opening output filestreams with string names

Question

Hi I have some C++ code that uses user defined input to generate file-names for some output files:

std::string outputName = fileName;
for(int i = 0; i < 4; i++)
{
    outputName.pop_back();
}
std::string outputName1 = outputName;
std::string outputName2 = outputName;
outputName.append(".fasta");
outputName1.append("_Ploid1.fasta");
outputName2.append("_Ploid2.fasta");

Where fileName could be any word the user can define with .csv after it e.g. '~/Desktop/mytest.csv'

The code chomps .csv off and makes three filenames / paths for 3 output streams.

It then creates them and attempts to open them:

std::ofstream outputFile;
outputFile.open(outputName.c_str());
std::ofstream outputFile1;
outputFile1.open(outputName1.c_str());
std::ofstream outputFile2;
outputFile2.open(outputName2.c_str());

I made sure to pass the names to open as const char* with the c_str method, however if I test my code by adding the following line:

std::cout << outputFile.is_open() << " " << outputFile1.is_open() << " " << outputFile2.is_open() << std::endl;

and compiling and setting fineName as "test.csv". I successfully compile and run, however,

Three zeros's are printed to screen showing the three filestreams for output are not in fact open. Why are they not opening? I know passing strings as filenames does not work which is why I thought conversion with c_str() would be sufficient.

Thanks, Ben W.

And you provide absolute paths to these files? If you output the actual filenames, are they correct then? — Some programmer dude, Feb 20 '14 at 18:17
They are correct - the files don't yet exist, they should be created and then written to. — SJWard, Feb 20 '14 at 18:21

score 2 · Answer 1 · edited May 23 '17 at 11:57

2

Your issue is likely to be due to the path beginning with ~, which isn't expanded to /{home,Users}/${LOGNAME}.

ifstream open file C++

This answer to How to create a folder in the home directory? may be of use to you.

Unfortunately, there is no standard, portable way of finding out exactly why open() failed:

Detecting reason for failure to open an ofstream when fail() is true

I know passing strings as filenames does not work which is why I thought conversion with c_str() would be sufficient.

std::basic_ofstream::open() does accept a const std::string & (since C++11)!

edited May 23 '17 at 11:57

Community

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answered Feb 20 '14 at 18:29

johnsyweb

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Re the last point: only in C++11. Older compilers might not support it. – James Kanze Feb 20 '14 at 18:42

opening output filestreams with string names

1 Answers1