《C++PrimePlus》第9章 内存模型和名称空间

9.1 单独编译

Visual Studio中新建头文件和源代码

通过解决方案资源管理器，如图所示：

分成三部分的程序（直角坐标转换为极坐标）

头文件coordin.h

#ifndef __COORDIN_H__ // 如果没有被定义过
#define __COORDIN_H__

struct polar {
	double distance;
	double angle;
};

struct rect {
	double x;
	double y;
};

polar rect_to_polar(rect xypos);
void show_polar(polar dapos);

#endif // 如果被定义过了（啥也不做）

源代码file1（放置main函数，调用其他函数）

#include 
#include "coordin.h" // 尖括号表明去系统目录找，双引号表明去当前目录找
using namespace std;

int main(){
	rect rplace;
	polar pplace;
	cout << "Enter the x and y values: ";
	while (cin >> rplace.x >> rplace.y) {
		pplace = rect_to_polar(rplace);
		show_polar(pplace);
		cout << "Next two numbers (q to quit): ";
	}
	cout << "Bye!" << endl;
	return 0;
}

源代码file2（其他函数的定义）

#include 
#include 
#include "coordin.h"

polar rect_to_polar(rect xypos) {
	using namespace std;
	polar answer;
	answer.distance = sqrt(xypos.x * xypos.x + xypos.y * xypos.y);
	answer.angle = atan2(xypos.y, xypos.x);
	return answer;
}

void show_polar(polar dapos) {
	using namespace std;
	const double Rad_to_deg = 57.29577951;
	cout << "distance = " << dapos.distance;
	cout << ", angle = " << dapos.angle * Rad_to_deg;
	cout << " degrees" << endl;
}

9.2 存储持续性、作用域和链接性

全局变量和局部变量

头文件support.h

#ifndef __SUPPORT_H__
#define __SUPPORT_H__
#include 

extern double warming; // 声明外部变量(不要赋值)

void update(double dt);
void local(void);

#endif

源代码external.cpp

#include 
#include "support.h"
using namespace std;

double warming = 0.3;

int main(){
	cout << "Global warming is " << warming << endl;
	update(0.1);
	cout << "Global warming is " << warming << endl;
	local();
	return 0;
}

源代码support.cpp

#include "support.h"
using namespace std;

void update(double dt) {
	warming += dt;
	cout << "Updating global warming to " << warming << endl;
}

void local(void) {
	double warming = 0.8; // 局部变量，只在local内部可见
	cout << "Local warming is " << warming << endl;
	// 作用域解析运算符(::),放在变量名前表示使用变量的全局版本
	cout << "But global warming is " << ::warming << endl;
}

static限定符用于全局变量

源代码twofile1.cpp

#include 
using namespace std;

int tom = 3;
int dick = 30;
static int harry = 300; // 仅在twofile1.cpp可见
void remote_access(void);

int main() {
	cout << "main() reports the following addresses: " << endl;
	cout << "&tom = " << &tom << endl;
	cout << "&dick = " << &dick << endl;
	cout << "&harry= " << &harry << endl;
	remote_access();
	return 0;
}

源代码twofile2.cpp

#include 
using namespace std;

extern int tom; // 声明为外部变量(来自twofile1.cpp)
static int dick = 10; // 只在twofile2.cpp中可见
int harry = 200; // 新建一个全局变量

void remote_access(void) {
	cout << "remote_access() reports the following addresses:" << endl;
	cout << "&tom = " << &tom << endl;
	cout << "&dick = " << &dick << endl;
	cout << "&harry= " << &harry << endl;
}

static限定符用于局部变量（统计字符串的字符个数）

#include 
using namespace std;

const int ArSize = 10;
void strcount(const char * str);

int main(){
	char input[ArSize];
	char next;
	cout << "Enter a line: " << endl;
	cin.get(input, ArSize);
	while (cin) {
		cin.get(next);
		// 如果输入的内容大于10个字符，则用cin.get全部消耗掉
		while (next != '\n')
			cin.get(next);
		strcount(input);
		cout << "Enter next line (empty line to quit):" << endl;
		cin.get(input, ArSize);
	}
	cout << "Bye!" << endl;
	return 0;
}

void strcount(const char * str) {
	// 局部变量加static，无论调用多少次这个函数，该变量只会在第一次初始化
	static int total = 0;
	int count = 0;
	while (*str) {
		count++;
		str++;
	}
	total += count;
	cout << count << " characters." << endl;
	cout << total << " characters total." << endl;
}

