std :: function分配在Visual C ++ 2015 Win32应用程序中不起作用

Question

Here's my scenario:

main.cpp

#include <Windows.h>
#include <functional>

std::function<void()> OnPrepare;

int WINAPI WinMain(HINSTANCE inst, HINSTANCE preInst, TCHAR*, int) {
    if (OnPrepare) {
        OnPrepare();
    }

    return 0;
}

other.cpp

#define _AFXDLL
#include <afx.h>  // TRACE
#include <functional>

extern std::function<void()> OnPrepare;

class Init {
public:
    Init() {
        OnPrepare = []() {
            TRACE("hello, world!\n");
        };
    }
};

Init g_init;

This code does not work in a Win32 application, but works well in a Console application. I don't know why. Can anyone point out what's wrong with my code? If I can't do it like this, is there a better way?

EDIT:

OnPrepare is always null in a Win32 application, so no "hello, world" will appear.

Answer 1

The code for constructing g_init modifies OnPrepare . This is, of course, only legal if OnPrepare has already been constructed. But what if g_init is constructed before OnPrepare ? Then you'll be modifying an object that hasn't been constructed yet and then later constructing an object you've already modified. Ouch. Doing real work in the constructors of static objects is never a good idea.

It's not clear what your outer problem is, but this code isn't a good way to solve it. An ugly workaround is to replace your global std::function with a global function that returns a reference to a function static std::function and use that. That ensures the object is constructed before being assigned to.

std::function<void()>& getOnPrepare()
{
    static std::function<void()> OnPrepare;
    return OnPrepare;
}

Then the constructor of Init can call getOnPrepare , ensuring that OnPrepare is constructed before it's assigned to:

Init() {
    getOnPrepare() = []() {
        TRACE("hello, world!\n");
    };