Error while implementing a C-API text block class in C++

Question

class Text
{
public :   
       char& operator[](int pos) const   
       {return text[pos];}

        char* get() const
       {return text;}
private: 
     char* text = "hello";
};
int main() 
{
     Text a;
    char * x = &a[0];
    *x = 's';
    
    cout << a.get() << endl;

}

I was following Scott Meyers Effective C++ book, there was a class that I had to implement myself, so I tried implementing it my self, but this program keeps crashing.

this might help you https://stackoverflow.com/questions/8302290/why-does-char-cause-undefined-behaviour-while-char-doesnt — Alberto Sinigaglia, Sep 14 '20 at 00:36
Literal strings are really arrays of **constant** characters. Attempting to modify a literal string leads to *undefined behavior*. — Some programmer dude, Sep 14 '20 at 00:55

Paul Sanders · Accepted Answer · 2020-09-14T01:17:57.053

2

This line:

char* text = "hello";

is not valid C++, since string literals decay to a const char *. With the correct command line flags, your compiler should have warned you about this.

Following on from there, *x = 's'; results in undefined behaviour, because x is pointing into said string literal. In practise, you get a seg fault, since you are trying to write to read-only memory.

Edit: Here's a fixed version using std::string:

#include <string>

class Text
{
public :   
     char& operator[](int pos) {return text[pos];}
     const char* get() const {return text.c_str ();}
private: 
     std::string text = "hello";
};

int main() 
{
    Text a;
    char * x = &a[0];
    *x = 's';
    
    std::cout << a.get() << std::endl;
}

Live demo

edited Sep 14 '20 at 01:17

answered Sep 14 '20 at 00:36

Paul Sanders

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Oh I get it, thanks, is there any way to fix it? because in the book it was given that way – X caliber Sep 14 '20 at 00:43
You can probably fashion something with [`std::string`](https://en.cppreference.com/w/cpp/string/basic_string). – Paul Sanders Sep 14 '20 at 00:44
@Xcaliber Is the book really using a pointer, or an array (as in `char text[] ="hello";`)? – Some programmer dude Sep 14 '20 at 00:57
@Someprogrammerdude OP said he had to implement that class himself. – Paul Sanders Sep 14 '20 at 00:59

Error while implementing a C-API text block class in C++

1 Answers1