_beginthreadex cannot convert from 'overloaded

问题:

So I was making a function to print text layered across a different window, and I wanted it to be in a separate thread so I can run a timer for the text to display while leaving the user open to continue using the program. However, when I compile I get this error:

error C2664: '_beginthreadex' : cannot convert parameter 3 from 'overloaded-function' to 'unsigned int (__stdcall *)(void *)'

Here's the main cpp file:

#include "stdafx.h"
#include "Trial.h"

int main()
{
wchar_t* text = L"Message!";
HWND hwnd = FindWindowW(0, L"Halo");
unsigned threadID;
_beginthreadex(0, 0, DrawText,(void *)(hwnd, 175, 15, text, 8), 0 , &threadID);
// Other function here
}

And here's the header file Trial.h: (it's a little sloppy, but works fine and since most monitors are around 2ms updates, sleep(2) should help prevent flicker).

#pragma once    
#include <Windows.h>
#include <string>
#include <process.h>

void DrawText(HWND hWnd, float x, float y, wchar_t* mybuffer, float DisplayTime)
{
SetForegroundWindow(hWnd);
HDC hdc = GetDC(hWnd);
SetBkColor(hdc,RGB(255, 255, 255));                   // While Background color...
SetBkMode(hdc, TRANSPARENT);                        // Set background to transparent so we don't see the white...

int howmany = sizeof(mybuffer) * 2;

DisplayTime *= 500;
int p = 0;
while(p < DisplayTime)
{
            // Shadow Offset
    SetTextColor(hdc,RGB(0, 0, 0)); 
    TextOut(hdc,x+2,y+2, (LPCWSTR)mybuffer,howmany);

    // Primary text
    SetTextColor(hdc,RGB(255, 0, 0));   
    TextOutW(hdc,x,y,(LPCWSTR)mybuffer,howmany);

    UpdateWindow(hWnd);
    p++;
    Sleep(2);
}
ReleaseDC(hWnd,hdc);
_endthreadex(0);
}

I've looked through multiple examples, checked the syntax, and made sure that I didn't mess up on _beginthreadex, but can't seem to find the cause of the problem :|

回答1:

In a nutshell thread startup functions need to follow an exact prototype, and not the one you used.

They can accept a function that takes a single void *.

There are several solutions.

1. Change your function to accept a void*. Right away cast it to a 'struct *' of some type you have created and has the data you want. You would typically create the struct in main with new/malloc, and then delete/free it when you don't need it in the thread function.
2. The somewhat cleaner alternative is to 'new' up an object of a class you have made. Give that class a public static method that takes said void *. Use the static method as the thread starter, and pass the address of the object as 'this'. Then have the static cast the void * to the object type and call some 'start/run' routine on the object proper. Have the object delete itself before returning from the thread routine unless you have a more coordinated solution across the threads.

回答2:

_beginthreadex(0, 0, DrawText,(void *)(hwnd, 175, 15, text, 8), 0 , &threadID);

According to MSDN, third argument must be pointer to a function whose argument is of type void*. In your case, DrawText is a function whose argument is not void* but (HWND hWnd, float x, float y, wchar_t* mybuffer, float DisplayTime). Hence the error and take a look at the examples in the link.