Store c_str() as char *

Question

I'm trying to use the function with the following declaration: extern int stem(struct stemmer * z, char * b, int k)1

I'm trying to pass a C++ string to it, so I thought I'd use the c_str() function. It returns const char *. When I try to pass it to the stem() function, I get this error: error: invalid conversion from 'const char*' to 'char*' [-fpermissive].

How can I store the result of c_str() such that I can use it with the stem function?

Here is the code I'm running:

    struct stemmer * z = create_stemmer();
    char * b = s.c_str();
    int res = stem(z, b, s.length()); //this doesn't work
    free_stemmer(z);
    return s.substr(0,res);

Does `stem` modify the string? If so, why are you trying to pass `const char*` to it? If not, why did you declare the parameter `char *`? — David Heffernan, Nov 30 '13 at 21:09
`char * b = s.c_str();` this should fail, possibly a typo in snippet? ...and can you please add a few words about how your `stem()` function works? — Wolf, Jun 02 '14 at 13:09
Why should it fail? Here is the function header of c_str(): `const char* c_str() const noexcept;` — muttley91, Jun 02 '14 at 15:12

score 8 · Accepted Answer · answered Nov 30 '13 at 21:08

The problem you are having is that c_str() returns a buffer that can not be modified (const), while stem() may modify the buffer you pass in (not const). You should make a copy of the result of c_str() to get a modifiable buffer.

The page http://www.cplusplus.com/reference/string/string/c_str/ has more information on the C++ 98 and 11 versions. They suggest replacing char * b = s.c_str(); with the following:

char * b = new char [s.length()+1];
std::strcpy (b, s.c_str());

score 2 · Answer 2 · answered Nov 30 '13 at 21:12

2

You shouldn't try to remove constness of a string returned by c_str():

char * b = s.c_str();

but you can pass an address of std::string's internal buffer directly:

int res = stem(z, static_cast<char*>(&s[0]), s.length());

answered Nov 30 '13 at 21:12

LihO

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score 1 · Answer 3 · answered Nov 30 '13 at 21:11

1

If stem() is going to modify the string, then make a copy of it:

char * scpy= strdup( s.c_str()) ;
int res = stem(z, scpy, strlen( scpy)); 
free( scpy) ;

answered Nov 30 '13 at 21:11

woolstar

5,063
20
31

score 0 · Answer 4 · answered Nov 30 '13 at 21:08

Use const_cast:

int res = stem(z, const_cast<char*>(s.c_str()), s.length()+1);
free_stemmer(z);
return s.substr(0,res);

Note the length+1 expression which might (or might not) be needed. C-style strings (char*) have an additional null terminator (zero byte, equivalent "\0") at the end. Your stem function may (or may not) expect a null terminator at the end of the string - try both variants.

Note also that "stem" function should not try to modify the string, otherwise bad things may happen (warning based on @David Heffernan's comment)

if stem modifies the c string this is undefined behaviour and will potentially causes bugs or crashes the application — Sebastian Hoffmann, Nov 30 '13 at 21:10

score 0 · Answer 5 · answered Nov 30 '13 at 21:12

.c_str()

Just returns a pointer to the data, I would update the stem function to accept a 'const char*' unless you are wanting to modify the data in the string, in that case you should pass it as a new string object.

If you can't edit the stem function you can cast it:

int res = stem(z, const_cast<char*>(s.c_str()), s.length());

score -1 · Answer 6 · answered Nov 30 '13 at 21:10

-1

It's not good to do this, but nothing stops you:

#include <iostream>
#include <string>
using namespace std;

void foo(char *ch)
{
  ch[0] = 'B';
}

int main()
{
  string str = "helo world";

  char *ch = const_cast<char *>(str.c_str());

  foo(ch);

  // Belo world
  cout << str << endl;

  return 0;
}

answered Nov 30 '13 at 21:10

gongzhitaao

6,566
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@downvoter? please why at least? I know it's a bad idea, but at least this works if you sure you don't mess up with `char *ch`. – gongzhitaao Nov 30 '13 at 21:18
My guess: Because he thought what you're doing is a bad idea. – Nov 30 '13 at 21:55
Sure, but the very start of the comment stated that. – muttley91 Nov 30 '13 at 21:56
Please downvoter, some comments and suggestion are very much appreciated. Or just follow suit to donwvote? – gongzhitaao Nov 30 '13 at 22:10

Store c_str() as char *

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