Thread: Problems when destroying a vector of strings

Thanks for the quick response anon. To answer your questions:

Or does this exact example have the problem (if it runs fine why did you post it?)

I posted this as an example of the operations I am performing on the vectors without bogging you down on the details. All I am doing is pushing some strings into the vector in func1 then reading them back from it after the func1 call.

vector2 is never modified after coming out of func1.

the exception always occurs when trying to deallocate the strings in vector2, whether by calling clear(), the vector going out of scope, or individually popping off each string.

The only thing I forgot to mention is that in my code, func1 is in a dll. Could Is there any special consideration I'm not managing in regards to modifying a vector across a dll?

>> Could Is there any special consideration I'm not managing in regards to modifying a vector across a dll?

Make sure all your allocations and deallocations are done in the same module (see C++ Coding Standards item 60). I don't know if that's your problem, but I do know that there can be problems if you create your vector in one place and fill it in the dll. Something tells me this isn't your problem, but it's worth a shot to reserve space for the vector before sending it to the function. There still could be a problem with the string, though.

Memory allocated in the DLL's heap is distinct from the executable's heap. This means that when you create memory inside the dll it must also be deleted inside the dll.

This is especially a problem in your case as there's a lot of stuff going on inside the string constructor and it's highly likely that it's creating local pointers when you push to the vector in the dll and therefore errors out when you release that memory inside your main program.

edit: I really like this term:

Originally Posted by brewbuck

Heap stomping.

Code:

#include <vector>
#include <string>
using std::string;
using std::vector;

//func1 is in a separate dll
void func1(const vector<string> &vector1, vector<string> &vector2)
{
//it doesn't matter what happens to vector1 because it is working properly
//disregard it for this example

//assign new values to the initialized strings
for (int i = 0; i < 8; i++)
{
vector2[i] = "Go Time!";
}
}

int main()
{
vector<string> vector1;
vector<string> vector2(20, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX");
func1(vector1, vector2);
//I was letting them go out of scope but I couldn't tell which vector was having trouble
//The problem manifests when clear() is called as well
vector1. clear();
vector2.clear(); //here is where the program breaks
return 1;
}

Even though I'm not allocating anything in the dll and not doing anything with the vector after getting it back I'm still getting an assert from IsValidHeapPointer in std::string's destructor. But if I take out the call to func1 it works just fine.

Originally Posted by brewbuck

Heap stomping. The problem lies in the code you think is not relevant.

Yep, nice choice of words there. =)

>> Even though I'm not allocating anything in the dll and not doing anything with the vector after getting it back I'm still getting an assert from IsValidHeapPointer in std::string's destructor. But if I take out the call to func1 it works just fine.

Well, in the code you posted you aren't checking the bounds of vector - try constraining it to vector2.size( ) and recheck.

I'm not sure where you mean for me to check the bounds unless it is in the "for" loop in func1. Unfortunately, checking the bounds isn't going to help because the error occurs in clear() which requires no bounds checking. I have even tried

Code:

for (int i = 0; i < vector2.size(); i++)
{
vector2.pop_back();
}

This makes it all the way up to the second or third element and then breaks with the same error.

Originally Posted by ryanrigdon

I'm not sure where you mean for me to check the bounds unless it is in the "for" loop in func1. Unfortunately, checking the bounds isn't going to help because the error occurs in clear() which requires no bounds checking. I have even tried

Code:

for (int i = 0; i < vector2.size(); i++)
{
vector2.pop_back();
}

This makes it all the way up to the second or third element and then breaks with the same error.

Can you provide a sample project file that that compiles?

Unfortunately simplifying my production code to do exactly what is shown in my last example func1 still has issues. I stripped everything else out of it and I'm still having deallocation issues. I don't have Visual Studio 6 at home so I won't be able to create a sample project this weekend but I'll make one when I get back to work on Monday. If anyone has a suggestion of another way to do this it would be welcomed with open arms. Thanks for all of your help so far.

Sorry for not reading the entire thread since the last time I checked in, but if you're assigning new strings in the dll and the vector is destroyed in the main codeline, then that will still be the same problem (strings allocate memory).

If you really wanted to verify that this isn't the problem, and you know the maximum size of your strings, try something like this:

Code:

const unsigned max_vec_size = 100; // or whatever
const unsigned max_string_size = 100; // or whatever
std::vector<std::string> my_vec(max_vec_size, std::string (max_string_size, '\0'));

my_func_in_dll(my_vec);

Hopefully that will avoid any string allocations in the dll.

Ok, so I found the solution to my problem. I was doing everything correctly except that I'm using an old microsoft product. This was in an obscure MS KB article STL std::string class causes crashes and memory corruption on multi-processor machines.

Long story short: the entire STL that shipped with VC++ 6 is not thread-safe, especially std:: string. Thanks for all of your help and sorry for the confusion.