CODEWITHSUNDEEP
c++luastdstring
Krystian
Krystian

Reputation: 3423

Passing const char* from std::string onto Lua stack turns into null

I've got this piece of code, where I gather device id from different types of devices supported by my game and set lua global to have value of id of current device. When I get id of the iOS device I receive a const char* from a mixed C++/Objective-C class and pass it on to the Lua stack. It all works fine. However I receive std::string from a piece of code responsible for getting Android device id. When I push deviceId.c_str() I get nil in Lua. I've tried passing const char* from the code responsible for getting the device id, but then it seems something wrong goes on with the pointer when it's returned from function [that's why I decided to return string, it works fine this way].

What should I do to allow passing const char* out of std::string without problems?

EDIT: I've tried using strcpy but it didn't work :/ still having the same problem.

So.. the code responsible for gathering deviceId from different devices looks like this:

#include "DeviceInfo.h"
#include "DeviceInfoIOS.h"
#include "DeviceInfoAndroid.h"
#include <string>


USING_NS_CC;

extern "C" {

const char *getDeviceId() {

    const char *deviceId;

    CCLog("test");

#if (CC_TARGET_PLATFORM == CC_PLATFORM_IOS)
    DeviceInfoIOS ios;
    deviceId = ios.getIOSDeviceId();
    CCLog("iOS platform %s", deviceId);

#endif  // CC_PLATFORM_IOS

#if (CC_TARGET_PLATFORM == CC_PLATFORM_ANDROID)

    CCLog("Android platform");
    std::string tempId = getAndroidDeviceId();
    CCLog("Android platform test %s", tempId.c_str());
    char y[tempId.size() + 1];
    strcpy(y, tempId.c_str());
    deviceId = (const char*) y;
    CCLog("Android platform %s", deviceId);


#endif  // CC_PLATFORM_ANDROID
    CCLog("Finished platform check");
    return deviceId;
}

}

Just a small note: All the logs look ok. Devie id is passed fine.

This is how I pass device id to Lua:

//deviceInfo
CCLog("DeviceInfo load");
const char *deviceId = getDeviceId();
CCLog("DeviceInfo %s", deviceId);
lua_pushstring(d_state, deviceId);
lua_setglobal(d_state, "DEVICE_ID");

Also in here, logfile contains the device id.

Upvotes: 0

Views: 2287

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473537

Your getDeviceId function is broken. Both tempId and y are stack variables. They will be destroyed once you return. Returning pointers to stack variables is always a bad idea.

Your function ought to return a std::string. Failing that, it should return a char* array that it allocates with new, and that the user is expected to deallocate with delete. That's generally why it's preferable to just return a std::string. Alternatively, you could delcare y as a static local variable using a fixed size (rather than one based on the string).

Upvotes: 2

Related Questions