Exemplo

#include "BaseClass.h"

struct ST
{
int a;
char c;
};

class Derived : Base{

public:
Derived(int newx, int newy);
Derived& operator=(const Derived& rhs);

private:
int x;
int y;

ST myStructure;
};

Derived& Derived::operator=(const Derived& rightValue)
{
if(this == &rightValue){
return *this;
}

x = rightValue.x;
y = rightValue.y;

myStructure.a = rightValue.myStructure.a;
myStructure.c = rightValue.myStructure.c;

return *this;
}

Solução

Chame o operador de designação da classe-base.

#include "BaseClass.h"

struct ST
{
int a;
char c;
};

class Derived : Base{

public:
Derived(int newx, int newy);
Derived& operator=(const Derived& rhs);

private:
int x;
int y;

ST myStructure;
};

Derived& Derived::operator=(const Derived& rightValue)
{
if(this == &rightValue){
return *this;
}

static_cast<Base&>(*this) = rightValue;

x = rightValue.x;
y = rightValue.y;

myStructure.a = rightValue.myStructure.a;
myStructure.c = rightValue.myStructure.c;

return *this;
}