getline(isstream, string) in C++

Question

I'm working on learning C++, and still keep running into stupid problems as I am yet unfamiliar with C++ libraries, and common errors, etc.

right now, the following piece of my code fails:

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

void getInt(int & val){
    string output;
    getline(cin output.c_str());
    val = atoi(output.c_str());
}

and gives me this error when I try to compile it:

test.cpp: In function void getInt(int&):
test.cpp:51: error: expected ) before output
test.cpp:51: error: no matching function for call to getline(std::istream&)

What am I doing wrong? I feel like I'm missing something obvious.

@Daniel Fischer Wow - easy fix. I hate working with emacs in the terminal lol. Can you post an answer so I can accept it? You were the first to it. =P — Jonathan, Feb 23 '13 at 18:47
Missed the opportunity. At this point, adding another answer would just be noise. — Daniel Fischer, Feb 23 '13 at 18:51
@LightnessRacesinOrbit I did debug - I just missed the obvious. I am working in an environment AND a language I am unfamiliar with. I simply misunderstood the error - I thought it was somehow grabbing the wrong "getline" function from some other location. — Jonathan, Feb 23 '13 at 18:53

Cornstalks · Answer 1 · 2013-02-23T19:19:47.123

4

It should be

getline(cin, output); // comma, no .c_str()

If you do .c_str(), you're going to invoke undefined behavior when you try to read data into an empty, read only area of memory. That's some bad mojo. Of course, as Lightness points out in the comments, it won't even compile because c_str() is a const char* and getline() needs a char* (non-const) (but even if you did manage to get it to compile, it's important to understand the undefined behavior).

edited Feb 23 '13 at 19:19

answered Feb 23 '13 at 18:48

Cornstalks

37,137
18
79
144

Thanks for pointing out I don't need .c_str(). I didn't have it originally, but I added it when I started trying to fix the problem. – Jonathan Feb 23 '13 at 18:49
2

It's not "don't need .c_str()" -- it's "_can't_ use .c_str()". – Lightness Races in Orbit Feb 23 '13 at 18:50
Actually, it'll result in a compilation error, since `std::string::c_str()` results in a `char const*`. – Lightness Races in Orbit Feb 23 '13 at 18:52
@LightnessRacesinOrbit: Yes, certainly, I wasn't trying to imply it would correctly compile. I added that last comment in case he tries to cast away the const-ness to get it to compile. – Cornstalks Feb 23 '13 at 18:53
@Cornstalks: You said "you're going to invoke undefined behavio[u]r", which is impossible in a non-compilable program. But okay. – Lightness Races in Orbit Feb 23 '13 at 18:56
@LightnessRacesinOrbit: There, added a comment. – Cornstalks Feb 23 '13 at 19:19

score 3 · Accepted Answer · answered Feb 23 '13 at 18:47

3

getline takes two arguments. When passing multiple arguments to a function, you must separate them with a comma. In addition, the second argument can be a std::string, so just pass output directly:

getline(cin, output);

answered Feb 23 '13 at 18:47

Joseph Mansfield

108,238
20
242
324

score 2 · Answer 3 · answered Feb 23 '13 at 18:49

2

getline(cin output.c_str());

You forgot a comma, skippy.

Also, your second argument is wrong! The string itself is just called output, and you should only apply .c_str() to it when you need to obtain a C-style character buffer for use with C APIs. i.e. not here.

getline(cin, output);

answered Feb 23 '13 at 18:49

Lightness Races in Orbit

378,754
76
643
1,055

I was about to say the same thing. He is correct that that was my problem, and I don't think he was meaning to be disrespectful with the "skippy" or anything? +1 – Jonathan Feb 23 '13 at 18:51
Thank you, @Jonathan. No, "skippy" is an affectionate pet name for only my favourite OPs. – Lightness Races in Orbit Feb 23 '13 at 18:51
@LightnessRacesinOrbit Highly suspicious... are you suuuure it's your favourite? – Feb 23 '13 at 18:52
@ThePhD As is your username, but you don't see me whining about it! :P – Lightness Races in Orbit Feb 23 '13 at 18:52
@ThePhD Just barely got your username reference, btw. LOVE that show. =) – Jonathan Feb 23 '13 at 19:19

getline(isstream, string) in C++

3 Answers3