Using string pointers in C++ and Objective-C - c++

Question

I have a sample project here on github where I created a c++ wrapper class for an external C++ library that I want to use in Objective-C.
I don't understand why my returned pointers are sometimes correct and sometimes wrong. Here's sample output:
Test Data = 43343008
In Compress 43343008
Returned Value = 43343008
Casted Value = 43343008
Test Data = 2239023
In Compress 2239023
Returned Value = 2239023
Casted Value = 2239023
Test Data = 29459973
In Compress 29459973
Returned Value = 29459973
Casted Value = l.remote
Test Data = 64019670
In Compress 64019670
Returned Value =
Casted Value = stem.syslog.master
In the above output you can see that the 1st and 2nd click of the button outputs the results I was expecting. In each of the other clicks either the returned value or casted value are invalid. I'm assuming this is because my pointer is pointing to an address I wasn't expecting. when running the app multiple times, any button click could be right or wrong.
I also tried with a single thread but experienced similar results.
The complete code is on github but here are the important bits.
ViewController.m
#import "ViewController.h"
extern const char * CompressCodeData(const char * strToCompress);
#implementation ViewController
...
// IBAction on the button
- (IBAction)testNow:(id)sender
{
[self performSelectorInBackground:#selector(analyze) withObject:nil];
}
- (void)analyze
{
#synchronized(self) {
const char *testData = [[NSString stringWithFormat:#"%d",
(int)(arc4random() % 100000000)] UTF8String];
NSLog(#"Test Data = %s", testData);
const char *compressed = CompressCodeData(testData);
NSLog(#"Returned Value = %s", compressed);
NSString *casted = [NSString stringWithCString:compressed
encoding:NSASCIIStringEncoding];
NSLog(#"Casted Value = %#\n\n", casted);
}
}
#end
SampleWrapper.cpp
#include <iostream>
#include <string.h>
#include <CoreFoundation/CoreFoundation.h>
using namespace std;
extern "C"
{
extern void NSLog(CFStringRef format, ...);
/**
* This function simply wraps a library function so that
* it can be used in objective-c.
*/
const char * CompressCodeData(const char * strToCompress)
{
const string s(strToCompress);
// Omitted call to static method in c++ library
// to simplify this test case.
//const char *result = SomeStaticLibraryFunction(s);
const char *result = s.c_str();
NSLog(CFSTR("In Compress %s"), result);
return result;
}
}

Answer 1

You are returning a pointer to at object that has been deallocated.
const string s(strToCompress);
…
const char *result = s.c_str();
NSLog(CFSTR("In Compress %s"), result);
return result;
s does not exist after CompressCodeData() function is over, so the pointer to it's internal memory is invalid.
You could allocate a chunk of memory to hold the response, but it would be up to the caller to release it.
char *compressed = CompressCodeData(testData);
NSLog(#"Returned Value = %s", compressed);
NSString *casted = [NSString stringWithCString:compressed
encoding:NSASCIIStringEncoding];
free(compressed);
NSLog(#"Casted Value = %#\n\n", casted);
…
const char * CompressCodeData(const char * strToCompress)
…
char *result = strdup(s.c_str());
Another solution is to pass in the memory to store the data into.
char compressed[2048]; // Or whatever!
CompressCodeData(testData, compressed, sizeof(compressed));
NSLog(#"Returned Value = %s", compressed);
NSString *casted = [NSString stringWithCString:compressed
encoding:NSASCIIStringEncoding];
NSLog(#"Casted Value = %#\n\n", casted);
…
void CompressCodeData(const char * strToCompress, char *result, size_t size)
…
s.copy(result, size - 1);
result[s.length() < size ? s.length() : size-1] = '\0';