CODEWITHSUNDEEP
c++
Viku
Viku

Reputation: 2973

Programme crashes while deallocating a character array

When I run the .exe being created with the below code in debug mode , it shows some assertion failure and the programme crashes But when i run the same exe created from the release mode of the below code , its working fine.

Please help to identify why I am geting the assertion failure in debug mode but not in release mode .

#include<iostream>
using namespace std;
#include<string.h>

void main()
{
    char *buf  = new char[5];   //pre-allocated buffer
    buf = "Hello";
    delete [] buf;
    getchar();
    //cout<<buf;
    //string *p = new (buf) string("hi");  //placement new
    //string *q = new string("hi");  //ordinary heap allocation
}

Upvotes: 2

Views: 1367

Answers (6)

nos
nos

Reputation: 229088

 char *buf  = new char[5];   //pre-allocated buffer

Here you define a pointer, and initialize it to point to a dynamically allocated buffer with room for 5 characters.

buf = "Hello";

Here you initialize the pointer to point to the beginning of a string literal.

 delete [] buf;

Here you delete[] the buf pointer, but the buf pointer no longer points to anything you new[]'d up, it points to the string literal. You can only delete/delete[] a pointer that points to something you got from new/new[]. So you get undefined behavior, and likely crash

You likely meant to copy the content of your string into the buffer you new[]'d. Remember to account for the nul terminator:

int main()
{
    char *buf  = new char[6];   //pre-allocated buffer
    strcpy(buf, "Hello");
    delete [] buf;
    getchar();
    //cout<<buf;
    //string *p = new (buf) string("hi");  //placement new
    //string *q = new string("hi");  //ordinary heap allocation
}

Though, in C++, you'd rather use std::string from #include <string>;

std::string = "Hello";

Upvotes: 4

Healer
Healer

Reputation: 290

 char *buf  = new char[6];   //pre-allocated buffer
 strncpy(buf, "hello", 6);
 delete [] buf;

buf = "hello"; would change the buf's value, from a pointer to new char[6] To a pointer point to "hello", a block of memory not in heap.

Upvotes: 2

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171127

You're trying to deallocate the character-string literal "Hello". This line:

buf = "Hello";

redirects the pointer buf to point at the literal "Hello". You probably meant to do this:

char *buf = new char[6]; //need one extra space for terminating NUL character
strcpy(buf, "Hello");

Upvotes: 2

Pete Becker
Pete Becker

Reputation: 76245

Because undefined behavior means anything can happen. The problem is that buf = "Hello" assigns the address of a string literal to buf, then tries to delete that literal. When compiled in debug mode the checking code sees that the address can't be deleted; in release mode that check isn't done, and the delete just stomps on something that isn't critical.

Upvotes: 2

melpomene
melpomene

Reputation: 85767

  1. void main is wrong. main returns int. No exceptions.
  2. You're doing delete[] "Hello". "Hello" is a string literal; you can't delete it.

Upvotes: 4

netcoder
netcoder

Reputation: 67695

When you do this:

buf = "Hello";

You're basically changing the pointer value (memory address) at which buf points to a read-only memory area, because "Hello" is a string literal and therefore is stored in read-only memory.

Then you attempt to free that memory, hence the crash.

Also, "Hello" is 6-bytes long, not 5.

Upvotes: 2

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