内置的C++数据类型分为两类：基本类型与复合类型。本章介绍基本类型及其变体。

3.1简单变量

变量名实际上是程序标记存放数据的内存的名称，C++中变量名的命名规则：

·只能使用字母字符、数字、下划线；

·第一个字符不能为数字；

·C++区分大小写；

·下划线开头的变量由保留实现（第四章详解）；

3.2整形变量及类别

整形变量有char,int,short,long,longlong，每种都分为有符号与无符号。

在新的标准钟，每种类型确保最小长度： （1 byte = 8 bit） 1位=8字节

short 16 bit        int 32 bit        long 32 bit        longlong 64 bit;

int可以选择为16 or 24 or 32 or 64；

/*******************************************/

// limits.cpp -- some integer limits
#include <iostream>
#include <climits>              // use limits.h for older systems
int main()
{
    using namespace std;
    int n_int = INT_MAX;        // initialize n_int to max int value
    short n_short = SHRT_MAX;   // symbols defined in climits file
    long n_long = LONG_MAX;
	long long n_llong = LLONG_MAX;

    // sizeof operator yields size of type or of variable
    cout << "int is " << sizeof (int) << " bytes." << endl;
    cout << "short is " << sizeof n_short << " bytes." << endl;
    cout << "long is " << sizeof n_long << " bytes." << endl;
    cout << "long long is " << sizeof n_llong << " bytes." << endl;
    cout << endl;

    cout << "Maximum values:" << endl;
    cout << "int: " << n_int << endl;
    cout << "short: " << n_short << endl;
    cout << "long: " << n_long << endl;
    cout << "long long: " << n_llong << endl << endl;

    cout << "Minimum int value = " << INT_MIN << endl;
    cout << "Bits per byte = " << CHAR_BIT << endl;
	// cin.get();
    return 0;
}

头文件limits定义了各种限制的符号名称。

sizeof可以获取变量的字节长度（byte），对类型名使用时需要()，对变量名使用时不需要。

变量的初始化有三种方式：直接初始化，间接初始化（用变量初始化变量），运算初始化。

/*******************************************/

// exceed.cpp -- exceeding some integer limits
#include <iostream>
#define ZERO 0      // makes ZERO symbol for 0 value
#include <climits>  // defines INT_MAX as largest int value
int main()
{
    using namespace std;
    short sam = SHRT_MAX;     // initialize a variable to max value
    unsigned short sue = sam;// okay if variable sam already defined

    cout << "Sam has " << sam << " dollars and Sue has " << sue;
    cout << " dollars deposited." << endl
         << "Add $1 to each account." << endl << "Now ";
    sam = sam + 1;
    sue = sue + 1; 
    cout << "Sam has " << sam << " dollars and Sue has " << sue;
    cout << " dollars deposited.\nPoor Sam!" << endl;
    sam = ZERO;
    sue = ZERO;
    cout << "Sam has " << sam << " dollars and Sue has " << sue;
    cout << " dollars deposited." << endl;
    cout << "Take $1 from each account." << endl << "Now ";
    sam = sam - 1;
    sue = sue - 1;
    cout << "Sam has " << sam << " dollars and Sue has " << sue;
    cout << " dollars deposited." << endl << "Lucky Sue!" << endl;
	// cin.get();
    return 0; 
}

关于无符号类型运行以上代码便可知其大概。

/*******************************************/

// hexoct1.cpp -- shows hex and octal literals
#include <iostream>
int main()
{
    using namespace std;
    int chest = 42;     // decimal integer literal
    int waist = 0x42;   // hexadecimal integer literal
    int inseam = 042;   // octal integer literal

    cout << "Monsieur cuts a striking figure!\n";
    cout << "chest = " << chest << " (42 in decimal)\n";
    cout << "waist = " << waist << " (0x42 in hex)\n";
    cout << "inseam = " << inseam << " (042 in octal)\n";
	// cin.get();
    return 0; 
}

该段代码是将42对应的十进制、十六进制、八进制用十进制表示。

// hexoct2.cpp -- display values in hex and octal
#include <iostream>
using namespace std;
int main()
{
    using namespace std;
    int chest = 42;
    int waist = 42; 
    int inseam = 42;

    cout << "Monsieur cuts a striking figure!"  << endl;
    cout << "chest = " << chest << " (decimal for 42)" << endl;
    cout << hex;      // manipulator for changing number base
    cout << "waist = " << waist << " (hexadecimal for 42)" << endl;
    cout << oct;      // manipulator for changing number base
    cout << "inseam = " << inseam << " (octal for 42)" << endl;
    // cin.get();
    return 0; 
}

cout<<控制符（dec，hex，oct）       可以指示cout以什么进制显示整数。

关于cout，显示数据时，若无特殊的后缀（L or l ; U or u ; UL or Ul or uL or ul ;LL ; ULL）或数值太大无法使用int，则通常将整形数据存储为int。

