C++基础学习教程（六）----类编写的前情回顾以及项目实战(1)

在开始类的编写之前我们依然需要回顾整理一下前面所说的内容,(前面虽然是一个自定义数据类型的实现过程,但是内容有点繁杂).

先看一段代码:

/** @file calssStruct.cpp *//** Member Functions for Class point */#include <cmath> // for sqrt and atanusing namespace  std;struct point{  point()  : x_(0.0), y_(0.0)  {}  point(double x, double y)  : x_(x), y_(y)  {}  point(point const& pt)  : x_(pt.x_), y_(pt.y_)  {}  /// Distance to the origin.  double distance()  {    return std::sqrt(x*x + y*y);  }  /// Angle relative to x-axis.  double angle()  {    return std::atan2(y, x);  }  /// Add an offset to x and y.  void offset(double off)  {    offset(off, off);  }  /// Add an offset to x and an offset to y  void offset(double  xoff, double yoff)  {    x = x + xoff;    y = y + yoff;  }  /// Scale x and y.  void scale(double mult)  {    this->scale(mult, mult);  }  /// Scale x and y.  void scale(double xmult, double ymult)  {    this->x = this->x * xmult;    this->y = this->y * ymult;  }  double x_;  double y_;};

上面代码中有构造函数,有成员函数,有数据成员.我们现在需要分析一个在上面和之前的成员函数中出现的一个this引用.

在C++中,每个成员函数都有一个隐含的形参this,当成员函数被调用时,其对象本身就被编译器作为实参隐式传入.在成员函数中可以使用*this表达式访问该对象.在C++语法中点操作符比星操作符优先级高,因此需要使用圆括号(如(*this).x).另一种等价的调用时使用”箭头”,即是this->x.

但是其实编译器可以自动识别那些是成员名字,所以this->是可选的,即是可以省略的.但是有时候为了代码清晰总是加上它.而一些程序员则为了表明是数据成员,有时候往往在数据成员名称前面加上前缀,或者在后面加上后缀.我倾向于后缀,如上面的代码.

关于构造函数,初始化时类和内置类型的一个重要区别,如果定义一个内置类型而不提供初始化序列,那么它的值就是无意义的;而定义类的对象时,该对象一定会被构造器初始化,因此总有机会初始化数据成员.

尽管构造函数的冒号后面的初始化列表是可选的,但是建议总是加上.

根据数据成员的类型是类或者内置类型,编译器会做出不同的处理.每个成员的初始化表有下面的三种情况:

下面再来一个构造函数的验证代码:

/** @file constructFun.cpp *//** Visual Constructors */#include <iostream>#include <ostream>using namespace std;struct demo{  demo()      : x_(0) { std::cout << "default constructor\n"; }  demo(int x) : x_(x) { std::cout << "constructor(" << x << ")\n"; }  demo(demo const& that)  : x_(that.x_)  {    std::cout << "copy constructor(" << x_ << ")\n";  }  int x_;};demo addone(demo d){  ++d.x_;  return d;}int main(){  demo d1;  demo d2(d1);  demo d3(42);  demo d4(addone(d3));}

结果如下:

但是是否是学习到这里我们就把构造函数什么的搞明白了呢,下面我们来测试一下,先看代码:

/** @file Test_Construct.cpp *//** Mystery Program */#include <iostream>#include <ostream>using namespace std;struct point{  point()  : x_(0.0), y_(0.0)  {    cout << "default constructor\n";  }  point(double x, double y)  : x_(x), y_(y)  {    cout << "constructor(" << x << ", " << y << ")\n";  }  double x_;  double y_;};int main(){  point pt();}

你认为这个编译后会输出什么呢?是default constructor么?

但是你错了……这就是输出:

即是根本没有定义变量,编译器认为你编写了一个无参数的返回值为point的名字是pt的函数声明!

如果你不理解那么把point改成int,现在是intpt();这样是不是就更像一个函数声明了呢.但是如果要定义一个变量应该怎么样的呢?

是 point pt;即是不带括号这个时候调用了默认的无参数构造函数初始化.

所以在定义变量的时候注意哪些括号是必须的,不然很可能就会误导编译器将你的所谓的”变量声明”当成函数声明.

练习项目一

身体质量指数BMI小程序

学习了那么多可以做个小项目练习一下了.要计算BMI需要知道一个人的身高体重,BMI的计算公式是体重/(身高^2),结果是一个无单位的值.现在的任务是编写一个程序,使其能够读取记录,打印记录并计算一些统计值.该程序以请求一个BMI上极限开始,仅打印BMI值大雨或等于此极限值的记录,每条记录包含姓名(可以有空格),体重,身高(单位cm),以及性别(M或F,不限大小写).读取完每个人的记录后要立即打印该记录的BMI值,手机所有的记录后,基于数据打印两个表------男性一个,女性一个.在BMI值后面用*标记超过界限的BMI记录.并打印BMI的均值和中值.

其中的一个示例代码如下.

