C++ classes, including structures and unions, are passed and returned by value. For Plain-Old-Data (POD) classes, the C++ compiler is required to pass the struct as would the C compiler. Objects of these classes are passed directly. For objects of classes with user-defined copy constructors, the compiler is effectively required to construct a copy of the object, pass a pointer to the copy, and destruct the copy after the return. Objects of these classes are passed indirectly. For classes that fall between these two requirements, the compiler can choose. However, this choice affects binary compatibility, so the compiler must choose consistently for every class.
For most compilers, passing objects directly can result in faster execution. This execution improvement is particularly noticeable with small value classes, such as complex numbers or probability values. You can sometimes improve program efficiency by designing classes that are more likely to be passed directly than indirectly.
A class is passed indirectly if it has any one of the following characteristics:
A user-defined copy constructor
A user-defined destructor
A base that is passed indirectly
A non-static data member that is passed indirectly
Otherwise, the class is passed directly.