[英]getline setting failbit along with eof
I am aware of the origin of this behavior since it has been very well explained in multiple posts here in SO, some notable examples are: 我知道这种行为的起源,因为它在SO中的多个帖子中得到了很好的解释,一些值得注意的例子是:
Why is iostream::eof inside a loop condition considered wrong? 为什么循环条件中的iostream :: eof被认为是错误的?
Use getline() without setting failbit 使用getline()而不设置failbit
std::getline throwing when it hits eof 当它击中eof时std :: getline投掷
C++ istream EOF does not guarantee failbit? C ++ istream EOF不保证failbit?
And it is also included in the std::getline
standard : 它也包含在
std::getline
标准中 :
3) If no characters were extracted for whatever reason (not even the discarded delimiter), getline sets failbit and returns.
3)如果由于某种原因没有提取任何字符(甚至没有丢弃的分隔符),getline设置failbit并返回。
My question is how does one deal with this behavior, where you want your stream to catch a failbit
exception for all cases except the one caused by reaching the eof
, of a file with an empty last line. 我的问题是如何处理这种行为,你希望你的流为所有情况捕获一个
failbit
异常,除了由于到达eof
而导致的最后一行空文件。 Is there something obvious that I am missing? 有什么明显的东西让我失踪吗?
A MWE: MWE:
#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
void f(const std::string & file_name, char comment) {
std::ifstream file(file_name);
file.exceptions(file.failbit);
try {
std::string line;
while (std::getline(file, line).good()) {
// empty getline sets failbit throwing an exception
if ((line[0] != comment) && (line.size() != 0)) {
std::stringstream ss(line);
// do stuff
}
}
}
catch (const std::ios_base::failure& e) {
std::cerr << "Caught an ios_base::failure.\n"
<< "Explanatory string: " << e.what() << '\n'
<< "Error code: " << e.code() << '\n';
}
}
int main() {
f("example.txt", '#');
}
where example.txt is a tab-delimited file, with its last line being only the \\n
char: 其中example.txt是制表符分隔的文件,其最后一行只是
\\n
char:
# This is a text file meant for testing
0 9
1 8
2 7
while(std::getline(file, line).good()){...}
replicates the problem. while(std::getline(file, line).good()){...}
复制问题。
Edit: I misunderstood the OP, refer to David's answer above. 编辑:我误解了OP,请参阅David的上述答案。 This answer is for checking whether or not the file has a terminating newline.
此答案用于检查文件是否具有终止换行符。
At the end of your while (getline)
loop, check for file.eof()
. 在
while (getline)
循环结束while (getline)
,检查file.eof()
。
Suppose you just did std::getline()
for the last line in the file. 假设你刚刚为文件的最后一行做了
std::getline()
。
If there is a \\n
after it, then std::getline()
has read the delimiter and did not set eofbit
. 如果后面有
\\n
,那么std::getline()
已经读取了分隔符并且没有设置eofbit
。 (In this case, the very next std::getline()
will set eofbit
.) (在这种情况下,下一个
std::getline()
将设置eofbit
。)
Whereas if there is no \\n
after it, then std::getline()
has read EOF and did set eofbit
. 然而,如果它之后没有
\\n
,那么std::getline()
已经读取了EOF并且确实设置了eofbit
。
In both cases, the very next std::getline()
will trigger failbit
and enter your exception handler. 在这两种情况下,下一个
std::getline()
将触发failbit
并输入您的异常处理程序。
PS: the line if ((line[0] != comment) && (line.size() != 0)) {
is UB if line
is empty. PS:行
if ((line[0] != comment) && (line.size() != 0)) {
如果line
为空则为UB。 The conditions' order needs to be reversed. 条件的顺序需要颠倒。
Another way to avoid setting failbit
, is simply to refactor your if
tests to detect the read of an empty-line . 避免设置
failbit
另一种方法是简单地重构if
测试以检测空行的读取。 Since that is your final line in this case, you can simply return
to avoid throwing the error, eg: 由于这是你在这种情况下的最后一行,你可以简单地
return
以避免抛出错误,例如:
std::ifstream file (file_name);
file.exceptions (file.failbit);
try {
std::string line;
while (std::getline(file, line)) {
// detect empty line and return
if (line.size() == 0)
return;
if (line[0] != comment) {
std::stringstream ss(line);
// do stuff
}
}
}
...
You other alternative is to check whether eofbit
is set in catch
. 另一种方法是检查
eofbit
是否设置为catch
。 If eofbit
is set -- the read completed successfully. 如果设置了
eofbit
- 读取成功完成。 Eg 例如
catch (const std::ios_base::failure& e) {
if (!file.eof())
std::cerr << "Caught an ios_base::failure.\n"
<< "Explanatory string: " << e.what() << '\n'
<< "Error code: " /* << e.code() */ << '\n';
}
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