You don't.* Keep track of it or use something like std::vector.

*without some horrible platform dependent hacks.

SDL is fundamentally a C API (although of course it works in C++ as well). It is a common trick for such APIs to behave like this: a parameter is passed in by pointer, for use as an "out parameter" (in C++ one would use references, but this option is not available in C). The function checks if the passed pointer is NULL, and skips any "reporting" in that case (so as not to cause a segfault - this may allow the function to do less work if needed). If it is not NULL, it will modify the passed-in thing via the pointer.

In the case of this SDL function, the modification is to write to the pointed-at integer value, writing the size of the array being returned (rather, the size of the allocated memory pointed at by the pointer being returned).

Here is how we may construct a std::vector instance which contains the same data as what was returned (and then the vector instance tracks the size internally, and is also capable of resizing itself when/if needed, and will also do all the rest of its own memory management - BTW, do you know whether or not you're supposed to free() that returned pointer when you're done with it? Or perhaps delete[] it?):

#include <vector>// ...int size;bool* tmp = SDL_GetKeyState(&size);vector<bool> copiedKeyState(tmp, tmp + size);

The vector constructor takes two pointers, representing a "range" of memory - from the beginning of the returned data (where the pointer is) to the end (the beginning plus the length - that points one past the end, but that's how these ranges are defined to work - Trust Us(TM), in the long run it's much more convenient that way). It will automatically copy the data over into a new memory allocation, which it "owns" (it will automatically be delete[]d when the vector falls out of scope).