You get this error when declaring a forward reference inside the wrong namespace thus declaring a new type without defining it. For example:
namespace X
{
namespace Y
{
class A;
void func(A* a) { ... } // incomplete type here!
}
}
...but, in class A was defined like this:
namespace X
{
class A { ... };
}
Thus, A was defined as X::A
, but I was using it as X::Y::A
.
The fix obviously is to move the forward reference to its proper place like so:
namespace X
{
class A;
namespace Y
{
void func(X::A* a) { ... } // Now accurately referencing the class`enter code here`
}
}