Using string pointers in C++ and Objective-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;
    }

}

Jeffery Thomas · Accepted Answer · 2012-06-13 20:32:39Z

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';

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