Opening File in C++ - getline fails on second iteration [closed] - c++

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Closed 10 years ago.
I cannot figure out why getline is working in one X-Code project but not in another. The error "No matching function for call to 'getline'.
When I make a single cpp file it compiles with no issues.
// reading a text file
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main () {
string line;
ifstream myfile ("example.txt");
if (myfile.is_open())
{
while ( myfile.good() )
{
getline (myfile,line);
cout << line << endl;
}
myfile.close();
}
else cout << "Unable to open file";
return 0;
}
I would really appreciate some assistance. I am just learning and the example above came from my test book.

Answer 1

The code is noisy and incorrect. The correct standard idiom is like this:
#include <fstream> // for std::ifstream
#include <string> // for std::getline and std::string
std::ifstream myfile("example.txt");
if (!myfile) { /* error, die */ }
for (std::string line; std::getline(myfile, line); )
{
std::cout << "Read one line: '" << line << "'\n";
}
Correctness:
You must check the success of the input operation before consuming the input. To do otherwise may be UB, and certainly never correct.
[Thanks #James for pointing this out:] good() doesn't check if a file was opened. You could use either !myfile.fail() or myfile.is_open(), but just don't bother (see below).
Noise:
The ifstream constructor takes the filename and opens the file already. Use it.
The ifstream cleans up in its destructor, no need to do that explicitly. Use tight scoping to close the file as soon as you're done with it.
Don't leak line into the ambient scope if you don't need it.
No need for good() (or any of the correct alternatives). Just evaluate the ifstream object in a boolean context to see if the file was opened successfully.

Answer 2

There is one actual error: you use the results of getline
without testing whether it has succeeded. The usual way of
writing your inner loop would be:
std::string line;
while ( std::getline( myfile, line ) ) {
std::cout << line << std::endl;
}
Others are more question of style, although some issues do
enjoy almost universal consensus: don't define a variable
before you need it, for example (and thus, the definition of
line should be immediately before the loop). Error output
goes to std::cerr, not std::out. And in case of an error,
you should returnEXIT_FAILURE, and not0`.
It's also generally accepted that you don't have to explicitly
close input (since the destructor will take care of it, and
you've already successfully read everything). The consensus
isn't as complete for others: I would never use using namespace
std;, for example, and I would never put the code following an
else on the same line. I also find code more readable if the
shorter condition in an if...else is the first, so I'd write:
if ( !myfile.is_open() ) {
std::cout << "Unable to open file" << std::endl
returnCode = EXIT_FAILURE;
} else {
std::string line;
while ( std::getline( myfile, line ) ) {
std::cout << line << std::endl;
}
returnCode = EXIT_SUCCESS;
}
Except that for anything more complicated, I'd factor the
processing of the file out into a separate function. So I'd end
up with something like:
if ( myfile.is_open() ) {
process( myfile );
} else {
// error handling...
}
But you seem to suggest that you're having problems with
std::getline in code you don't post. There are two obvious
possible explinations: you didn't include <string>, or you
forgot the std:: in front of it. There are other possible
explinations, but we'd have to actually see the code which is
failing to say exactly what is wrong.