Segmentation fault pointer to a reference

Question

I was trying out some code of Scott Meyers Effective C++, item 3 in particular.

The code in his book should be very similar to this (he left out the constructor)

#include <iostream>


class TextBlock {
public:
    TextBlock(char* ptr) : ptrText(ptr)
    {}
     char& operator[](std::size_t pos) const {
        return ptrText[pos];
    }
private :
    char* ptrText;
};

int main(int argc, char* argv[]) {
    const TextBlock block("Hello");
    std::cout << block[0] << std::endl;
    char* ptr = &block[0];
    *ptr = 'J';
    std::cout << block[0];
}

At the point where I change the contents in the pointer ptr (*ptr = 'J';), I get a segmentation fault (which normally happens when dereferencing an uninitialized or freed pointer). This is not happening here, what is going wrong at *ptr = 'J';

If you enable compiler warnings, it should tell you exactly what you're doing wrong. (And in C++11, it should be an error). — Mike Seymour, Jan 24 '14 at 03:36

score 4 · Answer 1 · answered Jan 24 '14 at 03:27

TextBlock does not own any storage. It was pointing at a constant string in memory "Hello" and you've tried to modify the read-only contents.

If you declared a local variable with its own storage, this would work, such as:

char test[32] ;
strcpy( test, "Hello") ;
const TextBlock block( test) ;
char * ptr= &block[0] ;

*ptr= 'J' ;

score 1 · Answer 2 · answered Jan 24 '14 at 04:01

You should manage

ptrText

member not as a pointer that point to a constant memory region where write is forbidden, but as a reference to a stack-based or heap-based object that you can fully control.

you can use char array instead

#include <iostream>

#define MAX_LENGTH = 1024

class TextBlock {
public:
TextBlock(char* ptr)
{
    memset(ptrText, 0, MAX_LENGTH);
    memcpy(ptrText, ptr, strlen(ptr) < MAX_LENGTH?strlen(ptr):MAX_LENGTH);
}

char& operator[](std::size_t pos) const {
    return ptrText[pos];
}
private :
char ptrText[MAX_LENGTH];

};

score 0 · Answer 3 · answered Jan 24 '14 at 03:35

#include <iostream>


class TextBlock {
public:
    TextBlock(char* ptr) : ptrText(ptr)
    {}
     char& operator[](std::size_t pos) const {
        return ptrText[pos];
    }
private :
    char* ptrText;
};

int main(int argc, char* argv[]) {
    char text[] = "Hello";
    const TextBlock block(text);
    std::cout << block[0] << std::endl;
    char* ptr = &block[0];
    *ptr = 'J';
    std::cout << block[0];
}

score 0 · Answer 4 · answered Jan 24 '14 at 03:53

0

yep this is true! ptrText is just record the ptr of "hello",and "hello" is const static text that you can't change.It didn't store anything~so you can't change it.

answered Jan 24 '14 at 03:53

jarry

26
2

Segmentation fault pointer to a reference

4 Answers4