writing an external function in std c++

**Kai-Uwe Bux** · Feb 18 '08, 06:25 AM

Re: writing an external function in std c++

Gerry Ford wrote:

I've been pecking away at writing a program that will calculate the inner
product of two double-width four-vectors.
>
Larry thinks I'm well started with the following source to populate a
vector:
#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <math.h>
>
int main() {
std::vector<dou blefour_vector;
>
for (double i=0.0; i<4.0; i++)
four_vector.pus h_back(sqrt(i)) ;
>
std::cout.preci sion(16);
>
std::copy(four_ vector.begin(), four_vector.end (),
std::ostream_it erator<double>( std::cout, "\n"));
return 0;
}
>
How do I imitate the following fortran source:
program vector2
implicit none
integer index, i
integer, parameter :: some_kind_numbe r = selected_real_k ind (p=16)
real (kind = some_kind_numbe r), dimension(4):: vec_a, vec_b
real (kind = some_kind_numbe r) :: res
>
>
index = 4
>
do i = 1, index
vec_a(i)= i**.5
>
vec_b(i)= (-1)*(i**2)
>
end do
>
res = dot_product(vec _a, vec_b)
>
>
write (*,*) vec_a, vec_b
write (*,*) res
>
>
end program vector2
>
! gfortran2 -o vector2 vector2.f95
! vector2 >text55.txt 2>text56.txt
//end source continue comment
, except making the inner product calculated externally. I have zero
chance
of getting it correct, so I'll spare you the flailing attempt. Screenshot
here: http://zaxfuuq.net/c++5.jpg
>
To imitate it, I believe the appropriate c++ inner product would be around
negative 25.
>

What about using

std::inner_prod uct( vec_a.begin(), vec_a.end(), vec_b.begin(), double() );

from the <numericheade r?

Best

Kai-Uwe Bux

**Jim Langston** · Feb 18 '08, 07:35 AM

Re: writing an external function in std c++

Gerry Ford wrote:

I've been pecking away at writing a program that will calculate the
inner product of two double-width four-vectors.
>
Larry thinks I'm well started with the following source to populate a
vector:
#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <math.h>
>
int main() {
std::vector<dou blefour_vector;
>
for (double i=0.0; i<4.0; i++)
four_vector.pus h_back(sqrt(i)) ;
>
std::cout.preci sion(16);
>
std::copy(four_ vector.begin(), four_vector.end (),
std::ostream_it erator<double>( std::cout, "\n"));
return 0;
}
>
How do I imitate the following fortran source:
program vector2
implicit none
integer index, i
integer, parameter :: some_kind_numbe r = selected_real_k ind (p=16)
real (kind = some_kind_numbe r), dimension(4):: vec_a, vec_b
real (kind = some_kind_numbe r) :: res
>
>
index = 4
>
do i = 1, index
vec_a(i)= i**.5
>
vec_b(i)= (-1)*(i**2)
>
end do
>
res = dot_product(vec _a, vec_b)
>
>
write (*,*) vec_a, vec_b
write (*,*) res
>
>
end program vector2
>
! gfortran2 -o vector2 vector2.f95
! vector2 >text55.txt 2>text56.txt
//end source continue comment
, except making the inner product calculated externally. I have zero
chance of getting it correct, so I'll spare you the flailing attempt.
Screenshot here: http://zaxfuuq.net/c++5.jpg
>
To imitate it, I believe the appropriate c++ inner product would be
around negative 25.

You are not actually showing the fortran function that calculates the dot
product. That fortran source code simply populates 2 arrays of 4 elements
then calls the function dot_product(). It is the function dot_product you
want to code, but you aren't showing the souce for that.

In fortran ** is the power symbol, in C and C++ you can use the pow
function. Although some number raised to the power of 2 it's just as simple
to multiply the number by itself. Raising a number to the power of .5 is
taking the square root of it. So basically all that fortran code is filling
one array of 4 elements with the square roots of 1 to 4, the second 4
element array with the squares of 1 to 4, then calling the function
dot_product on them which returns a single number.

--
Jim Langston
tazmaster@rocke tmail.com

**Gerry Ford** · Feb 18 '08, 11:05 AM

Re: writing an external function in std c++

"Jim Langston" <tazmaster@rock etmail.comwrote in message
news:Jwauj.420$ Pn5.79@newsfe05 .lga...