/** @file BMI_Again.cpp *//** New BMI Program */#include <algorithm>#include <cstdlib>#include <iomanip>#include <ios>#include <iostream>#include <istream>#include <limits>#include <locale>#include <ostream>#include <string>#include <vector>using namespace std;/// Compute body-mass index from height in centimeters and weight in kilograms.int compute_bmi(int height, int weight){   return static_cast<int>(weight * 10000 / (height * height) + 0.5);}/// Skip the rest of the input line.void skip_line(istream& in){  in.ignore(numeric_limits<int>::max(), '\n');}/// Represent one person’s record, storing the person’s name, height, weight,/// sex, and body-mass index (BMI), which is computed from the height and weight.struct record{  record() : height_(0), weight_(0), bmi_(0), sex_('?'), name_()  {}  /// Get this record, overwriting the data members.  /// Error-checking omitted for brevity.  /// @return true for success or false for eof or input failure  bool read(istream& in, int num)  {    cout << "Name " << num << ": ";    string name;    if (not getline(in, name))      return false;    cout << "Height (cm): ";    int height;    if (not (in >> height))      return false;    skip_line(in);    cout << "Weight (kg): ";    int weight;    if (not (in >> weight))      return false;    skip_line(in);    cout << "Sex (M or F): ";    char sex;    if (not (in >> sex))      return false;    skip_line(in);    sex = toupper(sex, locale());    // Store information into data members only after reading    // everything successfully.    name_ = name;    height_ = height;    weight_ = weight;    sex_ = sex;    bmi_ = compute_bmi(height_, weight_);    return true;  }  /// Print this record to @p out.  void print(ostream& out, int threshold)  {    out << setw(6) << height_        << setw(7) << weight_        << setw(3) << sex_        << setw(6) << bmi_;    if (bmi_ >= threshold)      out << '*';    else      out << ' ';    out << ' ' << name_ << '\n';  }  int height_;       ///< height in centimeters  int weight_;       ///< weight in kilograms  int bmi_;          ///< Body-mass index  char sex_;         ///< 'M' for male or 'F' for female  string name_; ///< Person’s name};/** Print a table. * Print a table of height, weight, sex, BMI, and name. * Print only records for which sex matches @p sex. * At the end of each table, print the mean and median BMI. */void print_table(char sex, vector<record>& records, int threshold){  cout << "Ht(cm) Wt(kg) Sex  BMI  Name\n";  float bmi_sum(0);  long int bmi_count(0);  vector<int> tmpbmis; // store only the BMIs that are printed                            // in order to compute the median  for (vector<record>::iterator iter(records.begin());       iter != records.end();       ++iter)  {    if (iter->sex_ == sex)    {      bmi_sum = bmi_sum + iter->bmi_;      ++bmi_count;      tmpbmis.push_back(iter->bmi_);      iter->print(cout, threshold);    }  }  // If the vectors are not empty, print basic statistics.  if (bmi_count != 0)  {    cout << "Mean BMI = "              << setprecision(1) << fixed << bmi_sum / bmi_count              << '\n';    // Median BMI is trickier. The easy way is to sort the    // vector and pick out the middle item or items.    sort(tmpbmis.begin(), tmpbmis.end());    cout << "Median BMI = ";    // Index of median item.    int i(tmpbmis.size() / 2);    if (tmpbmis.size() % 2 == 0)      cout << (tmpbmis.at(i) + tmpbmis.at(i-1)) / 2.0 << '\n';    else      cout << tmpbmis.at(i) << '\n';  }}/** Main program to compute BMI. */int main(){  locale::global(locale(""));  cout.imbue(locale());  cin.imbue(locale());  vector<record> records;  int threshold;  cout << "Enter threshold BMI: ";  if (not (cin >> threshold))    return EXIT_FAILURE;  skip_line(cin);  cout << "Enter name, height (in cm),"               " and weight (in kg) for each person:\n";  record rec;  while (rec.read(cin, records.size()+1))  {    records.push_back(rec);    cout << "BMI = " << rec.bmi_ << '\n';  }  // Print the data.  cout << "\n\nMale data\n";  print_table('M', records, threshold);  cout << "\nFemale data\n";  print_table('F', records, threshold);}

运行结果如下:

但是看到上面的这个函数:

你是否觉得函数参数传递应该是const类型更合适呢,我们修改了一下,测试,发现,,,不行,出现了错误.但是显然应该是const类型的引用传递.那怎么实现呢.

应该记得在每个成员函数的内部都有一个this隐藏参数,上面的代码段中,print_table调用print成员函数,但是经过修改,this引用了一个const对象.尽管你知道print函数不会修改任何的数据成员,但是编译器不知道,因此你需要让编译器知道,怎么做呢,只要在函数头和函数体之间加上一个const修饰符即可.如下:

而一个通用的规则是,为所有的不修改成员变量的函数添加const修饰符,它确保程序在有const对象的时候就可以调用成员函数.