What is the modern equivalent (C++) style for the older (C-like) fscanf method?

Question

What is the best option if I want to "upgrade" old C-code to newer C++ when reading a file with a semicolon delimiter:

/* reading in from file C-like: */
fscanf(tFile, "%d", &mypost.nr); /*delimiter ; */
fscanf(tFile, " ;%[^;];", mypost.aftername);/* delimiter ; */
fscanf(tFile, " %[^;]", mypost.forename);   /*delimiter ; */
fscanf(tFile, " ;%[^;];", mypost.dept);/*delimiter ; */
fscanf(tFile, " %[^;];", mypost.position);/* delimiter ; */
fscanf(tFile, "%d", &mypost.nr2);

//eqivalent best C++ method achieving the same thing?

I wonder how strict fscanf is? If you read all characters up to a semicolon on the line with forename, how can the next line read a space before the semicolon? - Also, `fscanf` probably wouldn't that bad if you could also specify the maximum number of characters to read somewhere. — UncleBens, Mar 29 '10 at 15:49

susmits · Accepted Answer · 2010-03-29T17:38:52.713

You could overload the right-shift operator on istream for your struct, so:

std::istream& operator>>(std::istream& is, mypost_struct& mps) {
    is >> mps.nr;
    is.ignore(1, ';');
    is.getline(mps.forename, 255, ';');
    is.getline(mps.aftername, 255, ';');
    is >> mps.dept;
    is.ignore(1, ';');
    is >> mps.position;
    is.ignore(1, ';');
    is >> mps.nr2;

    return is;
}

Subsequently, input is as simple as is >> mypost;, where is is the file that you have opened.

Edit: @UncleBens Thanks for pointing this out, I had forgotten to take spaces in account. I have updated the answer, assuming that forename and aftername are likely to contain spaces. And there was this rather embarrasing bit about the delimiters being double-quoted...

I just checked it using a struct definition as under:

struct mypost_struct {
    int nr;
    char forename[255], aftername[255];
    int dept, position, nr2;
};

.. and the result was as expected.

I wonder what all the upvotes are for. The idea of providing overloaded `>>` is fine but the implementation (**how** to read delimited input) is completely broken. `ignore` doesn't have this form, and even if you fix it this won't change the behavior of `istream.operator>>` and use ";" as a delimiter. It would sort of work if the input is also delimited with whitespace and none of the strings contain more than one word. — UncleBens, Mar 29 '10 at 14:56

score 3 · Answer 2 · answered Mar 29 '10 at 11:59

As @susmits says, but you can also use the returned stream as a conditional, like:

if (is >> mps.nr && is.ignore(1, ";") && is >> mps.aftername && ...) {
   // all is well ...
} else {
   // bad input format
}

or even:

if (is >> mps.nr >> ignore(";") >> mps.aftername >> ...) {
    // all is well ...
} else {
    // bad input format
}

score 1 · Answer 3 · answered Mar 29 '10 at 17:54

1

What is the best option if I want to "upgrade" old C-code to newer C++...?

IMHO, the best way to do this would be to read the file line-by-line and use regular expressions for parsing.

answered Mar 29 '10 at 17:54

Agnel Kurian

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What is the modern equivalent (C++) style for the older (C-like) fscanf method?

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