Where to place the '&' in a parameter to a function? [closed]

Question

Can anybody tell me what is the difference between

void fun(MyClass &mc);

and

void fun(MyClass& mc);

in C++?

Answer 1

As given none.

Originally, C would allow:

int x, *y;

To declare both an int, x and a pointer to int, y.

Hence part of the definition of the type - the bit that makes it a pointer - could be separated from another part.

C++ copied this wholesale.

Then references where added, and they got a similar style of declaration except with & rather than *. This meant that both MyClass &mc and MyClass& mc were allowed.

On the choice when it comes to *, 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.

By extension, when it comes to &, MyClass& mc matches the "typical C++" style.

Answer 2

To the compiler there's no difference.

The first is closer to usual C-syntax, the latter is more C++-ish.