計算函數(shù)參數(shù)的順序是未指定行為。(這不會使你的節(jié)目崩潰，爆炸，或訂購比薩餅.不像未定行為.)

唯一的要求是在調用函數(shù)之前必須對所有參數(shù)進行完全評估。

這是：

// The simple obvious one.callFunc(getA(),getB());

可以等效于以下內容：

int a = getA();int b = getB();callFunc(a,b);

或者這個：

int b = getB();int a = getA();callFunc(a,b);

這兩者都可以；這取決于編譯器。結果可能很重要，取決于副作用。

編譯器可以自由地重新排序表達式的計算部分(假設含義不變)。

根據(jù)原來的問題：

a[i] = i++;

// This expression has three parts:

(a) a[i]

(b) i++

(c) Assign (b) to (a)

// (c) is guaranteed to happen after (a) and (b)

// But (a) and (b) can be done in either order.

// See n2521 Section 5.17

// (b) increments i but returns the original value.

// See n2521 Section 5.2.6

// Thus this expression can be written as:

int rhs  = i++;

int lhs& = a[i];

lhs = rhs;

// or

int lhs& = a[i];

int rhs  = i++;

lhs = rhs;

雙重檢查鎖定。一個容易犯的錯誤。

A* a = new A("plop");

// Looks simple enough.

// But this can be split into three parts.

(a) allocate Memory

(b) Call constructor

(c) Assign value to 'a'

// No problem here:

// The compiler is allowed to do this:

(a) allocate Memory

(c) Assign value to 'a'

(b) Call constructor.

// This is because the whole thing is between two sequence points.

// So what is the big deal.

// Simple Double checked lock. (I know there are many other problems with this).

if (a == null) // (Point B)

{

    Lock   lock(mutex);

    if (a == null)

    {

        a = new A("Plop");  // (Point A).

    }

}

a->doStuff();

// Think of this situation.

// Thread 1: Reaches point A. Executes (a)(c)

// Thread 1: Is about to do (b) and gets unscheduled.

// Thread 2: Reaches point B. It can now skip the if block

//           Remember (c) has been done thus 'a' is not NULL.

//           But the memory has not been initialized.

//           Thread 2 now executes doStuff() on an uninitialized variable.

// The solution to this problem is to move the assignment of 'a'

// To the other side of the sequence point.

if (a == null) // (Point B)

{

    Lock   lock(mutex);

    if (a == null)

    {

        A* tmp = new A("Plop");  // (Point A).

        a = tmp;

    }

}

a->doStuff();

// Of course there are still other problems because of C++ support for

// threads. But hopefully these are addresses in the next standard.