C++ 解释有符号整数和无符号整数修饰符之间的差别

C++ 允许在char、int 和 double数据类型前放置修饰符。

修饰符是用于改变变量类型的行为的关键字，它更能满足各种情境的需求。

下面列出了数据类型修饰符：

• signed：表示变量可以存储负数。对于整型变量来说，signed 可以省略，因为整型变量默认为有符号类型。

• unsigned：表示变量不能存储负数。对于整型变量来说，unsigned 可以将变量范围扩大一倍。

• short：表示变量的范围比 int 更小。short int 可以缩写为 short。

• long：表示变量的范围比 int 更大。long int 可以缩写为 long。

• long long：表示变量的范围比 long 更大。C++11 中新增的数据类型修饰符。

• float：表示单精度浮点数。

• double：表示双精度浮点数。

• bool：表示布尔类型，只有 true 和 false 两个值。

• char：表示字符类型。

• wchar_t：表示宽字符类型，可以存储 Unicode 字符。

修饰符signed、unsigned、long 和 short可应用于整型，signed和unsigned可应用于字符型，long可应用于双精度型。

这些修饰符也可以组合使用，修饰符signed和unsigned也可以作为long或short修饰符的前缀。例如：unsigned long int。

C++ 允许使用速记符号来声明无符号短整数或无符号长整数。您可以不写 int，只写单词unsigned、short或long，int是隐含的。例如，下面的两个语句都声明了无符号整型变量。

signed int num1 = -10; // 定义有符号整型变量 num1，初始值为 -10 unsigned int num2 = 20; // 定义无符号整型变量 num2，初始值为 20 short int num1 = 10; // 定义短整型变量 num1，初始值为 10 long int num2 = 100000; // 定义长整型变量 num2，初始值为 100000 long long int num1 = 10000000000; // 定义长长整型变量 num1，初始值为 10000000000 float num1 = 3.14f; // 定义单精度浮点数变量 num1，初始值为 3.14 double num2 = 2.71828; // 定义双精度浮点数变量 num2，初始值为 2.71828 bool flag = true; // 定义布尔类型变量 flag，初始值为 true char ch1 = 'a'; // 定义字符类型变量 ch1，初始值为 'a' wchar_t ch2 = L'你'; // 定义宽字符类型变量 ch2，初始值为 '你'

为了理解 C++ 解释有符号整数和无符号整数修饰符之间的差别，我们来运行一下下面这个短程序：

实例

#include <iostream> using namespace std; /* * 这个程序演示了有符号整数和无符号整数之间的差别 */ int main() { short int i; // 有符号短整数 short unsigned int j; // 无符号短整数 j = 50000; i = j;

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cout << i << " " << j; return 0; }

当上面的程序运行时，会输出下列结果：

-15536 50000

上述结果中，无符号短整数 50,000 的位模式被解释为有符号短整数 -15,536。

C++ 中的类型限定符

类型限定符提供了变量的额外信息，用于在定义变量或函数时改变它们的默认行为的关键字。

const 实例

const int NUM = 10; // 定义常量 NUM，其值不可修改 const int* ptr = # // 定义指向常量的指针，指针所指的值不可修改 int const* ptr2 = # // 和上面一行等价

volatile 实例

volatile int num = 20; // 定义变量 num，其值可能会在未知的时间被改变

mutable 实例

class Example { public: int get_value() const { return value_; // const 关键字表示该成员函数不会修改对象中的数据成员 } void set_value(int value) const { value_ = value; // mutable 关键字允许在 const 成员函数中修改成员变量 } private: mutable int value_; };

static 实例

void example_function() { static int count = 0; // static 关键字使变量 count 存储在程序生命周期内都存在 count++; }

register 实例

void example_function(register int num) { // register 关键字建议编译器将变量 num 存储在寄存器中 // 以提高程序执行速度 // 但是实际上是否会存储在寄存器中由编译器决定 }

C++11 后 register 的变化：