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Why this is illegal?

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Started by Mkk July 28, 2005 11:46 AM

5 comments, last by smart_idiot 18 years, 8 months ago

Mkk

126

Author

July 28, 2005 11:46 AM


#include <iostream>
using namespace std;

#include <iostream>
using namespace std;

int replace(char * str, char c1, char c2);

int main()
{	
	char * str = "test";

	cout << replace(str, 't', 'p');

	cout << str;
	cin.get();
	return 0;
}

int replace(char * str, char c1, char c2)
{
	int num = 0;

	while (*str != '\0')
	{
		if (*str == c1)
		{
			*str = c2; // <- Unhandled exception
			num++;
		}

		str++;

	}

	return num;
}

SiCrane

11,840

July 28, 2005 11:48 AM

The memory you are trying to write into belongs to the "test" string, which is most likely located in a readonly segment of your executable image, which means that it's illegal to modify it.

You might consider changing your code to:

char str[] = "test";
Which should give you a modifiable buffer on the stack.

Fruny

1,658

July 28, 2005 11:49 AM

Even though C++ allows you to point to a string literal with a char* for historical and compatibility reasons, you are not allowed to modify such a string: it is hard-coded in your program. String literals should be treated as const char*€. A solution would be to declare your variable to be an array rather than just a pointer. Then the compiler would actually use the literal to initialize the array, allowing you to modify that copy of the string.

"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan

demonkoryu

980

July 28, 2005 11:50 AM

"test" is a string constant, so you shouldn't modify it because it can sit in read-only memory.
[edit]...lost the race [lol][/edit]

clapton

234

July 28, 2005 12:07 PM

It's great that this topic showed up. I've always had a question on this ...

Quote:Original post by Fruny
Even though C++ allows you to point to a string literal with a char* for historical and compatibility reasons, you are not allowed to modify such a string: it is hard-coded in your program.

What is the 'string literal' really? How is it related to pointers?

char *string = "abcABC";

How does 'string' appears to a C compiler? Is it a pointer whoose address is stored as 'abcABC'? It's quite confusing for me.

Thanks

___Quote:Know where basis goes, know where rest goes.

SiCrane

11,840

July 28, 2005 12:11 PM

A string literal is usually implemented as an array of characters in a read only data segment of the executable image. It has an implicit conversion to a char * that points to the first member of the string literal.

smart_idiot

1,298

July 28, 2005 12:24 PM

With GCC and it's decendants you can compile with the -Wwrite-strings option to have it give you a warning instead of silently casting away constness.

Chess is played by three people. Two people play the game; the third provides moral support for the pawns. The object of the game is to kill your opponent by flinging captured pieces at his head. Since the only piece that can be killed is a pawn, the two armies agree to meet in a pawn-infested area (or even a pawn shop) and kill as many pawns as possible in the crossfire. If the game goes on for an hour, one player may legally attempt to gouge out the other player's eyes with his King.

Why this is illegal?

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