from const char to char

This is a discussion on from const char to char within the C++ Programming forums, part of the General Programming Boards category; Hi All I'm writing a tool that uses C libraries. Which means that I have to convert string s into ...

  1. #1
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    from const char to char

    Hi All

    I'm writing a tool that uses C libraries.

    Which means that I have to convert strings into 'const char*'s

    So, this is how I think a string is converted into a char:
    Code:
    const char* c = x.c_str ;
    The problem I have now is that my C library functions need a char* not a const char*..

    Any suggestions what the best way is to go from a string to a char* ?

    Thanks a lot
    LuCa

  2. #2
    C++ Witch laserlight's Avatar
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    You should copy over the characters (anyway).
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    string to a char*
    strcpy() will make a copy that you can alter.

  4. #4
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    If the size of the string is variable, you will probably need a variable length character array. In that case, use a vector so that it will handle memory management for you. Then pass the vector to the code that expects a char*:
    Code:
    string s = "Some string.";
    vector<char> buf(s.begin(), s.end());
    buf += '\0';
    SomeCFunction(&buf[0], s.size()); // SomeCFunction takes a char* and an int size.
    That code isn't tested, but should be basically correct.

    (See 7stud's post below for corrections.)
    Last edited by Daved; 01-17-2007 at 11:21 AM.

  5. #5
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    Nice. I was trying to figure out how to handle the problem of variable lengths. Here's a tested version of Daved's solution:
    Code:
    void someCFunction(char* str)
    {
    	cout<<str<<endl;
    }
    
    int main ()
    {
    	string s = "Some string.";
    	vector<char> buf(s.begin(), s.end());
    	buf.push_back('\0');
    	
    	someCFunction(&buf[0]);
    
    	return 0;
    }

  6. #6
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    thanks a lot, this is all I need!!

    The vector seems perfect!

    LuCa

  7. #7
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    I guess this would be a better test:
    Code:
    void changeIt(char* str)
    {
    	str[0] = 'X';
    }
    
    int main ()
    {
    	string s = "Some string.";
    	vector<char> buf(s.begin(), s.end());
    	buf.push_back('\0');
    	
    	changeIt(&buf[0]);
    	cout<<&buf[0]<<endl;
    
    	return 0;
    }

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