How to see the last character in a stringstream object with O(1) constant complexity?

Question

I am trying to get ride of trailing newlines on a std::stringstream object after flushing some stuff into it. For now I would try something like this:

const char* inputvalue = "My line\n";
std::stringstream stream;

stream << std::string{inputvalue};
stream.seekp( -1, std::ios_base::end );

if( stream.peek() == '\n' ) {
    stream << " ";
} else {
    stream.seekp( 0, std::ios_base::end );
}

Is this the most efficient way to do so i.e., fewer CPU instructions and time?

As a side note, can I completely remove the last newline other than putting a space character to get rid of the new line without sacrificing performance?

I am reading a file by lines, and I am casting way the new line characters coming with the line itself. I am using [`PyUnicode_AsUTF8AndSize`](https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_AsUTF8AndSize) to get the line. The other problem is that [`PyUnicode_AsUTF8AndSize`](https://docs.python.org/3/c-api/unicode.html#c.PyUnicode_AsUTF8AndSize) returns a `const char*`. You can find a minimal code here on [my other](https://stackoverflow.com/questions/56482802) question. See the line `while( ( readline = PyObject_CallObject( fileiterator, NULL ) ) != NULL ) {` — Evandro Coan, Jun 06 '19 at 22:46
Maybe I can convert the `const char* pointer` to `char * pointer` and set `pointer[size-1] = '\0';`? — Evandro Coan, Jun 06 '19 at 22:51
The first problem seems to be the unnecessary `std::string{inputvalue};`. Having said that, `stringstream` is not particularly fast. The `iostream` methods are good for human-readable formatting, and humans just aren't fast. So speed was not really a design goal. — MSalters, Jun 06 '19 at 22:54
I am building a big string and when I tried `stringstream << const char*` it raised an exception, then, I added the `std::string{const char*}`. Is there a better wat to build a big string other than using `stringstream`? — Evandro Coan, Jun 06 '19 at 22:57
std::string has some pretty useful functions for finding last characters which you could then erase. If you had a char array you could use a while loop and remove the characters yourself. Also, don't set the character to a space set it to nul. — Bayleef, Jun 06 '19 at 23:34

score 0 · Accepted Answer · answered Jun 06 '19 at 23:09

Before adding the thing on the stringstream I am taking the const char* and checking whether it has the new line \n on it. If so, I am casting it to char* and ending the string with \0 on size-1.

Py_ssize_t linesize;
std::stringstream stream;

char* nonconstcharline;
const char* constcharline;

for( PyObject* linepy : linecache ) {
    constcharline = PyUnicode_AsUTF8AndSize( linepy, &linesize );

    if( constcharline[linesize-1] == '\n' ) {
        nonconstcharline = const_cast<char *>( constcharline );
        nonconstcharline[linesize-1] = '\0';
        stream << std::string{nonconstcharline};
    }
    else {
        stream << std::string{constcharline};
    }
}

I read on this other question How to use const_cast? I should not do const char* to char* and change it as I do not seem to own the const char* thing, but so far no problems.

Anyway, take a look at `std::string_view`. – Deduplicator Jun 06 '19 at 23:30 — Deduplicator, Jun 06 '19 at 23:30

How to see the last character in a stringstream object with O(1) constant complexity?

1 Answers1