Implementation dependent outputs of strings

Question

In the following code, I'm getting only string a as output on my mingw compiler but I get both strings as output on an online compiler. Why am I not getting appropriate output on my system?

#include<iostream>
#include<cstring>

using namespace std;

int main(){
    string a="Welcome";
    string b="HelloWorld";
    for(int i=0;i<=b.length();i++){
        a.push_back(b[i]);
        
    }
    cout<<a<<endl<<b;

     

    return 0;   
}

Output- WelcomeHelloWorld

Try `cout.flush();` before returning from main, maybe the last line just wasn't displayed. And with "maybe" I mean "most likely or something else is horribly wrong". — Lukas-T, Feb 03 '21 at 11:29
[Usual recommendation against `using namespace std;`](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice). — Quentin, Feb 03 '21 at 11:30
It's worh noting that you are copying a zero byte into `a` from `b[b.size()]`. It's well-defined, but maybe your terminal gets confused when outputting it. — Quentin, Feb 03 '21 at 11:31
@n.'pronouns'm. It's not wrong as in "causes UB"-wrong. It will only append a `\0` to `a`. But it's most likely not intended, yes. — Lukas-T, Feb 03 '21 at 11:32
@Scheff it is defined for the non-const version as well as long as you don't modify that null byte. — Quentin, Feb 03 '21 at 11:40
What exactly do you get as output? Is it `WelcomeHelloWord` , but you expected a second line? — 463035818_is_not_an_ai, Feb 03 '21 at 11:43
Sounds like a bug in mingw in regards of : https://stackoverflow.com/questions/15911517/is-there-a-guarantee-of-stdout-auto-flush-before-exit-how-does-it-work. add an `endl` — Devolus, Feb 03 '21 at 11:49
@largest_prime_is_463035818 ya.....that's my output. Only this line and I expected a 2nd line — Nityunj, Feb 03 '21 at 12:09
And what output did you expect? Why would there be two lines? — n. m. could be an AI, Feb 03 '21 at 15:28
@n.'pronouns'm. I have written both `a` and `b` in `cout`, hence I should get 2 lines — Nityunj, Feb 04 '21 at 06:52

score 2 · Answer 1 · answered Feb 03 '21 at 12:01

one thing you can do is, you can use cout.flush(). If it does not work you can do a+b or you can use substr()

#include<iostream>
#include<cstring>
using namespace std;
int main(){
    cout.flush();
    string a="Welcome";
    string b="HelloWorld";
    cout<<a+b<<endl<<b;
    // cout<<a+b.substr(0,9);
    return 0;   
}

score 1 · Answer 2 · answered Feb 11 '21 at 14:34

1

I think cout.flush() sometimes help when getting unwanted output. So, once you can try this also before printing the result.

answered Feb 11 '21 at 14:34

anonymous_frnd

73
7

Implementation dependent outputs of strings

2 Answers2