c_str like function implementation

This is a discussion on c_str like function implementation within the C++ Programming forums, part of the General Programming Boards category; If foo is a string-like class what additional modifications should c_str possibly perform? Adding a null terminator if the internal ...

  1. #16
    C++ Witch laserlight's Avatar
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    If foo is a string-like class what additional modifications should c_str possibly perform?
    Adding a null terminator if the internal string is not null terminated. I note that this is actually an option for implementations of std::basic_string. For example, the data() member function returns a pointer to the first character of an array of characters that is not guaranteed to be null terminated.
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  2. #17
    The larch
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    Quote Originally Posted by laserlight View Post
    Adding a null terminator if the internal string is not null terminated. I note that this is actually an option for implementations of std::basic_string. For example, the data() member function returns a pointer to the first character of an array of characters that is not guaranteed to be null terminated.
    Yes, but what would be a good reason not to keep the internal data null-terminated (even if it doesn't use the terminator for any internal purpose and allows null-characters within the string)?
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
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  3. #18
    C++ Witch laserlight's Avatar
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    [quoteYes, but what would be a good reason not to keep the internal data null-terminated (even if it doesn't use the terminator for any internal purpose and allows null-characters within the string)?[/quote]
    Saving space is the only reason I can come up with.
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  4. #19
    The larch
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    Saving space is the only reason I can come up with.
    Perhaps it has something to do with basic_string being a template (although I can't see why I'd make a string of something that is not a character-like)?

    Anyway, how can you save space (1 byte), if you probably need to add a whole char pointer to the class to be able to implement c_str()?
    I might be wrong.

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  5. #20
    CSharpener vart's Avatar
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    Quote Originally Posted by anon View Post
    Perhaps it has something to do with basic_string being a template (although I can't see why I'd make a string of something that is not a character-like)?

    Anyway, how can you save space (1 byte), if you probably need to add a whole char pointer to the class to be able to implement c_str()?
    You may want to store the string as a chain of buffers to avoid unneded reallocation on string growth... and only in the c_str() function reallocate the whole contents into one continues buffer and return pointer to the first char
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  6. #21
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    Quote Originally Posted by laserlight View Post
    By the way, built-in types do not have constructors.
    Code:
    #include <iostream>
    
    int main()
    {
        int i(10);  // default constructor, right?
    
        return 0;
    }

  7. #22
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    >> // default constructor, right?
    Actually, no. The default constructor for a class takes no arguments. Besides, that code works like a copy constructor, but it is still not considered a constructor call.

  8. #23
    (?<!re)tired Mario F.'s Avatar
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    Quote Originally Posted by robwhit View Post
    int i(10); // default constructor, right?
    It's called direct-initialization. As opposed to copy-initialization which would be int i = 10;

    The distinction becomes apparent with objects of a class type in which copy-initialization involves the construction of a temporary.

    Code:
    std::string str1 = "Hello"; // temporary is created
    std::string str2("Hello"); // direct initialization through constructor taking const char*
    For built-in types the notations are kept, although I think is treated in no different way by the compiler. Defintely, no constructors are involved
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  9. #24
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    If foo is a string-like class what additional modifications should c_str possibly perform?
    I'm not saying that it should, I yust added that so that if there would be a possibility of creating a pointer to a string + some aditional characters without malloc, return it without the bad pointer problem and then using it as another function parameter, someone would point it out.

    Btw while researching this I read somewhere that this is OK ( unfortunately I don't remember wher this was posted ):

    Code:
    const char* s = "hello";
    Howcome that is permited while this can create problems with the pointer:

    Code:
    const char* c_str()
    {
       char* s = i_string;  // char* i_string;
       s[ i_size ] = '\0';
       return s;
    }
    Last edited by DoMeN; 02-13-2008 at 01:19 PM.

  10. #25
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    Ah. Thanks.

  11. #26
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    "hello" is a string literal. It is constant and cannot be changed. Therefore, this second line of code is illegal:
    Code:
    const char* s = "hello";
    s[0] = 'b';
    Your second piece of code is also perfectly legal:
    Code:
    const char* c_str()
    {
       char* s = i_string;  // char* i_string;
       s[ i_size ] = '\0';
       return s;
    }
    Since your c_str() method is not a const function (there is no const at the end of the first line), you are allowed to modify the i_string member variable. Assigning it to a char* is allowed. You then modify the characters pointed to by s, which also modifies i_string, which is still allowed. Finally, you return the pointer s as a const char *, meaning whichever code saves the return value is not allowed to modify the characters.

    So I don't understand why you think that code "can create problems with the pointer". Can you elaborate?

  12. #27
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    hello" is a string literal. It is constant and cannot be changed. Therefore, this second line of code is illegal:

    const char* s = "hello";
    OK now I filaly get it. My stupid mistake that I thought that this is wrong because of the pointer to a constant string ( didn't know that that was legal).

    So I don't understand why you think that code "can create problems with the pointer". Can you elaborate?
    I once had some problems in one test case ( don't remember anymore what it was ). In all the other test cases it worked so perhaps I made a mistake somewhere else back then.

    I thouhth that the problem was caused because if you have a pointer of some length and then add aditional characters over the allocated size it could overwrite some other part of the memory that contains other data. Is that not so?
    Last edited by DoMeN; 02-13-2008 at 01:52 PM.

  13. #28
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    >> Is that not so?
    You are correct. My statement wasn't complete, I assumed that there was enough space. That code will cause problems if the allocated space for i_string is smaller than i_size+1.

    A common implementation for c_str() is to allocate at least size+1 characters for your character array. That way, data[size] = '\0' will be valid. This is what matsp was suggesting in the first response.

  14. #29
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    A common implementation for c_str() is to allocate at least size+1 characters for your character array. That way, data[size] = '\0' will be valid. This is what matsp was suggesting in the first response.
    I intend to keep the null character now that I know that I can't easily write it otherwise.
    Thanks to all for the explanations.

  15. #30
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    Did you mean you intend to keep room for the null character or that you intend to always add the null character?

    You can do it either way, there's nothing wrong with waiting until they call c_str() to add the null character if you always make sure there is enough space for it, and that isn't difficult either as long as you add one to whatever value you are allocating.

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