Gerry Ford wrote:

>To imitate it, I believe the appropriate c++ inner product would be
>around negative 25.

>
You are not actually showing the fortran function that calculates the dot
product. That fortran source code simply populates 2 arrays of 4 elements
then calls the function dot_product(). It is the function dot_product you
want to code, but you aren't showing the souce for that.
>
In fortran ** is the power symbol, in C and C++ you can use the pow
function. Although some number raised to the power of 2 it's just as
simple to multiply the number by itself. Raising a number to the power of
.5 is taking the square root of it. So basically all that fortran code is
filling one array of 4 elements with the square roots of 1 to 4, the
second 4 element array with the squares of 1 to 4, then calling the
function dot_product on them which returns a single number.

You're prettymuch right on all this. I appreciate you talking through this
source for the benefit of those less familiar with my common C extension of
choice.

There isn't fortran code for dot_product, as it comes with the food over
there. That shocked the heck out of me the first time I realized it. I was
given to understand that c++ had likewise, but not to worry. With what we
have for headers, we can write it from scratch.

double float c++_dot_product (array vec_a , array vec_b, integer index)
{
// declare local vars
res = 0;
term=0;
sum=0;
for (i = 0; i < index; ++ i)
{
term=vec_a(i)*v _b(i);
sum = sum + term;

}
res = powl(sum, .5);

return res;
}

How does one correctly code the caller for this and the external function
itself?

--
Gerry Ford

"Er hat sich georgiert." Der Spiegel, 2008, sich auf Chimpy Eins komma null
beziehend.

**Jim Langston** · Feb 18 '08, 11:55 AM

Re: writing an external function in std c++

Gerry Ford wrote:

"Jim Langston" <tazmaster@rock etmail.comwrote in message
news:Jwauj.420$ Pn5.79@newsfe05 .lga...

>Gerry Ford wrote:

>

>>To imitate it, I believe the appropriate c++ inner product would be
>>around negative 25.

>>
>You are not actually showing the fortran function that calculates
>the dot product. That fortran source code simply populates 2 arrays
>of 4 elements then calls the function dot_product(). It is the
>function dot_product you want to code, but you aren't showing the
>souce for that. In fortran ** is the power symbol, in C and C++ you can
>use the pow
>function. Although some number raised to the power of 2 it's just as
>simple to multiply the number by itself. Raising a number to the
>power of .5 is taking the square root of it. So basically all that
>fortran code is filling one array of 4 elements with the square
>roots of 1 to 4, the second 4 element array with the squares of 1 to
>4, then calling the function dot_product on them which returns a
>single number.

You're prettymuch right on all this. I appreciate you talking
through this source for the benefit of those less familiar with my
common C extension of choice.
>
There isn't fortran code for dot_product, as it comes with the food
over there. That shocked the heck out of me the first time I
realized it. I was given to understand that c++ had likewise, but
not to worry. With what we have for headers, we can write it from
scratch.
double float c++_dot_product (array vec_a , array vec_b, integer
index) {
// declare local vars
res = 0;
term=0;
sum=0;
for (i = 0; i < index; ++ i)
{
term=vec_a(i)*v _b(i);
sum = sum + term;
>
}
res = powl(sum, .5);
>
return res;
}
>
How does one correctly code the caller for this and the external
function itself?

Output of the follow program is:
0
1
1.4142135623730 95
1.7320508075688 77
0
1
4
9
Dot Product: 4.7164935617058 02

#include <cmath>
#include <vector>
#include <iostream>

double dot_product (const std::vector<dou ble>& vec_a, const
std::vector<dou ble>& vec_b)
{
if ( vec_a.size() != vec_b.size() )
{
std::cerr << "Vectors for dot product are not same size!\n";
return 0.0;
}

double sum = 0;
for ( std::size_t i = 0; i < vec_a.size(); ++i )
{
sum += vec_a[i] * vec_b[i];
}
return std::pow(sum, .5);
}

