Memory Management with returning char* function

问题

Today, without much thought, I wrote a simple function return to a char* based on a switch statement of given enum values. This, however, made me wonder how I could release that memory. What I did was something like this:

char* func()
{
    char* retval = new char[20];
    // Switch blah blah - will always return some value other than NULL since default:
    return retval;
}

I apologize if this is a naive question, but what is the best way to release the memory seeing as I cannot delete the memory after the return and, obviously, if I delete it before, I won't have a returned value. What I was thinking as a viable solution was something like this

void func(char*& in)
{
    // blah blah switch make it do something
}

int main()
{
    char* val = new char[20];

    func(val);
    // Do whatever with func (normally func within a data structure with specific enum set so could run multiple times to change output)

    delete [] val;
    val = NULL;
    return 0;
}

Would anyone have anymore insight on this and/or explanation on which to use?

Regards,
Dennis M.

回答1:

You could write such functions in pair, like

Xyz* CreateXyz();
void DestroyXyz(Xyz *xyz);


Abc* NewAbc();
void DeleteAbc(Abc *abc);

Or you simply can transfer the responsibilty of deleting Xyz/Abc to the clients, i.e ones who call the function must also do delete on the returned object after using it.

Whatever you choose, make it clear in your documentation how the created object should be destroyed.

I would prefer pair-functions, especially if there is lot of things to consider before deleting!

By the way, you should prefer using std::string, instead of char*. Make use of STL as much as you can. They can solve most of your problems! The above advice is for situations where STL doesn't fit! In general, prefer STL!

回答2:

Have you considered using an STL type or other class instead of returning a raw pointer? For instance, if your char * is a string, use std::string instead and avoid any risk of leaks:

std::string func()
{
    std::string retval("");

    // Switch blah blah - will always return some value other than NULL since default:

    return retval;
}

回答3:

If you plan to return raw pointers from a function, you must make really clear on the documentation whose is the responsibility of deleting the pointer, i.e. who owns it. In this case, you should state explicitly that the pointer ownership is transferred to the caller, who is responsible to delete it.

Although many people are OK with just specifying the ownership in the documentation, often it's better to enforce this policy in the code. In particular, smart pointers are often used for this: the current C++ standard provides the std::auto_ptr, a smart pointer which transfers ownership on copy, i.e. when you return it to the caller, you're transferring the ownership to the target std::auto_ptr. Notice that std::auto_ptr automagically deletes the pointed memory on its destruction if it still owns it. The upcoming C++ standard provides std::unique_ptr that works in a similar fashion, but using move semantic.

Unfortunately, std::auto_ptr isn't thought for arrays (which need delete [] instead of delete), so you cannot use for your your purpose. I think that the decision of not including an auto_ptr for arrays was made deliberately, because the STL already provides all the containers you may need if you need to return a collection of items which handle by their own memory management and copy.

In particular, for strings you should simply use std::string and forget completely about this kind of memory management and pointer ownership problems.