Stream exceptions

[Tobruk]

std::istringstream iss;
iss.str(*line);
// stream syntax: value>value
for (int z=0; z<2; z++) {
	iss.clear(); // clear flags from previous iterations
	std::getline(iss, container[z], '>');
	if (iss.good()) // goodbit
	if (iss.bad()) // badbit
	if (iss.eof()) // eofbit
	if (iss.fail()) // failbit
}

I run this code for this stream: "\n" (just a newline character to test how it would react). These are the results:

First iteration (contents: "\n"):
good() == false;
bad() == false;
eof() == false;
fail() == true;
At this point, the newline should already be discarded from the stream.

Second iteration (supposedly with a completely empty stream):
good() == false;
bad() == false;
eof() == FALSE (sic!);
fail() == true;

According to cplusplus.com/reference/string/string/getline/ - getline should stop before the newline, discard it from the stream and return empty string. The first iteration's results could be understood like this - internal logic of the operation was disrupted (it didn't found the delimeter (although it found the newline)). I could argue that the logic wasn't disrupted, because it succesfully returned a string, empty, but a string nonetheless.

However, what struck me the most, were the second iterations' results. eof() still returned FALSE on an empty stream. Why?

Could somebody clarify to me:
- what happens to the newline character after it's found (apart from the fact that it stops the extraction)
- is not finding the delimeter/newline critical for the function or not?

[Peter87]

Please post real code because it's not easy to guess what is going on in your code that we cannot see.

[Tobruk]

// --------------TOKENIZE PHRASES INTO MATRIXES--------------
	// SYNTAX FOR FILE (each line): word,word,...>word,word,word,...
	selected_dir_stream.close();
	selected_dir_stream.open(dir_chosen);
	std::string* container = new std::string[2];
	std::string* line = new std::string;
	std::string* token = new std::string;
	bool bad_segment=false;
	unsigned bad_segments=0, units_accepted=0;

	for (size_t i=0; i<number_of_lines; i++) {
		bad_segment=false;
		for (int y=0; y<2; y++)
			container[y].erase(0,std::string::npos);
		// get line from std::ifstream to *line
		std::getline(selected_dir_stream, *line);
		// convert it to istd::stringstream object
		std::istringstream language_split;
		language_split.str(*line);
		// two languages are separated by '>' sign
		for (int z=0; z<2; z++) {
			// read line anyway, but if line is not properly formatted - omitt it
			language_split.clear();
			std::getline(language_split, container[z], '>');
			if (language_split.fail()) {
				bad_segment=true;
				bad_segments++;
				break;
			}
		}
		// less vocabulary units, but at least they'll be all valid
		if (!bad_segment) {
			// nest vector<vector>
			source_words.push_back(std::vector<std::string>());
			target_words.push_back(std::vector<std::string>());
			// first language synonyms
			std::istringstream synonym_source_split(container[0]);
			while (std::getline(synonym_source_split, *token, ','))
				source_words.at(units_accepted).push_back(*token);
			// second language synonyms
			std::istringstream synonym_target_split(container[1]);
			while (std::getline(synonym_target_split, *token, ','))
				target_words.at(units_accepted).push_back(*token);
			units_accepted++;
		}
	}

[dhayden]

I suspect that *line doesn't contain what you think it does. Here is your code in a program that works as I'd expect it:

#include <sstream>
#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::istringstream;
using std::string;

int main()
{
    string line = "\n";
    cout << "Line is: " << line << '\n';

    std::istringstream iss;
    iss.str(line);
    // stream syntax: value>value
    for (int z=0; z<2; z++) {
	cout << "==== Z=" << z << " ====\n";
	iss.clear(); // clear flags from previous iterations
	string str;
	std::getline(iss, str, '>');
	cout << "After getline, str = \"" << str << "\"\n";
	cout << "good = " << iss.good() << '\n';
	cout << " bad = " << iss.bad() << '\n';
	cout << " eof = " << iss.eof() << '\n';
	cout << "fail = " << iss.fail() << '\n';
    }
}

Output:

Line is:

==== Z=0 ====
After getline, str = "
"
good = 0
 bad = 0
 eof = 1
fail = 0
==== Z=1 ====
After getline, str = ""
good = 0
 bad = 0
 eof = 1
fail = 1