/*******************************************/

// chartype.cpp -- the char type
#include <iostream>
int main( )
{
    using namespace std;
    char ch;        // declare a char variable

    cout << "Enter a character: " << endl;
    cin >> ch;
    cout << "Hola! ";
    cout << "Thank you for the " << ch << " character." << endl;
    // cin.get();
    // cin.get();
    return 0;
}

char类型用于存储字母，一个字节大小（1byte=8bit），对应ASCII码，关于如何显示char类型变量的ASCII值可以看下面的程序：

// morechar.cpp -- the char type and int type contrasted
#include <iostream>
int main()
{
    using namespace std;
    char ch = 'M';       // assign ASCII code for M to ch
    int i = ch;          // store same code in an int
    cout << "The ASCII code for " << ch << " is " << i << endl;

    cout << "Add one to the character code:" << endl;
    ch = ch + 1;          // change character code in ch
    i = ch;               // save new character code in i
    cout << "The ASCII code for " << ch << " is " << i << endl;

    // using the cout.put() member function to display a char
    cout << "Displaying char ch using cout.put(ch): ";
    cout.put(ch);

    // using cout.put() to display a char constant
    cout.put('!');

    cout << endl << "Done" << endl;
	// cin.get();
    return 0;
}

char默认情况下不是无符号也不是有符号，可以显示设置。

宽字符类型wcahr_t有足够的空间，对应使用wcin与wcout。（涉及到时会补充）

···bool类型···

true 1  false 0

3.2 const限定符

使用const限定符可以定义常量，即不可修改的变量。（关于全部变量将在后面介绍）

3.3 浮点数

浮点数可以表示带小数部分的数字，表示范围也比整形更大。

浮点数的表示方法分为小数点表示法与科学计数表示法。

// floatnum.cpp -- floating-point types
#include <iostream>
int main()
{
    using namespace std; 
    cout.setf(ios_base::fixed, ios_base::floatfield); // fixed-point
    float tub = 10.0 / 3.0;     // good to about 6 places
    double mint = 10.0 / 3.0;   // good to about 15 places
    const float million = 1.0e6;

    cout << "tub = " << tub;
    cout << ", a million tubs = " << million * tub;
    cout << ",\nand ten million tubs = ";
    cout << 10 * million * tub << endl;

    cout << "mint = " << mint << " and a million mints = ";
    cout << million * mint << endl;
    // cin.get();
    return 0;
}

浮点数分为三种类型：float  、double 、 long double

cout会删除浮点数结尾的0，而cout.setf()则不会。

在定义浮点数常量时，使用后缀可以指定浮点数的类型: f F l L ，默认为double。

// fltadd.cpp -- precision problems with float
#include <iostream>
int main()
{
    using namespace std;
    float a = 2.34E+22f;
    float b = a + 1.0f;

    cout << "a = " << a << endl;
    cout << "b - a = " << b - a << endl;
    // cin.get();
    return 0; 
}

浮点数在运算时精度会降低，并且运行慢。关于精度问题可以参考算法竞赛入门这本书。

3.4 C++算术运算符

// arith.cpp -- some C++ arithmetic
#include <iostream>
int main()
{
    using namespace std;
    float hats, heads;

    cout.setf(ios_base::fixed, ios_base::floatfield); // fixed-point
    cout << "Enter a number: ";
    cin >> hats;
    cout << "Enter another number: ";
    cin >> heads;

    cout << "hats = " << hats << "; heads = " << heads << endl;
    cout << "hats + heads = " << hats + heads << endl;
    cout << "hats - heads = " << hats - heads << endl;
    cout << "hats * heads = " << hats * heads << endl;
    cout << "hats / heads = " << hats / heads << endl;
    // cin.get();
    // cin.get();
    return 0;
}

