3.1 优先级与结合性

来自：http://www.learncpp.com/cpp-tutorial/31-precedence-and-associativity/

为了正确的计算类似下面这样的式子，4 + 2 * 3，我们必须知道操作符做了什么，以及它们执行的顺序，也就是说首先要明白操作符的优先级。使用常规的数学上的优先级规则，我们知道上面的式子可以这么求解 4 + （2 * 3） = 10

在C++中，所有的操作符都分配有自己的优先级。具有高优先级的步骤先被计算。你可以从下面的表中看出乘法与除法（优先级序列5）比加减法（优先级序列6）高。编译器通过这些标签确定如何对它遇到的算式求解。

如果两个操作符具有相同的优先级序列，并在表达式中相邻，结合规则告诉编译器是否从左向右或从右向左进行求值。比如，3*4/2，乘法和除法都是在等级5.具有从左向右的结合性，因此表达式可以被解释为（3*4）/2=6.

Prec/Ass	Operator	Description	Example
1 None	:: ::	Global scope (unary) Class scope (binary)	::g_nGlobalVar = 5; Class::m_nMemberVar = 5;
2 L->R	() () () [] . -> ++ –– typeid const_cast dynamic_cast reinterpret_cast static_cast	Parenthesis Function call Implicit assignment Array subscript Member access from object Member access from object ptr Post-increment Post-decrement Run-time type information Cast away const Run-time type-checked cast Cast one type to another Compile-time type-checked cast	(x + y) * 2; Add(x, y); int nValue(5); aValue[3] = 2; cObject.m_nValue = 4; pObject->m_nValue = 4; nValue++; nValue––; typeid(cClass).name(); const_cast<int*>(pnConstValue); dynamic_cast<Shape*>(pShape); reinterpret_cast<Class2>(cClass1); fValue = static_cast<float>(nValue);
3 R->L	+ - ++ –– ! ~ (type) sizeof & * new new[] delete delete[]	Unary plus Unary minus Pre-increment Pre-decrement Logical NOT Bitwise NOT C-style cast Size in bytes Address of Dereference Dynamic memory allocation Dynamic array allocation Dynamic memory deletion Dynamic array deletion	nValue = +5;M nValue = -1; ++nValue; ––nValue; if (!bValue) nFlags = ~nFlags; float fValue = (float)nValue; sizeof(int); address = &nValue; nValue = *pnValue; int *pnValue = new int; int *panValue = new int[5]; delete pnValue; delete[] panValue;
4 L->R	->* .*	Member pointer selector Member object selector	pObject->*pnValue = 24; cObject->.*pnValue = 24;
5 L->R	* / %	Multiplication Division Modulus	int nValue = 2 * 3; float fValue = 5.0 / 2.0; int nRemainder = 10 % 3;
6 L->R	+ -	Addition Subtraction	int nValue = 2 + 3; int nValue = 2 – 3;
7 L->R	<< >>	Bitwise shift left Bitwise shift right	int nFlags = 17 << 2; int nFlags = 17 >> 2;
8 L->R	< <= > >=	Comparison less than Comparison less than or equals Comparison greater than Comparison greater than or equals	if (x < y) if (x <= y) if (x > y) if (x >= y)
9 L->R	== !=	Equality Inequality	if (x == y) if (x != y)
10 L->R	&	Bitwise AND	nFlags = nFlags & 17;
11 L->R	^	Bitwise XOR	nFlags = nFlags ^ 17;
12 L->R	|	Bitwise OR	nFlags = nFlags | 17;
13 L->R	&&	Logical AND	if (bValue1 && bValue2)
14 L->R	||	Logical OR	if (bValue1 || bValue2)
15 L->R	?:	Arithmetic if	return (x < y) ? true : false;
16 R->L	= *= /= %= += -= <<= >>= &= |= ^=	Assignment Multiplication assignment Division assignment Modulus assignment Addition assignment Subtraction assignment Bitwise shift left assignment Bitwise shift right assignment Logical AND assingment Logical OR assignment Logical XOR assignment	nValue = 5; nValue *= 5; fValue /= 5.0; nValue %= 5; nValue += 5; nValue -= 5; nFlags <<= 2; nFlags >>= 2; nFlags &= 17; nFlags |= 17; nFlags ^= 17;
17 L->R	,	Comma operator	iii++, jjj++, kkk++;

随着章节的深入，对于上面操作符的认识也会越来越多的。