Question

Label_Info is just a trivial class with some int members:

class Label_Info {
public:
    Label_Info();
    unsigned int x, x1, x2, y, y1, y2, size;
};

Label_Info::Label_Info() {
    this->x = this->y = this->size = 0;
    this->x1 = this->y1 = -1;
    this->x2 = this->y2 = 0;
}

Then, compiled with gcc:

std::map<unsigned int, Label_Info> labels_info;
unsigned int label = 1;
Label_Info *label_info = &labels_info[label];

Very rarely, I am stopped by an access violation on the third line. As I understand it, what’s going on is the std::map inserts a new Label_Info for the key 1 and then a pointer to that is label_info. Then (at least when it doesn’t crash!) I can access the members like this:

label_info->x = 25;

… and so on. I receive no access violation for perhaps 1,000,000 inserts, then, suddenly, bam. I checked the usual suspects: 32-bit threshold and out of memory, and everything looks OK on the insert when it detonates. I know it’s an insertion because I can see it going through that STL std::map code in the debugger but I can’t read it well enough to tell what it’s trying to access for the insertion.

So is this code bad news? Any insight from folks who know more about C++ than me would be appreciated. Thanks.

Answer 1

The code that you’ve shown us should be perfectly fine. However there is almost certainly other code you aren’t showing us that either directly causes the problem, or more likely just causes memory corruption in certain circumstances and when it hits just right the maps breaks.

If you can compile your codebase on Linux there’s a good chance that valgrind can find the memory bug for you.