Chapter 4 C++: Expression Concept-Application of Operators

Introduces the operators defined by the language itself and used for built-in type operands, and briefly introduces several operators defined by the standard library.

Chapter 4 Expressions

Basic concepts

• Unary operator: Acts on an operand Operator

• Binary operator: Operator that acts on two operands

• The function call is also the same A special operator that has no limit on the number of operands

• Lvalue: When an object is used as an lvalue, the identity of the object (location in memory) is used ).

• Rvalue: When an object is used as an rvalue, the value (content) of the object is used.

Evaluation order

• In most cases, the evaluation order is not explicitly specified. There are only four operators that explicitly specify the order in which operands can be evaluated:

  1. 逻辑与运算符（&&）：先求左侧才求右侧，只有当左侧为真时才计算右侧的值2. 逻辑或运算符（||）3. 条件运算符（?:）4. 逗号运算符（,）

Arithmetic operators

bool b = true;bool b2 = -b;   //b2是true。-1不等于0，所以b2的值为真

• C 11 stipulates that quotients are the same Round to 0 (that is, cut off the decimal part directly).

• If m%n is not equal to 0, its sign is the same as m:

  (-m)/n 和 m/(-n) 都等于 -(m/n)
  m%(-n) 等于 m%n(-m)%n 等于 -(m%n)

Logical and relational operator

Relational operators act on arithmetic or pointer types, and logical operators act on any type that can be converted into a Boolean value.
- Their return values ​​are all Boolean type.

Logical AND&&, Logical OR||

• Short-circuit evaluation

Relational operators

The evaluation results of relational operators are Boolean values, and using them together will produce unexpected results:

if(i < j < k)   //若k大于1则为真

should be changed to:

if(i < j && j < k)

Assignment operator

• The left operand of the assignment operator must be a modifiable lvalue.

• If the left and right operands of the assignment operator have different types, the right operand will be converted to the type of the left operand.

The assignment operation satisfies the right associative law

int i, j;
i = j = 0;  //正确，都被赋值为0

Conditional operator?:

cond ? expr1 : expr2

Satisfies the right associative law, and the operands are generally from right to left sequence combination.

Bit operators

act on operands of integer type and a standard library type called bitset.

~   ：位求反
<<  ：左移
>>  ：右移
&   ：位与
^   ：位异或|   ：位或

sizeof operator

Returns the number of bytes occupied, and the resulting value is a size_t type.

有如下两种形式：sizeof(type)sizeof expr

• satisfies the right associative law.

Type conversion

Implicit type conversion

In most casesarrays are converted to pointers.
- When an array is used as a decltype keyword argument, or as the operand of operators such as the address operator (&), sizeof, and typeid, the above conversion does not occur.

Display conversion

• Named forced type conversion

cast-name<type>(expression)

cast-name has the following types:

dynamic_cast        //运行时类型识别static_cast         //只要不包含底层const，均可进行强制类型转换，告诉编译器不在乎精度损失const_cast          //只能改变运算对象的底层const，称为去掉cast性质（cast away the const）。将常量对象转换为非常量对象。reinterpret_cast    //

Reference: C Primer Fifth Edition

Introduces operators defined by the language itself and used for built-in type operands, and briefly introduces several operators defined by the standard library.

Related articles:

Chapter 1 C: Function return values, GNU compiler commands

Chapter 2 C: Variables and basic type

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