C++ Append character to std::string from int

Question

I've got a character from the keyboard, by integer:

int c = getch();

an I want to append it to a string only if it isn't a return:

void somefunction()
{
    std::string str = "you pressed: ";
    int c;
    while ( 1 )
    {
        c = getch();
        if ( c == 10 ) break;
        char* ch;
        sprintf(ch,"%c",c);
        str += std::string(ch);
    }
}

however, this creates a segmentation error when the scope of somefunction is left. I'm geussing that when the dtor for str is called the pointer to ch isn't available any more.

How can I remedy this?

Your problem here is that the pointer ch is not initialized and could point anywhere. — sstn
– sstn, Commented Jun 12, 2012 at 11:38
The segmentation error happens because you don't actually allocate any memory for character array sprintf prints to. — jrok
– jrok, Commented Jun 12, 2012 at 11:39
Your other problem is that you are mixing C style string manipulation with C++ style thereof. This can be done, of course, but isn't necessary and non-trivial to get right (as indicated by sstn). — Sebastian Mach
– Sebastian Mach, Commented Jun 12, 2012 at 11:39
The segfault is because sprintf is writing into random data. Change char *ch; to char ch[2]; - but really, use any of the proper std::string methods in the answers... — Roddy
– Roddy, Commented Jun 12, 2012 at 11:39
the strange thing is that when I comment out the str += ... line, no segfault occurs, leading me to believe the ch initialisation is done properly. — romeovs
– romeovs, Commented Jun 12, 2012 at 11:49

James M · Accepted Answer · 2012-06-12 11:35:59Z

13

This is much easier than you think:

str.push_back(c);

answered Jun 12, 2012 at 11:35

James M

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3 Comments

James M Over a year ago

@Nawaz What makes you say that?

Sarfaraz Nawaz Over a year ago

@JamesMcLaughlin: Ohh... I mistakenly thought it as + rather than +=.

Jaywalker Over a year ago

stringstream could be another class of interest here.

LihO · Accepted Answer · 2012-06-12 11:39:12Z

7

You are getting segmentation fault, because you are trying to sprintf string into unknown (not yet allocated) memory:

char* ch;
sprintf(ch,"%c",c);

possible fix of your code would be to replace char* ch; with char ch[2]; which would cause ch to become an statically allocated array with an automatic storage duration.

But note that since you are programming in C++ it would be wiser to use streams and methods of std::string rather than C-style (char*) strings and C-style functions like sprintf.

answered Jun 12, 2012 at 11:39

LihO

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1 Comment

Sarfaraz Nawaz Over a year ago

+1. Actually it is a complete answer, as it explains the cause of the segfault.

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