CODEWITHSUNDEEP
c++functionpointersreferencecomposition
kaaldir
kaaldir

Reputation: 61

Pointer in function argument seems not working

I'm developing in C++. I have 2 objects : which are the StandaloneAgent and the ConfigManager. I want from the Standalone Agent to reach a char * variable in the ConfigManager with a function .


int32_t StandaloneAgent::getConfigFile(char * file){
    int32_t code = mConfigMgr->getConfigFile(file);
    return code;
}

int32_t ConfigManager::getConfigFile(char * file){
    file = mConfigFile;
    return 0;
}

Here, the ConfigManager objet is an attribute of the StandaloneAgent with the ConfigManager * mConfigMgr; variable. And the file I want to reach is located in the mConfigFile attribute of the ConfigManager.

The problem is :

While putting std::cout, the mConfigFile in the is OK and the file variable after the

file = mConfigFile;

is OK

But when I return in the getConfigFile() function of the standalone agent, the file variable is a null pointer and I don't know why.

Is there a C++ specification that I am missing ?

Upvotes: 0

Views: 452

Answers (1)

Ted Lyngmo
Ted Lyngmo

Reputation: 117433

Arguments to functions are either passed by-value or by-reference. Any variable passed by-value will be local to the function and any changes made to it is local to the function too. This means that when to assign a new value to the pointer (file = mConfigFile) you only change the pointer in the function. The pointer you passed as an argument is unaffected.

When taking arguments by-reference, the variable inside the function references the variable used to call the function. Any changes made to that variable will be directly made to the variable used in the call to the function.

In your case, that means that you need to change the ConfigManager::getConfigFile function:

int32_t ConfigManager::getConfigFile(char*& file){
    file = mConfigFile;  // the change is made to the variable used as an argument
    return 0;
}

If the pointer passed to StandaloneAgent::getConfigFile is also supposed to be changed (I assume it is), you need the same change there:

int32_t StandaloneAgent::getConfigFile(char*& file){
    int32_t code = mConfigMgr->getConfigFile(file);
    return code;
}

Upvotes: 2

Related Questions