// Following prototype is not needed in this program since
// the definition is above, but would be used in another
// source file without the previous definition.
double dot_product (const std::vector<dou ble>& vec_a, const
std::vector<dou ble>& vec_b);

int main()
{
std::vector<dou blevec_a;
std::vector<dou blevec_b;

for ( int i = 0; i < 4; ++i )
{
vec_a.push_back ( std::sqrt( static_cast<dou ble>( i )) );
vec_b.push_back ( i * i );
}

std::cout.preci sion(16);

std::copy(vec_a .begin(), vec_a.end(),
std::ostream_it erator<double>( std::cout, "\n"));
std::copy(vec_b .begin(), vec_b.end(),
std::ostream_it erator<double>( std::cout, "\n"));

std::cout << "Dot Product: " << dot_product(vec _a, vec_b) << "\n";
return 0;
}

An alternative to the static_cast<dou ble>( i ) is chaning the for loop with
a double.

for ( double i = 0; i < 4.0; i += 1.0 )
{
vec_a.push_back ( std::sqrt( i ) );
vec_b.push_back ( i * i );
}

--
Jim Langston
tazmaster@rocke tmail.com

**Michael DOUBEZ** · Feb 18 '08, 12:55 PM

Re: writing an external function in std c++

Jim Langston a écrit :

Gerry Ford wrote:

>"Jim Langston" <tazmaster@rock etmail.comwrote in message
>news:Jwauj.420 $Pn5.79@newsfe0 5.lga...

>>Gerry Ford wrote:
>>>To imitate it, I believe the appropriate c++ inner product would be
>>>around negative 25.
>>You are not actually showing the fortran function that calculates
>>the dot product. That fortran source code simply populates 2 arrays
>>of 4 elements then calls the function dot_product(). It is the
>>function dot_product you want to code, but you aren't showing the
>>souce for that. In fortran ** is the power symbol, in C and C++ you can
>>use the pow
>>function. Although some number raised to the power of 2 it's just as
>>simple to multiply the number by itself. Raising a number to the
>>power of .5 is taking the square root of it. So basically all that
>>fortran code is filling one array of 4 elements with the square
>>roots of 1 to 4, the second 4 element array with the squares of 1 to
>>4, then calling the function dot_product on them which returns a
>>single number.

>You're prettymuch right on all this. I appreciate you talking
>through this source for the benefit of those less familiar with my
>common C extension of choice.
>>
>There isn't fortran code for dot_product, as it comes with the food
>over there. That shocked the heck out of me the first time I
>realized it. I was given to understand that c++ had likewise, but
>not to worry. With what we have for headers, we can write it from
>scratch.
>double float c++_dot_product (array vec_a , array vec_b, integer
>index) {
>// declare local vars
>res = 0;
>term=0;
>sum=0;
>for (i = 0; i < index; ++ i)
>{
>term=vec_a(i)* v_b(i);
>sum = sum + term;
>>
>}
>res = powl(sum, .5);
>>
>return res;
>}
>>
>How does one correctly code the caller for this and the external
>function itself?

>
Output of the follow program is:
0
1
1.4142135623730 95
1.7320508075688 77
0
1
4
9
Dot Product: 4.7164935617058 02
>
#include <cmath>
#include <vector>
#include <iostream>
>
double dot_product (const std::vector<dou ble>& vec_a, const
std::vector<dou ble>& vec_b)
{
if ( vec_a.size() != vec_b.size() )
{
std::cerr << "Vectors for dot product are not same size!\n";
return 0.0;
}
>
double sum = 0;
for ( std::size_t i = 0; i < vec_a.size(); ++i )
{
sum += vec_a[i] * vec_b[i];
}
return std::pow(sum, .5);
}
>
// Following prototype is not needed in this program since
// the definition is above, but would be used in another
// source file without the previous definition.
double dot_product (const std::vector<dou ble>& vec_a, const
std::vector<dou ble>& vec_b);
>
int main()
{
std::vector<dou blevec_a;
std::vector<dou blevec_b;
>
for ( int i = 0; i < 4; ++i )
{
vec_a.push_back ( std::sqrt( static_cast<dou ble>( i )) );
vec_b.push_back ( i * i );
}
>
std::cout.preci sion(16);
>
std::copy(vec_a .begin(), vec_a.end(),
std::ostream_it erator<double>( std::cout, "\n"));
std::copy(vec_b .begin(), vec_b.end(),
std::ostream_it erator<double>( std::cout, "\n"));
>
std::cout << "Dot Product: " << dot_product(vec _a, vec_b) << "\n";
return 0;
}
>
An alternative to the static_cast<dou ble>( i ) is chaning the for loop with
a double.
>
for ( double i = 0; i < 4.0; i += 1.0 )
{
vec_a.push_back ( std::sqrt( i ) );
vec_b.push_back ( i * i );
}
>

