Qt and VS C++ dll function returning value

Question

I'm trying to get std::string/std::wstring returned value from connected DLL in Qt and I having problem with this.

code from DLL:

using namespace std;
extern "C++" __declspec(dllexport) string test()
{
    return "Passed!";
}

code in my Qt application (Qt Creator):

typedef std::string (*Test)();
QLibrary *lib = new QLibrary("dllname");
lib->load();
.... dll load check ....
Test test = (Test) lib->resolve("test");
std::string s = test();
QString name = QString::fromStdString(s);

In result "name" variable will have "H" insted of "Passed!" What I'm doing wrong?

Thanks in advance

Your Qt app needs to link against MSVC runtime too. Oh, and there's no extern "C++". — David Heffernan, Oct 31 '11 at 22:01
@DavidHeffernan Yes there is, though being the default it is not needed (unless you're trying to override another linkage): http://stackoverflow.com/questions/610934/why-would-you-use-extern-c — HostileFork says dont trust SE, Oct 31 '11 at 23:37

Nek · Accepted Answer · 2011-11-01T16:14:19.623

1

Thanks for your comments, I've made it:

extern "C" __declspec(dllexport) int test(wchar_t* out)
{
    wcscpy_s(out, MAX_PATH, L"Passed!"); // I'm using sys paths in my app, 
                                           // so MAX_PATH is ok for me
    return 0;
}

Qt side:

typedef int (*Test)(wchar_t*);
QLibrary *lib = new QLibrary("dllname");
lib->load();
.... dll load check ....
Test test = (Test) lib->resolve("test");
wchar_t s[MAX_PATH];
test(s);
QString name = QString::fromWCharArray(s);

Variable "name" now should be "Passed!"

In QLibrary class reference says about support only

extern "C" __declspec(dllexport)

directive.

UPDATED Thanks @MSalters

edited Nov 01 '11 at 16:14

answered Nov 01 '11 at 09:17

Nek

41
5

I think you're mistaken on the assumption. You _can_ use `std::string` in `extern "C"` functions. – MSalters Nov 01 '11 at 11:53
I was confused by compiler errors and I did wrong assumption. Thanks :) – Nek Nov 01 '11 at 15:27

score 0 · Answer 2 · answered Nov 01 '11 at 15:37

The problem is that extern "C++" functions have their name mangled. This allows overloading. extern "C" functions cannot be overloaded.

QLibrary cannot deal with overloading, nor with name mangling. It therefore needs extern "C" functions. However, these may still use C++ types.

If you fail, you will get Undefined Behavior. You're unlucky, it would have been better if it would just have crashed.

Qt and VS C++ dll function returning value

2 Answers2