Re: What's the point of passing parameter by value (vs. const ref)

From:
mtlung@gmail.com
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 3 Jul 2008 03:28:03 CST
Message-ID:
<bd78f442-151b-4849-84f9-eca05ef5382e@s21g2000prm.googlegroups.com>
On 7???3???, ??????1???58???, "Martin T." <0xCDCDC...@gmx.at> wrote:

Hi all.

When writing new C++ code, what is the point of passing any (input)
parameter by value when passing by const reference will just work as
well? (Even and especially PODs, I would not do it with a complex type
anyway.)
(Given that in 90% of the code you will never want to modify the
parameter anyway.)

br,
Martin

[EXAMPLE_CODE]
class foo {

};

void f_val_int(int);
void f_ref_int(const int&);
void f_val_foo(foo);
void f_ref_foo(const foo&);

int main(int,)
{
  double d = 1.2;
  f_val_int(d);
  f_ref_int(d);
  f_val_int(1.4);
  f_ref_int(1.5);

  foo x;
  f_val_foo(x);
  f_ref_foo(x);
  f_val_foo(foo());
  f_ref_foo(foo());

  return 0;

};

void f_val_int(const int x) {
  int y = x;
  y++;

}

void f_ref_int(const int& x) {
  int y = x;
  y++;

}

void f_val_foo(const foo f) {
  foo ff = f;
  ff;

}

void f_ref_foo(const foo& f) {
  foo ff = f;
  ff;}

[/EXAMPLE_CODE]


{ Edits: quoted clc++m banner removed, since it's available at the end of this
article (and indeed, every clc++m article). There's no need to quote it. -mod }

It is because passing by reference/pointer will introduce the alias
problem, which hinder the compiler to perform a numbers of
optimizations.
Therefore, in a performance point of view, primitive types should pass
by value while large data structure should pass by reference/pointer.
More information on the aliasing problem:
http://www.ddj.com/cpp/184404273;jsessionid=FAZXOZJKL5K24QSNDLPCKHSCJUNN2JVN?_requestid=161084

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