In fact, in C++, the only version of std::sqrt() is double sqrt(double);
the other variants with float and long double are C99 additions. In the
standard case, the integer should be promoted to double and the
static_cast<dou ble>() is not necessary.
for ( int i = 0; i < 4; ++i )
{
vec_a.push_back ( std::sqrt( i ) );
vec_b.push_back ( i * i );
}

I guess that in the case of C99 extension, an integer in parameter is
forwarded to the double flavor otherwise, there would be an ambiguity.

Michael

**James Kanze** · Feb 18 '08, 05:55 PM

Re: writing an external function in std c++

On Feb 18, 1:49 pm, Michael DOUBEZ <michael.dou... @free.frwrote:

[...]

In fact, in C++, the only version of std::sqrt() is double
sqrt(double); the other variants with float and long double
are C99 additions.

Overloads for float and long have been present in C++ at least
since the 1998 version of the standard.

In the standard case, the integer should be promoted to double
and the static_cast<dou ble>() is not necessary.

for ( int i = 0; i < 4; ++i )
{
vec_a.push_back ( std::sqrt( i ) );
vec_b.push_back ( i * i );
}

I guess that in the case of C99 extension, an integer in
parameter is forwarded to the double flavor otherwise, there
would be an ambiguity.

Why? I think that there will be an ambiguity if std::sqrt() is
called with an int. That's what the standard (and Sun CC) say,
anyway.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

**peter koch** · Feb 18 '08, 07:15 PM

Re: writing an external function in std c++

On 18 Feb., 06:41, "Gerry Ford" <inva...@invali d.netwrote:

I've been pecking away at writing a program that will calculate the inner
product of two double-width four-vectors.

Why? There are libraries that do this better than you or I could
reasonably hope to do unless we really took the time (and perhaps even
then! ;-)).
I am quite sure of that even if I don't really know what double-width
and four-vector is. These are neither terms of C++ nor of mathematics.

>
Larry thinks I'm well started with the following source to populate a
vector:
#include <vector>
#include <algorithm>
#include <iterator>
#include <iostream>
#include <math.h>
>
int main() {
std::vector<dou blefour_vector;
>
for (double i=0.0; i<4.0; i++)
four_vector.pus h_back(sqrt(i)) ;

This loop is really dangerous. You risk having five elements in your
vector. Also it is inefficient as you (presumably!) know that there
should be four elements in the vector.
Use reserve or prefer a fixed-size vector which imo is the best
solution provided that you will always have a fixed number of elements
in your vector.

>
std::cout.preci sion(16);
>
std::copy(four_ vector.begin(), four_vector.end (),
std::ostream_it erator<double>( std::cout, "\n"));
return 0;
>
}
>
How do I imitate the following fortran source:
program vector2
implicit none
integer index, i
integer, parameter :: some_kind_numbe r = selected_real_k ind (p=16)
real (kind = some_kind_numbe r), dimension(4):: vec_a, vec_b
real (kind = some_kind_numbe r) :: res
>
index = 4
>
do i = 1, index
vec_a(i)= i**.5
>
vec_b(i)= (-1)*(i**2)

this would be something like

for (size_t i(0); i < 4; ++i)
{
a[i] = sqrt(i);
b[i] = -i*i;
}

>
end do
>
res = dot_product(vec _a, vec_b)
>
write (*,*) vec_a, vec_b
write (*,*) res

I can't imagine you having problems with the dot-product and I do not
understand the fortran output formats.

>
end program vector2
>

/Peter

**Gerry Ford** · Feb 19 '08, 03:55 AM

Re: writing an external function in std c++

"Jim Langston" <tazmaster@rock etmail.comwrote in message
news:_eeuj.3$qv 2.1@newsfe02.lg a...

