How to properly empty stringstream buffer?

Question

I have read How do you clear a stringstream variable? to clear the buffer by stringstream.str(""), or in other words - setting empty string. But if I do so, it won't clear it:

date.hpp:

#include <sstream>
#include <time.h>

#define fmt "%m/%d/%y"

class Date
{
    std::stringstream buffer;
    struct tm date;

    template <class T>
    void insertbuf(const T &);
    void inserttime();
    std::string getFromIn(std::istream &);

public:
    //ctors
    Date();
    Date(const char *str);
    Date(const std::string &s);
    Date(std::istream &in);
    Date(const Date &d);

    //operators
    Date &operator=(const Date &rhs);

    //general
    const std::string getAll() const;
    int getMonth() const;
    int getDay() const;
    int getYear() const;
};

date.cpp:

template <class T>
void Date::insertbuf(const T &val)
{
    if (!buffer.rdbuf()->in_avail())
    {
        buffer << val;
    }
}

void Date::inserttime()
{
    buffer >> std::get_time(&date, fmt);
}

Date &Date::operator=(const Date &rhs)
{
    if (&rhs != this)
    {
        buffer.str("");
        insertbuf<std::string>(rhs.buffer.str());
        inserttime();
    }
    return *this;
}

Now in function insertbuf, I will << to the buffer only if there is no other data. So in the operator=, which its left-hand-side (the object itself, or *this), has some data in the buffer and thus I have to clear them. I am trying to do so by setting the buffer to empty string buffer.str("") or equivalently buffer.str(std::string()), but it seems, that will not set it. From here:

main.cpp:

int main()
{
    Date bar = "11/23/2020";
    Date foo = "11/21/2020";
    cout << "before operator= " << foo.getAll() << endl;
    foo = bar;
    cout << "after operator= " << foo.getAll() << endl;
}

output:

before operator= date: 11/21/2020
after operator= date: 11/21/2020

As I can see the buffer was not cleared in the operator= function, because the buffer did not changed (The output should be 11/23/2020, the insertbuf function probably did not get over the if statement because the buffer was not empty, even though I set it to empty string), why? And how to properly clear the buffer of std::stringstream then?

An exmaple: https://godbolt.org/z/h6zofr

The reason, behind the question is here: https://codereview.stackexchange.com/questions/252456/how-to-implement-simple-date-class-in-c

The condition is to avoid `foo = foo`, or self-assignment. I am not doing self-assignment, so it should be ok — milanHrabos, Nov 21 '20 at 13:18
Please post a [mcve]. Based on what you posted I cannot reproduce the problem: https://godbolt.org/z/v84oKa (forget my last comment, it was non-sense ;) — 463035818_is_not_an_ai, Nov 21 '20 at 13:20
Ok I did not want to mess here with full implementation, but I will try to do so to godbolt an paste link to post — milanHrabos, Nov 21 '20 at 13:22
please include a [mcve] in the question. If you don't want to post too much code, you can reduce it, but please make sure that what you post is complete, such that others can reproduce your problem — 463035818_is_not_an_ai, Nov 21 '20 at 13:23
please [edit](https://stackoverflow.com/posts/64943474/edit) your [mre] into your question rather than posting as a link in the comments — Alan Birtles, Nov 21 '20 at 13:27
the code in the link is also incomplete btw. I had to add more to make it compile: https://godbolt.org/z/h6zofr. Consider that any piece of code that others have to add, means that others are not looking at the same code you do. — 463035818_is_not_an_ai, Nov 21 '20 at 13:29
this is the code that is currently in your question: https://godbolt.org/z/nsK59x — 463035818_is_not_an_ai, Nov 21 '20 at 13:31

score 2 · Accepted Answer · answered Nov 21 '20 at 13:42

2

The problem you are having is that calling buffer.str("") doesn't reset the stream state flags so if a previous operation failed or reached the end of the stream no other reads will succeed. The following should fix the issue:

buffer.str("");
buffer.clear();

Your code seems unnecessarily convoluted. If all you are trying to do is convert dates to and from strings I recommend using Howard Hinnant's date library which is now also part of c++20.

answered Nov 21 '20 at 13:42

Alan Birtles

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It was just for learning purposes of implementing classes, I would use std-lib class for date in real project of course – milanHrabos Nov 21 '20 at 13:49
Anyway, I am not yet very familiar with streambuf to know, which its flags are important (otherwise I would not care about clear(), which clears the flags). You said, the previous operation failed. I see no place in the code, where I am creating object from `const char*`, where would the buffer of `stringstream` should fail. So why do I have to clear the flags, when in nowhere the buffer should not fail. Can you please explain? – milanHrabos Nov 21 '20 at 14:00
And also, in what place (line) from the main function does the buffer "reach the end of stream"? Where? (just to want to understand it) – milanHrabos Nov 21 '20 at 14:01
`inserttime` sets the `eof` and `fail` bits as your format string is incorrect (should be `%m/%d/%Y`), even with the corrected format string it still sets the `eof` flag as it reads to the end of the stream – Alan Birtles Nov 21 '20 at 14:48
The reason behind the question, is here – milanHrabos Nov 21 '20 at 14:50
https://codereview.stackexchange.com/questions/252456/how-to-implement-simple-date-class-in-c – milanHrabos Nov 21 '20 at 14:50

How to properly empty stringstream buffer?

1 Answers1