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convert basic_string to int

Posted on 2001-08-15
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Last Modified: 2008-01-09
How do I convert a basic_string to an int?
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Question by:navarros
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10 Comments
 
LVL 86

Accepted Solution

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jkr earned 50 total points
ID: 6389357
Use

basic_str _str = "123";
int _i;

_i = atoi ( _str.c_str());

or

#include <stdlib.h>
char* pc;
_i = strtol ( str.c_str(), &pc, 10);
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LVL 86

Expert Comment

by:jkr
ID: 6389364
To elaborate:

'basic_str::c_str()' gives you access to the contents of the string as a NULL-terminated C-style string, so you can use the usual conversion functions like 'strtol()' or 'atoi()'
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Expert Comment

by:nietod
ID: 6389372
>> basic_str::c_str()' gives you access to the
>> contents of the string as a NULL-terminated
>> C-style string,
But this has to be done with great caution.  You should familarize yourself with the limitations of this function if you intend to use it.   The code jkr published is a safe use of the function, but you have to be careful if you use it in other ways.
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LVL 86

Expert Comment

by:jkr
ID: 6389389
>>but you have to be careful if you use it in other ways.

Absolutely - altering the contents of the 'const char*' (note the 'const'!) is an absolute no-no, even if a cast operation would allow you to remove the 'const' qualifier...
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Expert Comment

by:nietod
ID: 6389428
But also you have to be careful that array the pointer poitns to i not invalidated.  The pointer returned from c_str() points to an array of a (potentially) temporay nature.  The string object may delete ro alter this array whenever a non-cosntant member function of the string is called, this includes the destructor.  So for exanmple the code

string S = "ABC";
const char *Ptr = S.c_str();
S[0] = 'X'; // changes string, invalidates ptr.

cout << Ptr; // Error.  Ptr is invalid.
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Expert Comment

by:jkr
ID: 6389458
Well, now we're silghtly OT, so just the way I'd handle the 'potential temporary nature':

string S = "ABC";
const char *Ptr = strdup ( S.c_str());
S[0] = 'X'; // changes string, Ptr remains valid, but needs 'free()'.
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LVL 22

Expert Comment

by:nietod
ID: 6389512
I agree its off topic, I just didn't "like" the way you explained c_str() briefly and didn't mention its dangers.  Its too tempting to ose it for toher things and then run into problems.
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LVL 4

Expert Comment

by:AssafLavie
ID: 6391904
template<class T, class E>
T StringTo(const basic_string<E>& source)
{
     basic_istringstream<E> iss(source);
     T ret;
     iss >> ret;
     return ret;
};

int main(int, char*[])
{
     int i = StringTo<int>(wstring(L"123"));
     i = StringTo<int>(string("456"));
     return 0;
}
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LVL 1

Expert Comment

by:mail2yogi1
ID: 6392079
try this
#include <iostream.h>
#include <string.h>

void main()
{
char *str = "1234";
int a1;
a1=atoi(str);
cout<<a1<<endl;
}
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LVL 4

Expert Comment

by:AssafLavie
ID: 6421648
navaross, you got several solutions here as comments. Please accept one of them and close this question.
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