Gerry Ford wrote:

Thanks, Jim. In particular I appreciate the absence of disparaging remarks
concerning the god-awful syntax on my first function in c++ in a decade.

std::copy(vec_a .begin(), vec_a.end(),
std::ostream_it erator<double>( std::cout, "\n"));
std::copy(vec_b .begin(), vec_b.end(),
std::ostream_it erator<double>( std::cout, "\n"));

I've got 2 compilers that object to the above, to wit:
42 C:\Program Files\gfortran\ fortran source\vector3. cpp `ostream_iterat or'
is not a member of `std'
,with dev cpp and
vector3.cpp:52: 12: warning: no newline at end of file
vector3.cpp: In function 'int main()':
vector3.cpp:42: error: 'ostream_iterat or' is not a member of 'std'
vector3.cpp:42: error: expected primary-expression before 'double'
vector3.cpp:44: error: 'ostream_iterat or' is not a member of 'std'
vector3.cpp:44: error: expected primary-expression before 'double'
with g++.

What gives?
--
Gerry Ford

"Er hat sich georgiert." Der Spiegel, 2008, sich auf Chimpy Eins komma null
beziehend.

**Gerry Ford** · Feb 19 '08, 04:05 AM

Re: writing an external function in std c++

"peter koch" <peter.koch.lar sen@gmail.comwr ote in message
news:bcf7a69f-fe17-42ac-bc8a-11da4622cb75@s1 3g2000prd.googl egroups.com...
On 18 Feb., 06:41, "Gerry Ford" <inva...@invali d.netwrote:

I've been pecking away at writing a program that will calculate the inner
product of two double-width four-vectors.

Why? There are libraries that do this better than you or I could
reasonably hope to do unless we really took the time (and perhaps even
then! ;-)).
I am quite sure of that even if I don't really know what double-width
and four-vector is. These are neither terms of C++ nor of mathematics.

--->Do you know how to get these libraries hooked into a garden-variety
console app?

As to the rest, you sound german, so I'll address you so. Ein Vierervektor
soll etwa nicht in der Mathematik sein? Sie ercheinen im ersten Paragraph
in Einstein 1916 Mathematische Hilfsmittel fuer die Aufstellung allgemein
kovarianter Gleichungen.

Double width is twice the width of a float.

--
Gerry Ford

"Er hat sich georgiert." Der Spiegel, 2008, sich auf Chimpy Eins komma null
beziehend.

**Michael DOUBEZ** · Feb 19 '08, 08:55 AM

Re: writing an external function in std c++

James Kanze a écrit :

On Feb 18, 1:49 pm, Michael DOUBEZ <michael.dou... @free.frwrote:
>
[...]

>In fact, in C++, the only version of std::sqrt() is double
>sqrt(double) ; the other variants with float and long double
>are C99 additions.

>
Overloads for float and long have been present in C++ at least
since the 1998 version of the standard.

Funny thing. I really believed it was an extension.

>In the standard case, the integer should be promoted to double
>and the static_cast<dou ble>() is not necessary.

>

>for ( int i = 0; i < 4; ++i )
>{
> vec_a.push_back ( std::sqrt( i ) );
> vec_b.push_back ( i * i );
>}

>

>I guess that in the case of C99 extension, an integer in
>parameter is forwarded to the double flavor otherwise, there
>would be an ambiguity.

>
Why? I think that there will be an ambiguity if std::sqrt() is
called with an int. That's what the standard (and Sun CC) say,
anyway.

I have found the 26.5-5 overload of cmath functions with float and long
double but where does the standard say it must trigger an ambiguity ?
Although it seems logical, wouldn't that break old C code making the
distinction between sqrt and sqrtf ?
I guess it is not that much a change.

Michael

**James Kanze** · Feb 19 '08, 11:05 AM

Re: writing an external function in std c++

Michael DOUBEZ wrote:

James Kanze a ?crit :

On Feb 18, 1:49 pm, Michael DOUBEZ <michael.dou... @free.frwrote:

[...]

In fact, in C++, the only version of std::sqrt() is double
sqrt(double); the other variants with float and long double
are C99 additions.

Overloads for float and long have been present in C++ at least
since the 1998 version of the standard.

Funny thing. I really believed it was an extension.

In the standard case, the integer should be promoted to double
and the static_cast<dou ble>() is not necessary.

for ( int i = 0; i < 4; ++i )
{
vec_a.push_back ( std::sqrt( i ) );
vec_b.push_back ( i * i );
}

I guess that in the case of C99 extension, an integer in
parameter is forwarded to the double flavor otherwise, there
would be an ambiguity.

