How to copy strings from ifstream to vector elegantly?

Question

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
    ifstream fin("test.txt");

    vector<string> coll;

    typedef istreambuf_iterator<char> IStrBufIter;
    copy(IStrBufIter(fin), IStrBufIter(), back_inserter(coll)); // Error! Why?
}

The question is commented in the code.

The error I get is:

oi.cpp:15:62:   required from here
error: invalid user-defined conversion from ‘std::istreambuf_iterator<char, std::char_traits<char> >::char_type {aka char}’ to ‘std::vector<std::basic_string<char> >::const_reference {aka const std::basic_string<char>&}’ [-fpermissive]

The error messages that follow are too many to paste here. Please refer to the Error msg on gcc 4.7 http://pastebin.com/anKQqL00

Do you want to copy individual characters, or strings? If strings, what strings? — jogojapan, Feb 06 '13 at 02:22
Your istreambuf_iterator is giving you chars, your back_inserter is expecting std::strings. — André Puel, Feb 06 '13 at 02:24
@xmllmx What strings? Strings containing single characters? Or strings containing words? If words, how to you define the boundaries between words? — jogojapan, Feb 06 '13 at 02:24
@jogojapan, the boundaries are identified by '\n', which can be recognized by the iostream automatically. — xmllmx, Feb 06 '13 at 02:31
@xmllmx There is a misunderstanding about what `istreambuf_iterator` does. It does not read lines (neither by recognizing `\n` nor in any other way). It iterates through the individual characters of the input stream. If you want lines, you need to explain that in the question. — jogojapan, Feb 06 '13 at 02:35
@jogojapan, the title of the post has said I want to copy strings from fin to vector. — xmllmx, Feb 06 '13 at 02:38
@xmllmx But strings could be anything. _Lines_ is a very special case of strings. — jogojapan, Feb 06 '13 at 02:38

David · Answer 1 · 2013-02-06T02:53:31.493

4

Consider what your copy is doing. It's repeatedly calling coll.push_back(someChar);

coll is a vector of strings. You are trying to push a char into it. You can't construct a string from a char.

Also, you should be using istream_iterator here, not istreambuf_iterator. You can either make coll a vector of chars, or if you are aiming to get each word in the file into your vector of strings (which I suspect you might be) you can change istream_iterator<char> to istream_iterator<string>.

Or, better yet, you can construct the vector in the first place using your iterators:

 vector<string> coll(istream_iterator<string>(fin), istream_iterator<string>());

edited Feb 06 '13 at 02:53

answered Feb 06 '13 at 02:23

David

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`istreambuf_iterator` wouldn't compile, I don't think, or at least [I can't get it to compile](http://ideone.com/fXaO2t). – Rapptz Feb 06 '13 at 02:32
I cannot compile istreambuf_iterator too. – xmllmx Feb 06 '13 at 02:35
@xmllmx corrected - see edited answer. – David Feb 06 '13 at 02:53
@Dave, I can not compile vector coll(istream_iterator(fin), istream_iterator());! – xmllmx Feb 06 '13 at 02:55
@xmllmx, Look up the most vexing parse. – chris Feb 06 '13 at 03:16

score 3 · Accepted Answer · answered Feb 06 '13 at 02:26

3

If you want to put the contents of a file into a vector just loop through it.

#include <iostream>
#include <fstream>
#include <string>
#include <vector>

using namespace std;

int main()
{
    ifstream fin("test.txt");

    vector<string> coll;
    string str;
    while(getline(fin,str))
        coll.push_back(str);
}

Probably easier to deal with and self-explanatory.

answered Feb 06 '13 at 02:26

Rapptz

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However, I want to use std::copy here. Any idea? – xmllmx Feb 06 '13 at 02:26
@xmllmx This puts the _lines_ of the input into the vector. Is that what you want? – jogojapan Feb 06 '13 at 02:27
@xmllmx: If you want lines, you'll have to develop a special input iterator that reads a line at a time, or maybe you can find a public implementation of something like that. The standard library doesn't have one. – Benjamin Lindley Feb 06 '13 at 02:30
1

@BenjaminLindley wouldn't `istream_iterator` rather than `istreambuf_iterator` work instead? – Rapptz Feb 06 '13 at 02:30
1

@Rapptz: Not for getting lines. `istream_iterator` uses `operator>>`, and with strings, that means reading whitespace delimited tokens. Any whitespace, not just newlines. – Benjamin Lindley Feb 06 '13 at 02:31
@xmllmx why do you *want* to use `std::copy`? It's much easier to use this approach for what you're doing. – Rapptz Feb 06 '13 at 02:34
1

Another option would be defining a special type that encapsulates a string, and has an implicit conversion to `std::string`, but its overload of `operator>>` reads in a whole line instead of just a word. Then you could use `istream_iterator`. That would probably be simpler than implementing an iterator, but less interesting and less useful. – Benjamin Lindley Feb 06 '13 at 02:35

How to copy strings from ifstream to vector elegantly?

2 Answers2