Call by value vs. Call by reference

**Ron AF Greve** · Jul 11 '08, 08:45 AM

Re: Call by value vs. Call by reference

Hi,

I would expect you see a large difference since if you pass by value it has
to copy the whole vecor, however..

In your code the compiler might have removed the whole vector copying during
optimization since you don't do anything with it. You wouldn't notice a
difference between a program with the vector passed in and one completely
without and without assignment

Regards, Ron AF Greve

http://www.InformationSuperHighway.eu

"Saeed Amrollahi" <s_amrollahi@ya hoo.comwrote in message
news:7159c59a-49aa-46b0-938f-e0cb4d003701@w7 g2000hsa.google groups.com...

Dear All
Hi
>
I have learned when an object is big, I should pass the object using a
reference or pointer to it rather than calling by value.
In the following code, I don't gain cosiderable performance with call-
by-reference vs. call-by-value. Indeed, the perforamnce is absolutely
unconsiderable:
>
#include <vector>
#include <iostream>
>
using namespace std;
void f1(std::vector< intv)
{
cout << "f1 called" << '\n';
for (std::vector<in t>::size_type sz = 0; sz < v.size(); sz++)
v[sz] = sz;
cout << "f1 end" << '\n';
}
>
void f2(std::vector< intv)
{
cout << "f2 called" << '\n';
for (std::vector<in t>::size_type sz = 0; sz < v.size(); sz++)
v[sz] = sz;
cout << "f2 end" << '\n';
}
>
void f3(std::vector< int>* pv)
{
cout << "f3 called" << '\n';
for (std::vector<in t>::size_type sz = 0; sz < pv->size(); sz++)
(*pv)[sz] = sz;
cout << "f3 end" << '\n';
}
int main()
{
vector<intv(500 00000);
f1(v);
f2(v);
f3(&v);
>
return 0;
}
>
I ran under Visual Studio 2005, Visual Studio 2008 and GCC. Is it
about some compiler switches that I should on/off? Did compilers
become more clever?
>
In advance, thank you for your comments.
- Saeed Amrollahi

**Soumen** · Jul 11 '08, 02:15 PM

Re: Call by value vs. Call by reference

On Jul 11, 1:37 pm, Saeed Amrollahi <s_amroll...@ya hoo.comwrote:

Dear All
Hi
>
I have learned when an object is big, I should pass the object using a
reference or pointer to it rather than calling by value.
In the following code, I don't gain cosiderable performance with call-
by-reference vs. call-by-value. Indeed, the perforamnce is absolutely
unconsiderable:
>
#include <vector>
#include <iostream>
>
using namespace std;
void f1(std::vector< intv)
{
cout << "f1 called" << '\n';
for (std::vector<in t>::size_type sz = 0; sz < v.size();sz++)
v[sz] = sz;
cout << "f1 end" << '\n';
>
}
>
void f2(std::vector< intv)
{
cout << "f2 called" << '\n';
for (std::vector<in t>::size_type sz = 0; sz < v.size();sz++)
v[sz] = sz;
cout << "f2 end" << '\n';
>
}
>
void f3(std::vector< int>* pv)
{
cout << "f3 called" << '\n';
for (std::vector<in t>::size_type sz = 0; sz < pv->size(); sz++)
(*pv)[sz] = sz;
cout << "f3 end" << '\n';}
>
int main()
{
vector<intv(500 00000);
f1(v);
f2(v);
f3(&v);
>
return 0;
>
}
>
I ran under Visual Studio 2005, Visual Studio 2008 and GCC. Is it
about some compiler switches that I should on/off? Did compilers
become more clever?
>
In advance, thank you for your comments.
- Saeed Amrollahi

Probably because your vector is empty (may be it's just holding a
pointer and few other int)and you're inserting elements inside the
function.
Try inserting elements before calling the function and call the
function 1000 times or so. Also try vector of string and let it have
all same
elements. Let me know what you find.

Regards,
~ Soumen

**Juha Nieminen** · Jul 11 '08, 02:45 PM

Re: Call by value vs. Call by reference

Saeed Amrollahi wrote:

I have learned when an object is big, I should pass the object using a
reference or pointer to it rather than calling by value.
In the following code, I don't gain cosiderable performance with call-
by-reference vs. call-by-value. Indeed, the perforamnce is absolutely
unconsiderable:

How do you measure the speed of the program? Maybe copying that vector
is relatively so fast that you don't notice the difference. Try calling
each function in a loop to see if there's a difference.

