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From: James Pau on 17 Jul 2010 01:18
Hi all,

I'm having trouble displaying the value of string variables created in a c++ mex file. A sample situation is shown below:

[code]
#include "mex.h"
#include <iostream>
#include <string.h>

using namespace std;

void mexFunction(
int nlhs,
mxArray *[],
int nrhs,
const mxArray *prhs[]
)
{
string a;
a = "test";
mexPrintf("%s \n",a);
mexPrintf("%s \n","test");
}
[\code]

The output is:
r
test

What this means is that the string variable 'a' is not being read correctly by the mexPrintf function and I have no idea why. Any thoughts would be greatly appreciated, I'm completely stuck!

TIA,
James
From: Jan Simon on 17 Jul 2010 07:16
Dear James,

> string a;
> a = "test";
> mexPrintf("%s \n",a);
> mexPrintf("%s \n","test");
>
> The output is:
> r
> test

Not surprising. Look in the documentation of mexPrintf (or printf) and you find out, that the "%s" format displays character arrays and takes the pointer to the first character of the string. "string a" is not a character array, but a "string". Do you find a conversion from a "string" to a char vector?

Good luck, Jan
From: James Pau on 17 Jul 2010 08:45
"Jan Simon" <matlab.THIS_YEAR(a)nMINUSsimon.de> wrote in message <i1s3e0$e6n$1(a)fred.mathworks.com>...
> Dear James,
>
> > string a;
> > a = "test";
> > mexPrintf("%s \n",a);
> > mexPrintf("%s \n","test");
> >
> > The output is:
> > r
> > test
>
> Not surprising. Look in the documentation of mexPrintf (or printf) and you find out, that the "%s" format displays character arrays and takes the pointer to the first character of the string. "string a" is not a character array, but a "string". Do you find a conversion from a "string" to a char vector?
>
> Good luck, Jan

Hi Jan,

Thank you for your response. I did read the documentation for printf before and sort of assumed that a character array was equivalent to a string. My mistake, but I have managed to get it to work. For those who are interested the amended code is as follows:

string a;
char b[260];
a = "test";
strcpy(b,a.c_str());
mexPrintf("%s \n",b);

Output for b will correctly be "test'.

Thanks again!
James
From: Jan Simon on 17 Jul 2010 11:53
Dear James,

> strcpy(b,a.c_str());
> mexPrintf("%s \n",b);
> Output for b will correctly be "test'.

Thanks. This is the way a newsgroup takes advantage from the solved questions of users. You are my personal poster of the week!

Jan
From: Praetorian on 17 Jul 2010 19:04
On Jul 17, 6:45 am, "James Pau" <the_manti...(a)hotmail.com> wrote:
> "Jan Simon" <matlab.THIS_Y...(a)nMINUSsimon.de> wrote in message <i1s3e0$e6....(a)fred.mathworks.com>...
> > Dear James,
>
> > >     string a;
> > >     a = "test";
> > >     mexPrintf("%s \n",a);
> > >     mexPrintf("%s \n","test");
>
> > > The output is:
> > > r
> > > test
>
> > Not surprising. Look in the documentation of mexPrintf (or printf) and you find out, that the "%s" format displays character arrays and takes the pointer to the first character of the string. "string a" is not a character array, but a "string". Do you find a conversion from a "string" to a char vector?
>
> > Good luck, Jan
>
> Hi Jan,
>
> Thank you for your response. I did read the documentation for printf before and sort of assumed that a character array was equivalent to a string. My mistake, but I have managed to get it to work. For those who are interested the amended code is as follows:
>
>     string a;
>     char b[260];
>     a = "test";
>     strcpy(b,a.c_str());
>     mexPrintf("%s \n",b);
>
> Output for b will correctly be "test'.
>
> Thanks again!
> James

James,
You shouldn't have to perform the copy to variable b either. The
string::c_str() function returns a const char * to the character array
so mexPrintf() should be able to print it. If mexPrintf() has a
problem with it being a const pointer a const_cast should be able to
fix that.

HTH,
Ashish.
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