Adding character arrays to strings.

This is a discussion on Adding character arrays to strings. within the C++ Programming forums, part of the General Programming Boards category; Hi, I'm writing a simple program to encrypt files ...and I want it to give it's output as enc_filename, or ...

  1. #1
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    Adding character arrays to strings.

    Hi, I'm writing a simple program to encrypt files ...and I want it to give it's output as enc_filename, or dec_filename when encrypting or decrypting, respectively. I get an error message:
    scrypt2.cpp:90: error: invalid operands of types ‘const char [5]’ and ‘char*’ to binary ‘operator+’

    for the line of code:
    Code:
    std::ofstream out("dec_" + argv[1]);
    any idea how I can get this to work?
    thanks.

    edit:: i also get that error for another line, where the only difference is that it has "enc_" instead of "dec_".....

  2. #2
    Registered User Tonto's Avatar
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    If you don't mind std::string's:

    Code:
    std::ofstream out((std::string("dec_") += argv[1]).c_str());

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    The hard way is to use strcat.

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Code:
    std::ofstream out("dec_" + string(argv[1]));
    Will that work?
    Last edited by dwks; 02-04-2006 at 02:33 PM.
    dwk

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    No, because the result will be an instance of string and the ofstream constructor takes a const char *. To fix it, do what Tonto did and call c_str() on the result of the +.

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    tonto's method works. but i have no clue what it does.... could you explain tonto?

    daved's would probably work, didn't try.
    dwk's doesn't work..

    hmm.
    also, a new problem has arisen:

    when it encrypts something it now appends enc_ to the output filename.
    when i tell it to decrypt that file, it appends dec - meaning it is now dec_enc_(filename).
    i don't want it to do that........... although i know why it does.

    any ideas on how to check if that's there and remove it from the..... umm... argv[2]?

  7. #7
    carry on JaWiB's Avatar
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    Instead of prepending dec_/enc_, why not store the file extension inside the file and encrypt it along with the rest of the data. Then, when you encrypt something, save it with a new extension (eg, *.enc) and when you decrypt it, you just read the extension and save it again with its original extension.
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  8. #8
    Frequently Quite Prolix dwks's Avatar
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    Use &argv[2][4].
    dwk

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    hmm jawib's idea has some merit.... and dwks, your idea might not work for everything - for example, picture files like pic_03.jpg or whatever.....
    i can throw the extension in at the beginning of the file.

    and does anybody know how tonto's idea works / what it is?

  10. #10
    Frequently Quite Prolix dwks's Avatar
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    Here's a breakdown:
    Code:
    std::ofstream out((std::string("dec_") += argv[1]).c_str());
    ->
    Code:
    std::string filename("dec_");
    filename += argv[1];
    std::ofstream out(filename.c_str());
    dwk

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  11. #11
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    actually jawib's idea is kinda hard...
    hmm...........
    dwks, can i say :
    if (argv[2][0] == "enc_") or would i have to do like ((argv[2][0] == 'e') && (argv[2][1] == 'n').... etc

    hmm
    double hmm.

  12. #12
    Frequently Quite Prolix dwks's Avatar
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    strcmp()

    [edit]
    Code:
    #include <cstring>
    
    char astring[100];
    
    if(strcmp(astring, "hello") == 0) {
        // astring is "hello"
    }
    [/edit]
    dwk

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  13. #13
    Frequently Quite Prolix dwks's Avatar
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    Or you could use strncpy:
    Code:
    #include <cstring>
    
    char s[100] = "enc_file";
    
    cout << s << endl;
    strncpy(s, "dec", 3);
    cout << s << endl;
    
    /* output:
    enc_file
    dec_file
    */
    dwk

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  14. #14
    Frequently Quite Prolix dwks's Avatar
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    Quote Originally Posted by JaWiB
    Instead of prepending dec_/enc_, why not store the file extension inside the file and encrypt it along with the rest of the data. Then, when you encrypt something, save it with a new extension (eg, *.enc) and when you decrypt it, you just read the extension and save it again with its original extension.
    jawib's idea is kinda hard..
    You could change a file like file.ext to file.encoded.ext to file_encoded.ext instead.
    dwk

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  15. #15
    Confused Magos's Avatar
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    stringstreams!
    Code:
    #include <sstream>
    
    std::stringstream Stream;
    Stream << "dec_" << argv[1];
    
    Stream.str() //Get an std::string from it
    Stream.str().c_str() //Get a char pointer from it
    Works with integers etc which std::string operator + won't.
    MagosX.com

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