How to pass pointer and pointer to a function? - c++

Question

I implement a function that acts like getline( .. ). So my initial approach is:
#include <cstdio>
#include <cstdlib>
#include <cstring>
void getstr( char*& str, unsigned len ) {
char c;
size_t i = 0;
while( true ) {
c = getchar(); // get a character from keyboard
if( '\n' == c || EOF == c ) { // if encountering 'enter' or 'eof'
*( str + i ) = '\0'; // put the null terminate
break; // end while
}
*( str + i ) = c;
if( i == len - 1 ) { // buffer full
len = len + len; // double the len
str = ( char* )realloc( str, len ); // reallocate memory
}
++i;
}
}
int main() {
const unsigned DEFAULT_SIZE = 4;
char* str = ( char* )malloc( DEFAULT_SIZE * sizeof( char ) );
getstr( str, DEFAULT_SIZE );
printf( str );
free( str );
return 0;
}
Then, I think I should switch to pure C instead of using half C/C++. So I change char*& to char**:
Pointer to Pointer version ( crahsed )
#include <cstdio>
#include <cstdlib>
#include <cstring>
void getstr( char** str, unsigned len ) {
char c;
size_t i = 0;
while( true ) {
c = getchar(); // get a character from keyboard
if( '\n' == c || EOF == c ) { // if encountering 'enter' or 'eof'
*( *str + i ) = '\0'; // put the null terminate
break; // done input end while
}
*( *str + i ) = c;
if( i == len - 1 ) { // buffer full
len = len + len; // double the len
*str = ( char* )realloc( str, len ); // reallocate memory
}
++i;
}
}
int main() {
const unsigned DEFAULT_SIZE = 4;
char* str = ( char* )malloc( DEFAULT_SIZE * sizeof( char ) );
getstr( &str, DEFAULT_SIZE );
printf( str );
free( str );
return 0;
}
But this version crashed, ( access violation ). I tried run the debugger, but I could not find where it crashed. I'm running Visual Studio 2010 so could you guys show me how to fix it?
Another weird thing I've encountered is that, if I leave the "&" out, it only works with Visual Studio, but not g++. That is
void getstr( char* str, unsigned len )
From my understanding, whenever we use pointer to allocate or deallocate a block of memory, we actually modify where that pointer are pointing to. So I think we have to use either ** or *& to modify the pointer. However, because it run correctly in Visual Studio, is it just luck or it should be ok either way?

Answer 1

Then, I think I should switch to pure C instead of using half C/C++.
I suggest the other direction. Go full-blown C++.

Answer 2

Your pointer crash is probably in the realloc
*str = ( char* )realloc( str, len )
Should be
*str = ( char* )realloc( *str, len )
As Steve points out, your code leaks the original if realloc fails, so maybe change it to something like:
char* tmp = (char*) realloc(*str, len)
if (tmp) {
*str = tmp
} else {
// realloc failed.. sigh
}

Answer 3

Well, running it in a debugger highlights this line
*str = ( char* )realloc( str, len ); // reallocate memory
where there is a mismatch between str - the pointer to the variable - and *str - the pointer to the memory.
I'd be tempted to rewrite it so it returns the string, or zero on error, rather than having a void return and an in/out parameter ( like fgets does, which seems to be the function you're sort-of copying the behaviour of ). Or wrap such a function. That style doesn't let you get confused as you're only ever dealing with a pointer to char, rather than a pointer to pointer to char.
char* getstr_impl ( char* str, unsigned len ) {...}
void getstr( char** str, unsigned len ) {
*str = getstr_impl ( *str, len );
}