IN/OUT Parameters and how to work with them in C++

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This part applies to all types of paramters – most library interfaces try to be C compatible, so it is more common to pass parameters by pointer, rather than by reference.

IN: When a parameter is listed as IN it is a guarantee being offered by the interface that it won’t modify that parameter. In my opinion, this is better conveyed by marking the parameter as const, then the language itself will prevent modifications to the value. If this parameter is being passed by value, it is inconsequential whether it is marked IN in the documentation (or const in the prototype) since the parameter is local to the function anyway. But to avoid copying it may be passed by reference or by pointer, in which case the const keyword becomes very important.

OUT: A parameter marked OUT usually means that the value of the parameter when it is being passed to the function is not of any importance. In fact, if it being passed by pointer, it may even be required to be NULL, and the function will allocate memory and pass a value back to you.

IN/OUT: An IN/OUT parameter usually indicates something where both the input and output values are meaningful. For instance, if you have a library function that fills a buffer, it may require you to pass a pointer to the buffer, along with another pointer indicating the length of the buffer. When the function returns, the second pointer may contain the actual number of bytes that have been written to the buffer.

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