Difference between char x[n] and char* x=new char[n]

This is a discussion on Difference between char x[n] and char* x=new char[n] within the C++ Programming forums, part of the General Programming Boards category; Hi. noob warning. Why does this work: Code: char* filename=new char[256]; and this does not: Code: char filename[256]; in this ...

  1. #1
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    Difference between char x[n] and char* x=new char[n]

    Hi. noob warning.

    Why does this work:
    Code:
    char* filename=new char[256];
    and this does not:
    Code:
    char filename[256];
    in this context:
    Code:
    filename=argv[2];
    (error is: 25 G:\proiecte\xorcrypt\main.cpp incompatible types in assignment of `char*' to `char[256]') . Aren't they both char ?

  2. #2
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    You aren't copying the string in argv[2] in that last line. You are assigning the pointer that points to the string to the filename variable. When you used new, filename was a pointer, when you did not it was an array. You can assign a pointer to a pointer but not a pointer to an array.

    However, it doesn't really matter, since it is doubtful that you want either. Presumably you want to copy the string, in which case you would use strcpy, not operator=.

    And of course, in C++, it is generally best to use C++ strings. Assigning with operator= would work if you used the C++ string class.

  3. #3
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    I can't pass that string to an ifstream filename if it's a string.

  4. #4
    Frequently Quite Prolix dwks's Avatar
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    Use this.
    Code:
    std::string filename = "file.txt";
    std::ifstream file;
    
    file.open(filename.c_str());
    dwk

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  5. #5
    CSharpener vart's Avatar
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    you will use c_str() member in this case
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  6. #6
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    hey, that works. thanks.

    just as a thing i hit right by mistake:
    Code:
            char ch;
            while( fin.get( ch ) ) {
                filedata.append( &ch );
                fsize++;
            }
    append() needs a & operator before a char? I wasn't able to pass a char, but all works fine this way.
    By the way, is this the right way to read everything from a file into a variable (ios::binary set) ?

  7. #7
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    >append() needs a & operator before a char?
    To append a single char, use push_back():
    Code:
                filedata.push_back( ch );
    Or you can also use:
    Code:
                filedata += ch;

  8. #8
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    >> I wasn't able to pass a char, but all works fine this way.
    You got "lucky". Passing a pointer to char (which is what the & did) means that append reads each char in memory one at a time until it finds the null character. The next char in memory was probably just the null character (which has the value 0), so it only appended one character.

    If you are reading a binary file into a variable, then reading character by character might be inefficient. If your program doesn't have any performance issues then it's fine. Otherwise, you might want to read into a buffer with read(), then append that buffer to your string.

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