**acehreli@gmail.com** · Jul 11 '08, 06:05 PM

Re: Call by value vs. Call by reference

On Jul 11, 7:08 am, Soumen <soume...@gmail .comwrote:

On Jul 11, 1:37 pm, Saeed Amrollahi <s_amroll...@ya hoo.comwrote:

void f1(std::vector< intv)
{
cout << "f1 called" << '\n';
for (std::vector<in t>::size_type sz = 0; sz < v.size(); sz++)
v[sz] = sz;

That is not "inserting" an element, rather assigning to an existing
one.

cout << "f1 end" << '\n';

>

}

vector<intv(500 00000);

That is a vector with 50 million ints, not empty.

Probably because your vector is empty (may be it's just holding a
pointer and few other int)and you're inserting elements inside the
function.

That would be this code:

vector<intv; // empty
/* ... */
v.push_back(sz) ; // insert

Ali

**Thomas J. Gritzan** · Jul 11 '08, 06:55 PM

Re: Call by value vs. Call by reference

Saeed Amrollahi schrieb:

Dear All
Hi
>
I have learned when an object is big, I should pass the object using a
reference or pointer to it rather than calling by value.
In the following code, I don't gain cosiderable performance with call-
by-reference vs. call-by-value. Indeed, the perforamnce is absolutely
unconsiderable:
>

[...]

void f1(std::vector< intv)

[...]

void f2(std::vector< intv)

[...]

void f3(std::vector< int>* pv)

[...]

There is no reference. You have two function that do a call-by-value and
one that does a call-by-reference (via pointer).

Obviously, the first two shouldn't show any measurable performance
difference.

--
Thomas

**Saeed Amrollahi** · Jul 11 '08, 07:35 PM

Re: Call by value vs. Call by reference

On Jul 11, 9:54 pm, "Thomas J. Gritzan" <phygon_antis.. .@gmx.de>
wrote:

Saeed Amrollahi schrieb:Dear All

Hi

>

I have learned when an object is big, I should pass the object using a
reference or pointer to it rather than calling by value.
In the following code, I don't gain cosiderable performance with call-
by-reference vs. call-by-value. Indeed, the perforamnce is absolutely
unconsiderable:

>
[...]

void f1(std::vector< intv)

[...]

void f2(std::vector< intv)

[...]

void f3(std::vector< int>* pv)

>
[...]
>
There is no reference. You have two function that do a call-by-value and
one that does a call-by-reference (via pointer).
>
Obviously, the first two shouldn't show any measurable performance
difference.
>
--
Thomas

Hi
Sorry for typo and confusion. I actually used reference in the code:
void f1(std::vector< intv)
void f2(std::vector< int>& v)
void f3(std::vector< int>* pv)

Based on

**KalleGuld** · Jul 15 '08, 09:15 PM

Re: Call by value vs. Call by reference

>
cout << "f1 called" << '\n';
for (int i = 0; i < 10; i++) {
vector<intv;
f1(v);
}
cout << "f1 end" << '\n';
v.clear();
cout << "f2 called" << '\n';
for (int i = 0; i < 10; i++) {
vector<intv;
f2(v);
}
cout << "f2 end" << '\n';
v.clear();
cout << "f3 called" << '\n';
for (int i = 0; i < 10; i++) {
vector<intv;
f3(&v);
}

The problem with your benchmark is that most of the work is done
within the functions themselves.

I have learned when an object is big, I should pass the object using a
reference or pointer to it rather than calling by value.

That's true, but you must understand why. The reason is that when
calling by value, you make a copy of the object, and copying that
object takes time.
But in your example, you just pass an empty vector, and that's quite
quick to copy.
Furthermore, it's only _starting_ the function that suffers when
calling by value, not the work done _inside_ it.

So, to make your benchmark work better, strip the body of your
functions and call your functions more times.
The code below should show the difference quite clearly (part 1 took
about 3 minutes, and part 2 was under a second on my system).

//speedtest.cpp
#include <vector>

#include <iostream>
using namespace std;

double f1(vector<doubl ein) {
return in[0];
}
double f2(vector<doubl e&in) {
return in[1];
}
int main() {

vector<doubleve c(1000000); //big vector
vec[0]=1.2;
vec[1]=3.4;
double db;
int bigNum = 10000;
cout << "start f1:\n" << endl; //remember to flush...
for (int i=0;i< bigNum ;i++) {
db = f1(vec);
}
cout << "start f2:\n" << endl;
for (int i=0;i< bigNum ;i++) {
db = f2(vec);
}
cout << "end" << endl;

return 0;
}

Call by value vs. Call by reference

Call by value vs. Call by reference

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