创建字符串向量：C ++

Question

I am trying to create a vector which should input strings (white space included) until user enters '!' .

Below is my code :

#include <iostream>
#include <vector>
#include <string>
#include <iterator>

using namespace std;


int main()
{

   char buffer[100];
   string temp;
   vector <string> vec;


    while(1)//Shows Segmentation fault error while running,
    //compiles succesfully
    {
    cout << "Enter String : ";
    cin.get(buffer,100,'\n');//To ensure that whitespaces are also input
    cout << "@@@@ " << buffer << endl ;//Shows correct input

    cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
    //Clearing cin stream for any residual characters
    temp.assign(buffer);//Assigning string the input "phrase"
    cout << "###" << temp << endl;//Shows correct output

    if (temp == "!") //if input is '!' character do not push on the vector a
    //come out of the loop
    break;

    vec.push_back(temp);//push on the vector
    temp.assign(NULL); //flush temp string

    }


  vector <string>::iterator p = vec.begin();
  //display thre vector
  while( p != vec.end())
   {
       cout << *p << endl;
       p++;
       }

    return 0;
    }

It compiles successfully but at run-time throws Segmentation fault error.

Not able to figure out why ? Can any one point out?

Also smarter solution for this, is appreciated along side pointing out what is wrong with my code.

Thanks

Answer 1

This will be the cause:

temp.assign(NULL);

as std::string::assign() will attempt to read until a null terminator is found and dereferencing a NULL pointer is undefined behaviour: in this case a segmentation fault. Use temp.clear() or just create a new object on each iteration.

Use std::getline() which reads lines including whitespace and avoids having to hardcode a fixed size array (ie buffer[100] ):

std::string line;
while (std::getline(std::cin, line) && line != "!")
{
    vec.push_back(line);
}

Answer 2

Other problems notwithstanding, running your program in gdb (or probably any other debugger) reveals the reason:

tikal@seven ~$ g++ -o temp temp.cpp
tikal@seven ~$ gdb temp
GNU gdb 6.3.50-20050815 (Apple version gdb-1822) (Sun Aug  5 03:00:42 UTC 2012)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries ... done

(gdb) run
Starting program: /Users/tikal/temp 
Reading symbols for shared libraries ++............................. done
Enter String : asd
@@@@ asd
###asd

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x00007fff9081e6b0 in strlen ()
(gdb) backtrace
#0  0x00007fff9081e6b0 in strlen ()
#1  0x00007fff9857ab95 in std::string::assign ()
#2  0x0000000100001642 in main ()
(gdb) 

Essentially, temp.assign( NULL ) is a bad idea. You could use temp.clear() instead, or don't bother clearing it (you'll just reassign it later).

Answer 3

while (getline(cin, temp) && temp != "!")
{
    vec.push_back(temp);
}

Using buffers like that is more a C thing to do than C++. In C++ there's usually a way to avoid such explicit memory management using classes -- this is a good example.