C++构造函数

// stock10.h ?Stock class declaration with constructors, destructor added
#ifndef STOCK1_H_
#define STOCK1_H_
#include <string>
class Stock
{
private:
    std::string company;
    long shares;
    double share_val;
    double total_val;
    void set_tot() { total_val = shares * share_val; }
public:
    Stock();        // 默认构造函数
    Stock(const std::string & co, long n = 0, double pr = 0.0);
    ~Stock();       // noisy destructor
    void buy(long num, double price);
    void sell(long num, double price);
    void update(double price);
    void show();
}；
#endif

// stock1.cpp ?Stock class implementation with constructors, destructor added
#include <iostream>
#include "stock1.h"
// constructors (verbose versions)
Stock::Stock()        // default constructor
{
    std::cout << "Default constructor called\n";
    company = "no name";
    shares = 0;
    share_val = 0.0;
    total_val = 0.0;
}

Stock::Stock(const std::string & co, long n, double pr)
{
    std::cout << "Constructor using " << co << " called\n";
    company = co;

    if (n < 0)
    {
        std::cout << "Number of shares can't be negative; "
            << company << " shares set to 0.\n";
        shares = 0;
    }
    else
        shares = n;
    share_val = pr;
    set_tot();
}
// class destructor
Stock::~Stock()        // verbose class destructor
{
    std::cout << "Bye, " << company << "!\n";
}

// other methods
void Stock::buy(long num, double price)
{
    if (num < 0)
    {
        std::cout << "Number of shares purchased can't be negative. "
            << "Transaction is aborted.\n";
    }
    else
    {
        shares += num;
        share_val = price;
        set_tot();
    }
}

void Stock::sell(long num, double price)
{
    using std::cout;
    if (num < 0)
    {
        cout << "Number of shares sold can't be negative. "
            << "Transaction is aborted.\n";
    }
    else if (num > shares)
    {
        cout << "You can't sell more than you have! "
            << "Transaction is aborted.\n";
    }
    else
    {
        shares -= num;
        share_val = price;
        set_tot();
    }
}

void Stock::update(double price)
{
    share_val = price;
    set_tot();
}

void Stock::show()
{
    using std::cout;
    using std::ios_base;
    // set format to #.###
    ios_base::fmtflags orig =
        cout.setf(ios_base::fixed, ios_base::floatfield);
    std::streamsize prec = cout.precision(3);

    cout << "Company: " << company
        << "  Shares: " << shares << '\n';
    cout << "  Share Price: $" << share_val;
    // set format to #.##
    cout.precision(2);
    cout << "  Total Worth: $" << total_val << '\n';

    // restore original format
    cout.setf(orig, ios_base::floatfield);
    cout.precision(prec);
}
int main()
{
        Stock stock1= Stock("NanoSmart", 12, 20.0);
        stock1.show();        

        Stock Stock2("Nano", 120, 40.0);
        Stock2.show();

        Stock Stock3;
        Stock3.show();

        Stock *Stock4 = new Stock("Smart",230,230);
        Stock4->show();

    system("pause");
    return 0;
}

 

构造函数中有一类成为默认构造函数，默认构造函数没有参数，因此在声明中不需要对其进行赋值。
当类中没有任何构造函数时，才会去编译默认构造函数。在定义构造函数后，必须也要为类定义默认构造函数，若是未定义默认构造函数，下面代码将会出错。
Stock Stock3;

 

构造函数的调用提供了两种方式，第一种：显式调用构造函数：
Stock stock1= Stock("NanoSmart", 12, 20.0);
另一种隐式构造函数：
Stock Stock2("Nano", 120, 40.0);