program crashing

Question

I m newbie to programming. Please help me with this question.

when i execute this program, program will crash. can anybody tell me the exact reason for the crash?

#include<stdio.h>    
#include<string.h>    
#include<vector>    
using namespace std;    

struct s  
{  
    char *str;  
};  

std::vector<struct s> v;  
int main()  
{  
    struct s s1;  
    strcpy(s1.str,"hi");  
    v.push_back(s1);  
    strcpy(s1.str,"hello");  
    v.push_back(s1);  
    strcpy(s1.str,"How are you");  
    v.push_back(s1);  
    strcpy(s1.str,"AMAZING");  
    v.push_back(s1);  
    for (int i=0;i<(int)v.size();i++)  
    {           
        printf("%s\n",v[i].str);  
    }  
    return 0;  
}  

i am compiling it in devc++. pls help.

Answer 1

No memory has been allocated for str and struct s is violating the rule of three : use std::string instead.

You don't need to specify struct s (you do in C) when declaring a type of s , just use s :

std::vector<s> v;

s s1;

You could make struct s more convenient to use by providing a constructor:

struct s
{
    s(const std::string& a_s) : str(a_s) {}
    std::string str;
};

v.push_back(s("hi"));

Answer 2

You haven't allocated any memory for s1.str to point at. You're writing via an unitialized pointer, giving undefined behavior.

Try something like:

struct s { 
    std::string str;
};

s s1;

s1.str = "hi";
v.push_back(s1);
// etc.

better still, just use a string directly, and a C++11 initializer list:

std::vector<std::string> v{"hi", "hello", "how are you", "AMAZING"};

std::copy(v.begin(), v.end(), std::ostream_iterator<std::string>(std::cout, "\n"));

Answer 3

You don't allocated memory for char *str; in your struct

just take a look at the strcpy documentation: http://www.cplusplus.com/reference/clibrary/cstring/strcpy/

The memory were you are copying must be allocated. But when you write:

char *str; 

memory is not allocated. You've just created a pointer (to garbage)

You can allocated the memory or better use std::string , because memory management will be done for you in that case

Answer 4

您没有在s.str分配内存。

Answer 5

You are experiencing Undefined Behavior. Your char * str is just a pointe,r with no memory allocated. Nevertheless, you try to modify the memory it points to. Since that memory doesn't belong to your program, you are getting a crash.

Since it's UB, you can't really tell what will happen. For example, for me this code runs.

Answer 6

You need to allocate memory for s1, before using it (uniniatialized pointers point to garbage).. use new for that once done don't forget to release the memory using delete operator..

I suggest you to have a look in some good C++ tutorials,

1. see this link
2. Another good C++ resource

Note I have edited (changed) this answer since all others answer correctly address the problem. I just wanted to add few cents to them.