Debug Assertion Failed! Expression: __acrt_first_block == header

I had a similar problem and it turned out that my unittest project was set to a different code generation runtime library - so by setting it to the same as the DLL project, then no heap exception

As this is a DLL, the problem might lie in different heaps used for allocation and deallocation (try to build the library statically and check if that will work).

The problem is, that DLLs and templates do not agree together very well. In general, depending on the linkage of the MSVC runtime, it might be problem if the memory is allocated in the executable and deallocated in the DLL and vice versa (because they might have different heaps). And that can happen with templates very easily, for example: you push_back() to the vector inside the removeWhiteSpaces() in the DLL, so the vector memory is allocated inside the DLL. Then you use the output vector in the executable and once it gets out of scope, it is deallocated, but inside the executable whose heap doesn't know anything about the heap it has been allocated from. Bang, you're dead.

This can be worked-around if both DLL and the executable use the same heap. To ensure this, both the DLL and the executable must use the dynamic MSVC runtime - so make sure, that both link to the runtime dynamically, not statically. In particular, the exe should be compiled and linked with /MD[d] and the library with /LD[d] or /MD[d] as well, neither one with /MT[d]. Note that afterwards the computer which will be running the app will need the MSVC runtime library to run (for example, by installing "Visual C++ Redistributable" for the particular MSVC version).

You could get that work even with /MT, but that is more difficult - you would need to provide some interface which will allow the objects allocated in the DLL to be deallocated there as well. For example something like:

__declspec(dllexport) void deallocVector(std::vector<std::string> &x);

void deallocVector(std::vector<std::string> &x) {
    std::vector<std::string> tmp;
    v.swap(tmp);
}

(however this does not work very well in all cases, as this needs to be called explicitly so it will not be called e.g. in case of exception - to solve this properly, you would need to provide some interface from the DLL, which will cover the vector under the hood and will take care about the proper RAII)

---

EDIT: the final solution was actually was to have all of the projects (the exe, dll and the entire googleTest project) built in Multi-threaded Debug DLL (/MDd) (the GoogleTest projects are built in Multi-threaded debug(/MTd) by default)