How to convert string into char * array

Question

I have changed my code, now while the compilation these errors occur:

`check.cpp: In function ‘int main()’:`

check.cpp:14:55: error: invalid conversion from ‘const char**’ to ‘char* const*’ [-fpermissive]

/usr/include/getopt.h:152:12: error: initializing argument 2 of ‘int getopt(int, char* const*, const char*)’ [-fpermissive]

int main() {

string text="-f  input.gmn -output.jpg";
int argc=text.length();
cout<<"argc: "<<argc<<endl;
char const * argv = text.c_str();
cout<<"argv: "<<argv<<endl;
int c = getopt (argc, &argv, "f:s:o:pw:h:z:t:d:a:b:?");
return 0;
}

Read this page: cplusplus.com/reference/string/string/c_str

besworland
– besworland

2012-05-08 12:43:05 +00:00
Commented May 8, 2012 at 12:43 — besworland
– besworland, Commented May 8, 2012 at 12:43

Community · Accepted Answer · 2017-05-23 12:20:17Z

5

You can use text.c_str() to convert a std::string into a const char*. See here.

To elaborate on my answer, there are many ways to create the array you need, but this is already described here, here, here and here. A simple solution to your problem that does not involve new/malloc or intensive uses of the STL and istringstream/back_inserter/copy what not and performs really fast could look like this:

/* variables. */
std::vector< char* > argv;
int i, argc, state;
char c;

/* convert string to char string with automatic garbage collection. */
std::vector< char > tokens(text.begin(), text.end());
tokens.push_back(0);

/* tokenize string with space. */
for (state=0, argc=0, i=0; (c=tokens[i]); i++) {
    if (state) {
        if (c == ' ') {
            tokens[i]=0;    
            state=0;        
        }           
    } else {
        if (c != ' ') {
            argv.push_back(&tokens[i]);
            argc++;         
            state=1;        
        }           
    }       
}   

/* print argv. */
std::cout << "argc: " << argc << std::endl;
for (i=0; i < argc; i++) {
    std::cout << "argv[" << i << "]: " << argv[i] << std::endl;
}   

/* call getopt. */
c = getopt(argc, &argv[0], "f:s:o:pw:h:z:t:d:a:b:?");

This is just an example, but one advantage of this kind of code is that you can use other characters as delimiter, not just space, and that you need not care about releasing the allocated memory since std::vector does this for you on function exit.

edited May 23, 2017 at 12:20

CommunityBot

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answered May 8, 2012 at 12:38

hochl

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2 Comments

John Dibling Over a year ago

A const char* is not what is needed. OP needs a const char*[]. Multiple strings.

hochl Over a year ago

I added working code that solves the problem. Might be more elegant but covers all corner cases I could think up.

Tom Tanner · Accepted Answer · 2012-05-08 12:43:31Z

4

In short, you have an array argv which contains 100 pointers to strings, of which only the first is set. argv[1] hasn't been set to anything, so is pointing somewhere random. And in this case, illegal.

Moreoever, what getoption expects is going to be more like this:

argv[0] = "progname";
argv[1] = "-f";
argv[2] = "input.gmn"
argv[3] = "-output.jpg"
argv[4] = 0

Note the =0 at the end to stop getoption chargins through random bits of memory

answered May 8, 2012 at 12:43

Tom Tanner

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Comments

Sarfaraz Nawaz · Accepted Answer · 2012-05-08 12:45:04Z

4

First bug with your code is the comparison:

for (int i=0; i<=stringLength; i++) {
   arv[i]=text[i]; 
}

Use i< stringLength instead of i<=stringLength.

The second bug is that arv is not null-terminated.

After fixing both bugs, your code should look like this:

for (int i=0; i < stringLength; i++) {
   arv[i]=text[i]; 
}
arv[stringLength] = '\0';

By the way, the correct function signature of getopt is this:

int getopt(int argc, char * const argv[], const char *optstring);

which takes second and third argument as const. That means, you can do this:

 char const * s = text.c_str();
 int c = getopt (argc, &s, "f:s:o:pw:h:z:t:d:a:b:?");

No need of any conversion, using manual loop.

edited May 8, 2012 at 12:45

answered May 8, 2012 at 12:40

Sarfaraz Nawaz

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