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主页 / user-381066

LShadow77's questions

Martin Hope
LShadow77
Asked: 2022-08-06 21:44:07 +0000 UTC

嵌套模板的问题

  • 1

假设我们有一些课程:

class Base
{
public:
    INLINE static int get_something()
    {
        return 777;
    }
    
    //....
};

并且还声明了以下模板类 A:

template <class TBase>
class A
{
public:
    template <typename TObject>
    INLINE static TObject get_something()
    {
        return (TObject)(TBase::get_something() + 1);
    }

    //....
};

现在我们可以在这样的代码中使用它:

double a = A<Base>::get_something<double>();
printf("A: %lf\n", a);

太好了,一切正常!但是,现在让我们添加一个模板类 B,它可以被认为是 A 的某种包装器:

template <class TBase>
class B
{
public:
    template <typename TObject>
    INLINE static TObject get_something()
    {
        return (TObject)(A<TBase>::get_something<TObject>() + 1); //Тут компилятор ругается!
    }

    //....
};

我们立即从编译器中得到:

main.cpp: In static member function 'static TObject B<TBase>::get_something()':
main.cpp:38:57: error: expected primary-expression before '>' token
   38 |         return (TObject)(A<TBase>::get_something<TObject>() + 1);
      |                                                         ^
main.cpp:38:59: error: expected primary-expression before ')' token
   38 |         return (TObject)(A<TBase>::get_something<TObject>() + 1);

同时,如果在表达式中我们A<TBase>::get_something<TObject>()将模板参数替换TBase为显式类型Base,那么一切都会再次起作用:

template <class TBase>
class B
{
public:
    template <typename TObject>
    INLINE static TObject get_something()
    {
        return (TObject)(A<Base>::get_something<TObject>() + 1); //А тут всё хорошо...
    }

    //....
};

//...и мы можем это использовать в коде
float b = B<Base>::get_something<float>();
printf("B: %f\n", b);

问题是,编译器不喜欢什么以及如何使其正确取消引用类模板参数A,其函数是从模板类B的模板函数调用的?类似的东西。

c++ классы
  • 1 个回答
  • 22 Views
Martin Hope
LShadow77
Asked: 2022-06-14 22:01:02 +0000 UTC

printf() 在 C++17 中不是线程安全的!

  • 0

万事如意!我在各种资料中读到 printf() 函数是线程安全的,并且可以由不同的线程调用,以便将无冲突的输出输出到控制台(当然,前提是参数传递正确)。事实上,这个函数总是处理多线程输出,我实际使用了它......直到我需要编译支持 C ++ 17 标准的程序!一般来说,这里有一个最小可重复的例子来说明问题的本质:

#include <stdio.h>
#include <conio.h>
#include <atomic>
#include <windows.h>

#define NUM_THREADS 4


std::atomic<bool> g_abExit(false);


DWORD WINAPI thread_func (LPVOID num)
{
    while (!g_abExit.load(std::memory_order_acquire))
    {
        printf("*** THREAD #%02u: This is the bug, it's not a feature!\n",(UINT)num);
        Sleep(1);
    }
    
    return 0;
}


int main()
{
    HANDLE threads[NUM_THREADS];

    printf("Enter q to quite...\n");
    Sleep(500);

    for (int i=0; i<NUM_THREADS; ++i)
        if (!(threads[i] = CreateThread(NULL,0,(LPTHREAD_START_ROUTINE)thread_func,(LPVOID)i,0,NULL))) return -1;

    while (getch()!='q');

    g_abExit.store(true,std::memory_order_release);
    for (int i=0; i<NUM_THREADS; ++i)
        WaitForSingleObject(threads[i],INFINITE);
    return 0;
}

我正在使用 MinGW GCC 编译器版本 9.2.0(构建 2)。我默认为 C++ 标准编译(我不知道默认情况下 MinGW 使用什么标准,我没有找到如何查看)并且我得到了预期的正确结果:

Enter q to quite...
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!

但是,当使用 -std=c++17 选项编译时,所有输出线程安全崩溃,导致一团糟:

Enter q to quite...
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a f*** THREAD #00: This is the bug, it's not a feature!
eature*** THREAD #!
01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a featu*** THREAD #re!
01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a featu*** THREAD #03: This is the bre!
ug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #*03: This is the bug, it's not a feature!
** THREAD #02: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #0*1:**  THThisR iEAD #00s the bu: This is the bg, ugit, it's not a fea's not a feature!
ture!
*** THREAD #02: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!
*** THREAD #00: This is the bug, it's not a feature!
*** THREAD #01: This is the bug, it's not a feature!
*** THREAD #03: This is the bug, it's not a feature!

请向我解释为什么会这样?由于哪个版本的 C++ 标准具有 printf() 函数不再是线程安全的,为什么?有没有办法在不编写自己的拐杖选项(例如,使控制台输出线程安全的编译器选项)的情况下恢复线程安全?

PS我读到了flockfile()、funlockfile()函数,试过了,但MinGW写道它们是“未声明的”。虽然使用它们也是一个马马虎虎的选择,但我仍然希望 printf() 本身来处理阻塞......

c++
  • 2 个回答
  • 10 Views

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