Access Violation When Accessing an STL Object Through A Pointer or Reference In A Different DLL or EXE

Question

I experience the following problem while using legacy VC6. I just cann't switch to modern compiler, as I am working on a legacy code base.

http://support.microsoft.com/kb/172396

Since there are no way to export map, my planned workaround is using static linking instead of dynamic linking.

I was wondering whether you all had encountered the similar situation? What is your workaround for this?

Another workaround is to create wrapper class around the stl map, to ensure creation and accessing stl map, are within the same DLL space. Note that, fun0, which uses wrapper class will just work fine. fun1 will crash.

Here is the code example :

// main.cpp. Compiled it as exe.
#pragma warning (disable : 4786)
#include <map>
#include <string>

template <class K, class V>
class __declspec(dllimport) map_wrapper {
public:
    map_wrapper();
    ~map_wrapper();    
    map_wrapper(const map_wrapper&);
    map_wrapper& operator=(const map_wrapper&);

    V& operator[](const K&); 
    const V& operator[](const K&) const;

    const V& get(const K&) const;
    void put(const K&, const V&);
    int size() const;

private:
    std::map<K, V> *m;
};

__declspec(dllimport) void fun0(map_wrapper<std::string, int>& m);
__declspec(dllimport) void fun1(std::map<std::string, int>& m);

int main () {
    map_wrapper<std::string, int> m0;
    std::map<std::string, int> m1;

    m0["hello"] = 888;
    m1["hello"] = 888;

    // Safe. The we create std::map and access map both in dll space.
    fun0(m0);
    // Crash! The we create std::map in exe space, and access map in dll space.
    fun1(m1);

    return 0;
}

---

// dll.cpp. Compiled it as dynamic dll.
#pragma warning (disable : 4786)
#include <map>
#include <string>
#include <iostream>

/* In map_wrapper.h */
template <class K, class V>
class __declspec(dllexport) map_wrapper {
public:
    map_wrapper();
    ~map_wrapper();    
    map_wrapper(const map_wrapper&);
    map_wrapper& operator=(const map_wrapper&);

    V& operator[](const K&); 
    const V& operator[](const K&) const;

    const V& get(const K&) const;
    void put(const K&, const V&);
    int size() const;

private:
    std::map<K, V> *m;
};
/* End */

/* In map_wrapper.cpp */
template <class K, class V>
map_wrapper<K, V>::map_wrapper() : m(new std::map<K, V>()) {
}

template <class K, class V>
map_wrapper<K, V>::~map_wrapper() {
    delete m;
}

template <class K, class V>
map_wrapper<K, V>::map_wrapper(const map_wrapper<K, V>& map) : m(new std::map<K, V>(*(map.m))) {
}

template <class K, class V>
map_wrapper<K, V>& map_wrapper<K, V>::operator=(const map_wrapper<K, V>& map) {
    std::map<K, V>* tmp = this->m;
    this->m = new std::map<K, V>(*(map.m));
    delete tmp;
    return *this;
}

template <class K, class V>
V& map_wrapper<K, V>::operator[](const K& key) {
    return (*this->m)[key];
}

template <class K, class V>
const V& map_wrapper<K, V>::operator[](const K& key) const {
    return (*this->m)[key];
}

template <class K, class V>
const V& map_wrapper<K, V>::get(const K& key) const {
    return (*this->m)[key];
}

template <class K, class V>
void map_wrapper<K, V>::put(const K& key, const V& value) {
    (*this->m)[key] = value;
}

template <class K, class V>
int map_wrapper<K, V>::size() const {
    return this->m->size();
}

// See : http://www.parashift.com/c++-faq-lite/templates.html#faq-35.15
// [35.15] How can I avoid linker errors with my template classes?
template class __declspec(dllexport) map_wrapper<std::string, int>;
/* End */

__declspec(dllexport) void fun0(map_wrapper<std::string, int>& m) {
    std::cout << m["hello"] << std::endl;
}

__declspec(dllexport) void fun1(std::map<std::string, int>& m) {
    std::cout << m["hello"] << std::endl;
}

Answer 1

I have never experienced this problem but I don't think that static linking is going to solve this issue for you. The access violation occurs because two compilation units are compiled with different STL implementations and are trying to access an STL object from the other unit. It shouldn't matter whether the two compilation units are statically or dynamically linked. If you cannot export map then you must export accessor functions.

Answer 2

Another thing to keep in mind is that MSVC's STL implementation comes wrapped in Microsoft's "safe" iterator checking (among other safety-related additions.) If these flags are not the same between your binary and the one you are linking with they will not be binary compatible, even if the symbols match up.

Answer 3

The KB article you linked is old, it was written before VS2008 came out. It doesn't solve the problem, it is a fundamental limitation of C++. There is no mechanism to verify that classes in separately built binaries have compatible memory layouts and were allocated with the same memory allocator.

Things you can do to avoid the problem:

• avoid exporting class objects
• build your DLLs and EXE with /MD so they'll share the memory allocator
• build all DLLs and EXE with the same compiler and CRT version
• carefully control the build settings to ensure they are the same for all projects, project property sheets are the best way to ensure this.