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Standard argument passing
In C it’s very common to pass by
value, and when you want to pass an address your only choice is to use a
pointer[43].
However, neither of these approaches is preferred in C++. Instead, your first
choice when passing an argument is to pass by reference, and by const
reference at that. To the client programmer, the syntax
is identical to that of passing by value, so there’s no confusion about
pointers – they don’t even have to think about
pointers. For the creator of the function, passing an
address is virtually always more efficient than passing an entire class object,
and if you pass by const reference it means your function will not change
the destination of that address, so the effect from the client
programmer’s point of view is exactly the same as pass-by-value (only more
efficient).
Because of the syntax of references (it
looks like pass-by-value to the caller) it’s possible to pass a
temporary object to a function
that takes a const reference, whereas you can never pass a temporary
object to a function that takes a pointer – with a pointer, the address
must be explicitly taken. So passing by reference produces a new situation that
never occurs in C: a temporary, which is always const, can have its
address passed to a function. This is why, to allow temporaries to be
passed to functions by reference, the argument must be a const
reference. The following example
demonstrates this:
//: C08:ConstTemporary.cpp
// Temporaries are const
class X {};
X f() { return X(); } // Return by value
void g1(X&) {} // Pass by non-const reference
void g2(const X&) {} // Pass by const reference
int main() {
// Error: const temporary created by f():
//! g1(f());
// OK: g2 takes a const reference:
g2(f());
} ///:~
f( ) returns an object of
class X by value. That means when you
immediately take the return value of f( ) and pass it to another
function as in the calls to g1( ) and g2( ), a temporary
is created and that temporary is const. Thus, the call in
g1( ) is an error because g1( ) doesn’t take a
const reference, but the call to g2( ) is
OK.
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