum == 0) ? glWidget->mesh() : glWidget->mesh(0));

Or you could just use a pointer and deference it later:

MyMesh *mesh = 0;
if(meshNum==0) {
    mesh = &glWidget->mesh();
}
else if (meshNum==1) {
    mesh = &glWidget->mesh(0);
}
else {
  return;
}

function(*mesh);