Dove posizionare il '&' in un parametro in una funzione?

Question

Qualcuno può dirmi qual è la differenza tra

void fun(MyClass &mc); 

e

void fun(MyClass& mc); 

in C++?

Answer 1

Come dato nessuno.

Originariamente, C permetterebbe:

int x, *y; 

Dichiarare sia un int, x e un puntatore a int, y.

Quindi parte della definizione del tipo - il bit che lo rende un puntatore - potrebbe essere separata da un'altra parte.

C++ ha copiato questo commercio all'ingrosso.

Quindi riferimenti aggiunti, e hanno ottenuto uno stile simile di dichiarazione tranne & anziché *. Ciò significava che erano consentiti sia MyClass &mc e MyClass& mc.

Sulla scelta quando si tratta di *, Strousup wrote:

Both are "right" in the sense that both are valid C and C++ and both have exactly the same meaning. As far as the language definitions and the compilers are concerned we could just as well say "int*p;" or "int * p;"

The choice between "int* p;" and "int *p;" is not about right and wrong, but about style and emphasis. C emphasized expressions; declarations were often considered little more than a necessary evil. C++, on the other hand, has a heavy emphasis on types.

A "typical C programmer" writes "int *p;" and explains it "*p is what is the int" emphasizing syntax, and may point to the C (and C++) declaration grammar to argue for the correctness of the style. Indeed, the * binds to the name p in the grammar.

A "typical C++ programmer" writes "int* p;" and explains it "p is a pointer to an int" emphasizing type. Indeed the type of p is int*. I clearly prefer that emphasis and see it as important for using the more advanced parts of C++ well.

The critical confusion comes (only) when people try to declare several pointers with a single declaration:

int* p, p1; // probable error: p1 is not an int*

Placing the * closer to the name does not make this kind of error significantly less likely.

int *p, p1; // probable error?

Declaring one name per declaration minimizes the problem - in particular when we initialize the variables. People are far less likely to write:

int* p = &i; int p1 = p; // error: int initialized by int*

And if they do, the compiler will complain.

Whenever something can be done in two ways, someone will be confused. Whenever something is a matter of taste, discussions can drag on forever. Stick to one pointer per declaration and always initialize variables and the source of confusion disappears. See The Design and Evolution of C++ for a longer discussion of the C declaration syntax.

Per estensione, quando si tratta di &, MyClass& mc corrisponde al "tipico C++" stile.

Answer 2

Al compilatore non c'è differenza.

Il primo è più vicino alla solita sintassi C, quest'ultimo è più C++ - ish.