How do declare an array in C++ as a parameter?

Question

I am working with propriety code in my iOS project and all I have access to is the header files for my project. How do you declare an array in C++ so that it will return an array when I make a call?

Here's the header file method,

 short WI_GetIDsFromList(int32_t module, int32_t *idsArray, uint32_t *count);

How do you declare an array to received an array of in32_t? I keep getting a parameter error for returnedIdsArray when I make this call? It works perfectly fine for count? I tried making it into a pointer but it did not work?

    //Array of ID's
    int32_t returnedIdsArray[] = {};

    // Array of ID's count
    uint32_t count;

 rc += WI_GetIDsFromList(mod, returnedIdsArray, &count);

Another Example

short dpCount;

//Get number of data points from the device
WI_GetDatapointCount(modIDHDS, &dpCount);

//dpCount now has returned value of method WI_GetDatapointCount
NSLog@"%d", int(dpCount);

@clyky invalid parameters. I don't think I am declaring these arrays right. Is a pointer needed for an array? — Mochi
– Mochi, Commented Dec 6, 2017 at 23:13
There is no memory there behind your pointers. Try to use int32_t **idsArray, if the function allocates memory to be returned, or provide an non zero length array as argument and indicate maximum length available in the count parameter. — Thinkeye
– Thinkeye, Commented Dec 6, 2017 at 23:17
do you mean "how do you declare a function to receive an array" ? — M.M
– M.M, Commented Dec 6, 2017 at 23:20
what is the meaning of the return value (being added to rc) ? — M.M
– M.M, Commented Dec 6, 2017 at 23:21

schmitze333 · Accepted Answer · 2017-12-06 23:42:37Z

1

I think Mochi's question is how to declare the array that it suits the need of the function given in the header. If I understand him right, he has no influence to the function taking the array as parameter.

Did you try:

int32_t returnedIdsArray[MaximumExpectedIds];

Maybe there is also a function in the API giving you the number of Ids that you could use to determine the array size.

answered Dec 6, 2017 at 23:42

schmitze333

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3 Comments

Mochi Over a year ago

Yes. Let me try this, but you are correct. Like I said these are header files that are propriety code, it helps me access a hardware device. I can only use these methods declared in a header file based on their parameters. Thank you for understanding.

schmitze333 Over a year ago

In case it does not work, please share the error message.

Mochi Over a year ago

It worked! Thank you! The maximumExpectedIds are actually returned in a different method. I didn't know I just had to change the declaration.

Chris Uzdavinis · Accepted Answer · 2017-12-06 23:17:57Z

You cannot pass an array in C or C++, because they will always decay to a pointer to the first element.

You can, however, pass a reference to an array. It retains its array type rather than decay to a pointer, so sizeof() will return the actual size of the array rather than the sizeof pointer, and so on.

void f(char(&charArray)[30])
{
}

Syntax is pretty ugly though. A type alias can help:

using CharArray30 = char(&)[30];
void f(CharArray30 charArray)
{
}

etc. It has restrictions, though. For example, you cannot pass arrays of a different size.

If you need your function to work with various sizes, you can use a function template with a non-type parameter for the size:

template <size_t SIZE>
void f(int32_t module, int32_t(&idArray)[SIZE])
{
  // ...
}

M.M · Accepted Answer · 2017-12-06 23:33:30Z

1

I guess that what you are trying to do is to have the function output a set of int values where the length is not known at compile-time.

In C++ an array has a fixed size that must be known at compile-time. The concept of "runtime-sized array" is called vector in C++.

Also, it is more natural to use the return value for values being returned. Your code could look like:

std::vector<int> WI_GetIDsFromList(int32_t mod);

and the calling code could be:

auto values = WI_GetIDsFromList(mod);

answered Dec 6, 2017 at 23:33

M.M

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