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C++ return array from function

I am trying to declare a function that returns an array of void pointers. I have the following code:

void *[] get_functions();

However I get the compilation error: expected unqualified-id before '[' token

Is what I'm trying to do valid, and if so what is my syntax error?

EDIT In reply to some of the comments, I am trying to return an array (which now will probably be a vector) of functions, which I can then randomly select one and call it. What would you suggest instead of void *?

EDIT 2 The type of functions returned will have a fixed signature (not decided yet), Let's for arguments sake say the signature will be int f(int i, int j) what would the return of my get_functions function look like, or will vector<void*> still be appropriate?

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  • Just use void * or std::vector<void*>. The [] doesn't add anything. (Think about it. What difference would it make? Also, think about what would happen if you did a sizeof on the returned value.) Commented Oct 11, 2012 at 11:50
  • 2
    Chances are, void pointers are the wrong thing to use here. Tell us more about your actual problem and we might be able to suggest a better solution. Commented Oct 11, 2012 at 11:51
  • If you need void*, you're probably doing something wrong. Commented Oct 11, 2012 at 11:51
  • I agree. Most likely there's a better way to solve your outer problem. Commented Oct 11, 2012 at 11:51
  • The correct syntax is void * get_functions()[], but it's not valid C++ (you cannot return arrays). You can however return pointers or references to arrays, like void * (& get_functions())[10]. Commented Oct 11, 2012 at 12:01

4 Answers 4

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8

C++ doesn't allow a function to return an array. You should probably return a vector instead:

std::vector<void *> get_functions();
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3 Comments

Next up, get rid of the void pointers.
What is the advantage to returning std::vector<void*> rather than a void**?
@Dasaru: All the usual advantages of using std::vector over handling all the work on your own. Basically, if you use void **, you probably need to dynamically allocate the data. That means the caller needs to free the data. Making it exception safe without an RAII wrapper is difficult at best. Bottom line: dynamic allocation is one of the leading causes of problems in C. Automating it is one of the major advantages of C++.
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There are two issues with your approach. The first of which is that you cannot return arrays from functions. In C you would return a pointer to the elements in the array, but that implies that you need to manage the memory. In C++ you can use a container, like std::vector instead of the array, and that can be returned by value.

The second issue is that you are returning function pointers, and the conversion from function pointer to void* is not guaranteed by the standard. The alternatives here start with returning a function pointer of the appropriate type (i.e. std::vector<int (*)(int,int)>) or using higher level constructs like std::function (C++11, or boost::function in C++03): std::vector<std::function<int(int,int)>>. The first approach is better suited for the description you provided as the types of the functions seem to be fixed and there will be little overhead in using the function pointers. The second approach is more generic as it can encapsulate anything that is callable with two int and return an int, including function pointers and function objects. That in turn allows you to adapt the signatures of other functions by means of std::bind or create lambdas with the appropriate signature: [](int x, int y){ return x*y;}

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1 
void **get_functions();

Later on you can then say:

void **pp = get_functions();
pp[5];     // This is the sixth pointer-to-void

If you don't already know the length of the array, you will need to pass it some other way -- for that reason, Jerry Coffin's answer is probably better.

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you can just return void pointer to first element of the array. and you can cast it inside the calling function. the array itself is passed/returned as a pointer.

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