Error while performing string copy operation in C++

Question

Please explain why is this giving an error but the other on is running fine The following code gives the error:

#include <iostream>
#include <string>
#include <conio.h>
#include <math.h>
#include <iomanip>
#include <string.h>

using namespace std;

int main()
{
    string s1,s2;
    int i;

    cout << "Enter the string to copy into another string : ";
    getline(cin,s1);

    for(i=0; s1[i]!='\0'; ++i)
    {
      s2[i]=s1[i];
    }
    s2[i]='\0';
    cout<<"\n\nCopied String S2 is : "<<s2;
    return 0;
}

Error looks like this

But this works perfectly fine

#include <iostream>
#include <string>
#include <conio.h>
#include <math.h>
#include <iomanip>
#include <string.h>

using namespace std;

int main()
{
    char s1[100], s2[100], i;

    cout << "Enter the string to copy into another string : ";
    cin>>s1;

    for(i=0; s1[i]!='\0'; ++i)
    {
      s2[i]=s1[i];
    }
    s2[i]='\0';
    cout<<"\n\nCopied String S2 is : "<<s2;
    return 0;
}

For std::string s2, s2[i] exhibits undefined behavior when i >= s2.size(). Which it is in your first example, ass2.size() == 0 — Igor Tandetnik
– Igor Tandetnik, Commented Aug 29, 2020 at 20:36
Instead of looping simply s2 = s1; would copy a std::string correctly — UnholySheep
– UnholySheep, Commented Aug 29, 2020 at 20:36
@UnholySheep My assignment says without using inbuilt functions please help me with another way to gain marks. — Omkar Arora
– Omkar Arora, Commented Aug 29, 2020 at 20:38
You cannot copy a std::string without using "inbuilt functions" - the internals of a std::string are not something you can just mess with, they are hidden from you (since there's a lot of optimizations going on in there). — UnholySheep
– UnholySheep, Commented Aug 29, 2020 at 20:41
s2[i] calls std::string::operator[]. Why is it OK to call that, but not std::string::operator=? What exactly is the definition of "inbuilt function", and how does it manage to distinguish between these two member functions of std::string class? — Igor Tandetnik
– Igor Tandetnik, Commented Aug 29, 2020 at 20:45

Aplet123 · Accepted Answer · 2020-08-29 20:45:54Z

1

In your case, s2 is initialized to an empty string with a length of 0, so you can't write past the bounds. If you want to, you must first resize it:

s2.resize(s1.length());
for(i=0; s1[i]!='\0'; ++i)
{
    s2[i]=s1[i];
}

Also, c++ std::string does not need a terminating nullbyte, unlike C strings.

answered Aug 29, 2020 at 20:45

Aplet123

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