结构消耗太多内存

Question

I'm doing a school assignment dealing with Structs in C++. The assignment does not allow the use of STL or new / delete .

I'm representing items in a magazine.

struct PLACE {
    int itemCount;
    int etiquette[2];
};

struct SHELF {
    int placesCount;
    PLACE places[128];
};

struct RACK {
    int shelvesCount;
    SHELF shelves[128]; 
};

struct MAG {
    int racksCount;
    RACK racks[5];
};

Now, when I run the following code, it works fine, but if I change the length of racks array to something like 20, I get a runtime error (code.exe has stopped working).

PLACE place1 = {1, {1,2}};
SHELF shelf1 = {2, {place1}};   
RACK rack1 = {3, {shelf1}};
MAG mag1 = {1, {rack1}};

It seems that I'm exceeding some sort of memory? I would have thought that unlikely since when I calculated it, (when racks length is 5) I'm only using (((12 * 128) + 4) * 128 + 4) * 5 + 4 bytes, which is around 1MB.

EDIT:

I asked my instructor for some clarification on this. He told me that it's possible than my heap is exceeded, not stack , because: (quote)

Stack deals with function calls and stack exceeded could be caused by for example recursion, while here we're dealing with large data, so it's rather the heap being exceeded.

He also told me that it's possible that my computer just has a lower limit (around 4MB), while the server that our code is being checked on has been allocated 1GB to handle this much data.

Don't know if this is true though, because from what I read online, both stack and heap can be allocated variables and stack isn't reserved purely for function calls, but also for variables inside of these functions...

Answer 1

You are using a lot of automatic storage duration here. It's unlikely your compiler and architecture will permit any more than about 1Mb of such stuff.

Fortunately, the solution is trivial.

Use std::vector in place of the arrays: eg

struct MAG {
    // int racksCount; no need for this if you use a std::vector
    std::vector<RACK> racks;
};

and so on. std::vector puts most of its payload in dynamic memory which is why this approach will work. There are other C++ standard library containers too, but a good rule of thumb is to use a std::vector unless you have a good reason not to.

Answer 2

Yes, you run out of stack.

This is especially the case because you are copying data from place1 in shelf1 and the same for all oothers, all that just for one element. Use a std::vector instead of static arrays.

If you can't because it's an assignment, you can still use new / delete to use heap instead of stack.

If not, cross your fingers, and try to use:

MAG mag1 = {1, {{3, {{2, {{1, {1,2}}}}}}}};

Ugly, and you don't know what objects you are populating, but should work on modern compilers.