在C++和Objective-C
问题描述:
使用字符串指针我有一个示例项目here on github,我创建了一个C++包装类,我想在Objective-C使用一个外部的C++库。在C++和Objective-C
我不明白为什么我返回的指针有时是正确的,有时是错误的。这里是示例输出:
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
在上面的输出中,您可以看到按钮的第一次和第二次点击输出我期待的结果。在其他每次点击中,返回值或铸造值都是无效的。我假设这是因为我的指针指向了一个我并不期待的地址。当多次运行该应用程序时,任何按钮点击都可能是正确的或错误的。
我也试过用单线程,但经历了类似的结果。
完整的代码是github但这里有重要的位。
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;
}
}
答
您在返回对象的指针已被释放。
const string s(strToCompress);
…
const char *result = s.c_str();
NSLog(CFSTR("In Compress %s"), result);
return result;
s
不存在后CompressCodeData()
功能是结束了,所以指针,它的内存是无效的。
你可以分配一块内存来保存响应,但是由调用者来释放它。
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());
另一种解决方案是传入内存来存储数据。
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';
谢谢你,这回答完美。 – JamesOR