定位new运算符的使用

#include 
#include 
using namespace std;

const int BUF = 512;
const int N = 5;
char buffer[BUF];

int main(){
	double *pd1, *pd2;
	int i;
	cout << "Calling new and placement new: " << endl;
	// 用常规new运算符为N个double类型开辟内存空间
	pd1 = new double[N];
	// 用定位new运算符为N个double类型开辟内存空间
	pd2 = new (buffer) double[N];
	// 赋值
	for (int i = 0; i < N; i++) {
		pd2[i] = pd1[i] = 1000 + 20.0 * i;
	}
	cout << "pd1 = " << pd1 << ", buffer = " << (void *)buffer << endl;
	// 打印pd1和pd2的地址
	for (int i = 0; i < N; i++) {
		cout << pd1[i] << " at " << &pd1[i] << ";";
		cout << pd2[i] << " at " << &pd2[i] << endl;
	}

	cout << "\nCalling new and placement new a second time: " << endl;
	double *pd3, *pd4;
	pd3 = new double[N];
	pd4 = new(buffer) double[N]; // 会覆盖掉原来地址里的值
	for (int i = 0; i < N; i++) {
		pd4[i] = pd3[i] = 1000 + 40.0 * i;
	}
	for (int i = 0; i < N; i++) {
		cout << pd3[i] << " at " << &pd3[i] << ";";
		cout << pd4[i] << " at " << &pd4[i] << endl;
	}

	cout << "\nCalling new and placement new a third time: " << endl;
	delete[] pd1;
	pd1 = new double[N]; // 删了重新new(申请和之前相同的地方)
	pd2 = new(buffer + N * sizeof(double)) double[N]; // 地址往后偏移5个double类型的长度
	for (int i = 0; i < N; i++) {
		pd2[i] = pd1[i] = 1000 + 60.0 * i;
	}
	for (int i = 0; i < N; i++) {
		cout << pd1[i] << " at " << &pd1[i] << ";";
		cout << pd2[i] << " at " << &pd2[i] << endl;
	}

	delete[] pd1;
	delete[] pd3; // pd2和pd4是定位new开辟出来的，delete不能用于定位new
	return 0;
}

9.3 名称空间

当名称空间和声明区域定义了相同名称（伪代码）

namespace Jill{
    double bucket(double n) {...}
    double fetch;
    struct Hill {...};
}
char fetch; // 全局变量
int main(){
    using namespace Jill; // 使用using编译指令
    Hill Thrill; // 创建一个Jill::Hill 的结构
    double water = bucket(2); // 使用Jill::bucket()
    double fetch; // 不会出错，Jill::fetch被隐藏
    cin >> fetch; // 读入一个数据到局部变量fetch
    cin >> ::fetch; // 读入一个数据到全局变量fetch
    cin >> Jill::fetch; // 读入一个变量到Jill::fetch
    ...
}

int foom(){
    Hill top; // 会出错
    Jill::Hill crest; // 可用
}

名称空间示例（打印人名及欠款）

头文件namesp.h

#pragma once
#include 

namespace pers {
	struct Person {
		std::string fname;
		std::string lname;
	};
	void getPerson(Person &rp);
	void showPerson(const Person &rp);
}

namespace debts {
	using namespace pers;
	struct Debt {
		Person name;
		double amount;
	};
	void getDebt(Debt &rd);
	void showDebt(const Debt &rd);
	double sunDebts(const Debt ar[], int n);
}

源代码namessp.cpp

#include 
#include "namesp.h"

int main() {
	using debts::Debt;
	using debts::showDebt;

	Debt golf = { {"Micheal", "Jordan"}, 120.0 };
	showDebt(golf);
	return 0;
}

源代码namesp.cpp

#include 
#include "namesp.h" // 头文件放结构体类型、函数原型声明

namespace pers {
	using std::cout;
	using std::cin;

	void getPerson(Person &rp) {
		cout << "Enter first name: ";
		cin >> rp.fname;
		cout << "Enter last name: ";
		cin >> rp.lname;
	}
	void showPerson(const Person &rp) {
		cout << rp.lname << ", " << rp.fname;
	}
}

namespace debts {
	void getDebt(Debt &rd) {
		getPerson(rd.name);
		std::cout << "Enter debt: ";
		std::cin >> rd.amount;
	}
	void showDebt(const Debt &rd) {
		showPerson(rd.name);
		std::cout << ": $" << rd.amount << std::endl;
	}
	double sunDebts(const Debt ar[], int n) {
		double total = 0;
		for (int i = 0; i < n; i++) {
			total += ar[i].amount;
		}
		return total;
	}
}