四则运算，乘除的优先级比加减高，两两平级时看结合性（涉及到自定义操作时，对于四则而言都是自左向右）。

// divide.cpp -- integer and floating-point division
#include <iostream>
int main()
{
    using namespace std;
    cout.setf(ios_base::fixed, ios_base::floatfield);
    cout << "Integer division: 9/5 = " << 9 / 5  << endl;
    cout << "Floating-point division: 9.0/5.0 = ";
    cout << 9.0 / 5.0 << endl;
    cout << "Mixed division: 9.0/5 = " << 9.0 / 5  << endl;
    cout << "double constants: 1e7/9.0 = ";
    cout << 1.e7 / 9.0 <<  endl;
    cout << "float constants: 1e7f/9.0f = ";
    cout << 1.e7f / 9.0f <<  endl;
    // cin.get();
    return 0;
}

除法运算则与变量类型相关，精度问题是除法的关注点。

// modulus.cpp -- uses % operator to convert lbs to stone
#include <iostream>
int main()
{
    using namespace std;
    const int Lbs_per_stn = 14;
    int lbs;

    cout << "Enter your weight in pounds: ";
    cin >> lbs;
    int stone = lbs / Lbs_per_stn;      // whole stone
    int pounds = lbs % Lbs_per_stn;     // remainder in pounds
    cout << lbs << " pounds are " << stone
         << " stone, " << pounds << " pound(s).\n";
    // cin.get();
    // cin.get();
    return 0; 
}

求模运算的操作数必须是整形，若是浮点数则会出错。、

若操作数存在负数，则符号满足（a/b）*b+a%b = a关系式。

3.5 类型转换

在对不同类型的变量进行运算时，会出现精度丢失等情况，为此C++内置有三种转换：

将一种算数类型的值赋值给另一种算数类型的变量时；

表达式中包含不同的类型时；

将参数传递给函数时。

/*****初始化和赋值进行的转换******/

C++允许将一种类型的值赋给另一种类型的变量，值将转换为接受变量的类型

short thirty=12；
long big = thirty；

但是从小范围转化为大范围通常没有问题，但是从大范围转化为小范围是会丢失数据（详情参考计算机组成结构）.

// assign.cpp -- type changes on assignment
#include <iostream>
int main()
{
    using namespace std;
    cout.setf(ios_base::fixed, ios_base::floatfield);
    float tree = 3;     // int converted to float
    int guess = 3.9832; // float converted to int
    int debt = 7.2E12;  // result not defined in C++
    cout << "tree = " << tree << endl;
    cout << "guess = " << guess << endl;
    cout << "debt = " << debt << endl;
    // cin.get();
    return 0;
}

运行之后就知道结果。

/********以{}方式初始化进行的转换********/

列表初始化通常用于为复杂的数据类型提供值列表，不允许缩窄narrowing，即变量的类型可能无法表示赋给它的值。EG:不允许将浮点数转换为整形等。（交代很少）

/*******表达式中的转换******/

将不同的类型进行算术运算时，涉及到两种数据类型时，较小转换为较大。

C++11版本校验表：

·if 一个数为long double类型，则将另一个也转换。

·else if 有一个为double，则另一个也转换。

·else if 有一个为float，则另一个也转换。

·else 都为整形，则执行整形提升

·在上述情况下，若两个操作数都是有符号或都是无符号，且一低一高，则转化为高。

·若一个无符号一个有符号，且无符号比有符号高，则有符号数转换为无符号数的类型。

·否则，若有符号类型可表示无符号类型的所有可能值（根据limits头文件可以判断出来，C++内置也可以），则将无符号数转换为有符号操作数。

·否则就将两个操作数都转换为有符号类型的无符号版本。

/*******传递参数时的转换*******/

C++原型函数控制下，char与short类型会进行整形提升，float提升为double

/*******强制类型转换**********/

// typecast.cpp -- forcing type changes
#include <iostream>
int main()
{
    using namespace std;
    int auks, bats, coots;

    // the following statement adds the values as double,
    // then converts the result to int
    auks = 19.99 + 11.99;

    // these statements add values as int
    bats = (int) 19.99 + (int) 11.99;   // old C syntax
    coots = int (19.99) + int (11.99);  // new C++ syntax
    cout << "auks = " << auks << ", bats = " << bats;
    cout << ", coots = " << coots << endl;

    char ch = 'Z';
    cout << "The code for " << ch << " is ";    // print as char
    cout << int(ch) << endl;                    // print as int
    cout << "Yes, the code is ";
    cout << static_cast<int>(ch) << endl;       // using static_cast
   // cin.get();
    return 0; 
}

由上述代码可以认识到在运算中数据存放与赋值的关系。