How to check if a certain memory address is available for use in c++?

Question

I'm working on my hobby project in c++, and want to test a continuous memory allocation for variables of different types, (Like array with variables of different types). How can i check if a specific memory address is available for use?

More Details:

Let's say we've got the following code: we have an integer int_var, (it doesn't matter in which memory address this variable seats), In order to allocate a variable of different type in the address right after the address of int_var i need to check if that address is available and then use it. i tried the following code:

int int_var = 5;
float* flt_ptr = (float*)(&int_var + (sizeof(int_var) / sizeof(int)));
// check if flt_ptr is successfully allocated
if (flt_ptr) { // successfully allocated
    // use that address
} else { // not successfully allocated
    cout << "ERROR";
}

The problem is: When i run the program, sometimes flt_ptr is successfully allocated and all right, and sometimes not - but when it is not successfully allocated it throws an exception that says "Read access violation ..." instead of printing "ERROR". Why is that? Maybe i missed something about checking if flt_ptr is successfully allocated? Or did something wrong? If so, How do i check if flt_ptr is successfully allocated before i use it?

Thanks!!

Answer 1

This memory model you are assuming was valid back in DOS, where, in real mode, the memory was a continuous stream of bytes.

Now that we have paging (either in x86 or in x64), this is not possible. Therefore, you can make no assumptions on the existance of memory "near" memory.

You have to allocate properly, which means using C++ shared_ptr/unique_ptr/STL. Or, new/malloc the old (bad) way.

If you want variables to be one near the other, allocate the whole memory at once (via a struct, for example).