Why? I think that there will be an ambiguity if std::sqrt() is
called with an int. That's what the standard (and Sun CC) say,
anyway.

I have found the 26.5-5 overload of cmath functions with float
and long double but where does the standard say it must
trigger an ambiguity ?

Where does it say which one should be called?:-)

Basically, in §13.3.3.1.1, all of the standard conversions are
ranked, and int->float, int->double and int->long double all
have the same rank (conversion). Even in cases where int->float
is lossy.

Although it seems logical, wouldn't that break old C code
making the distinction between sqrt and sqrtf ?

Yep. There's no such thing as an ambiguous function call in C:
"sqrt( i )" calls "sqrt( double )", because that's the only
function named sqrt.

Ideally, only std::sqrt() is overloaded; and ::sqrt() continues
to behave as in C. (I'm not too sure what the standard
requires/allows in this respect.)

Note too that there is no std::sqrtf.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

**Thomas J. Gritzan** · Feb 19 '08, 01:15 PM

Re: writing an external function in std c++

Gerry Ford schrieb:

Thanks, Jim. In particular I appreciate the absence of disparaging remarks
concerning the god-awful syntax on my first function in c++ in a decade.
>

> std::copy(vec_a .begin(), vec_a.end(),
> std::ostream_it erator<double>( std::cout, "\n"));
> std::copy(vec_b .begin(), vec_b.end(),
> std::ostream_it erator<double>( std::cout, "\n"));

>
I've got 2 compilers that object to the above, to wit:
42 C:\Program Files\gfortran\ fortran source\vector3. cpp `ostream_iterat or'
is not a member of `std'
,with dev cpp and
vector3.cpp:52: 12: warning: no newline at end of file
vector3.cpp: In function 'int main()':
vector3.cpp:42: error: 'ostream_iterat or' is not a member of 'std'
vector3.cpp:42: error: expected primary-expression before 'double'
vector3.cpp:44: error: 'ostream_iterat or' is not a member of 'std'
vector3.cpp:44: error: expected primary-expression before 'double'
with g++.
>
What gives?

#include <iterator>

--
Thomas

How do I quote correctly in Usenet?

http://www.netmeister.org/news/learn2quote.html

Sentences long extremely and notation Polish reverse in writing about

**Thomas J. Gritzan** · Feb 19 '08, 01:25 PM

Re: writing an external function in std c++

Gerry Ford schrieb:
[...]

There isn't fortran code for dot_product, as it comes with the food over
there. That shocked the heck out of me the first time I realized it. I was
given to understand that c++ had likewise, but not to worry. With what we
have for headers, we can write it from scratch.
>
double float c++_dot_product (array vec_a , array vec_b, integer index)
{
// declare local vars
res = 0;
term=0;
sum=0;
for (i = 0; i < index; ++ i)
{
term=vec_a(i)*v _b(i);
sum = sum + term;
>
}
res = powl(sum, .5);
>
return res;
}
>
How does one correctly code the caller for this and the external function
itself?

Two questions:
1) Why do you take the root from the result? A dot product is the sum of
the product of the elements.
2) Why don't you use std::inner_prod uct?

#include <numeric>

double res = std::inner_prod uct(vec1.begin( ), vec1.end(), vec2.begin(), 0.0);

--
Thomas

How do I quote correctly in Usenet?

http://www.netmeister.org/news/learn2quote.html

An ideal world is left as an excercise to the reader.
--- Paul Graham, On Lisp 8.1

**James Kanze** · Feb 19 '08, 09:35 PM

Re: writing an external function in std c++

On Feb 19, 7:28 pm, peter koch <peter.koch.lar ...@gmail.comwr ote:

On 19 Feb., 11:41, Richard Herring <junk@[127.0.0.1]wrote:

[...]

You really think it will help? The very first push_back will do
something internally equivalent to reserve(N) for some N which quite
likely exceeds 4.

Perhaps. I have not examined my implementation but would expect it to
have reserves of 1, 2 and four.

Why? (That's certainly not what my implementation would do, if
I were writing it.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
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writing an external function in std c++

writing an external function in std c++

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