定义函数后主文件出错？ "undefined reference to 'sequence::sequence()'"

Question

在main (sequence test;)的第一行代码中出现错误，说它是对sequence::sequence()的未定义引用。我不能改变main里面的代码。有人知道我该如何修复这个错误吗?

#include <cctype>       // Provides toupper
#include <iostream>     // Provides cout and cin
#include <cstdlib>      // Provides EXIT_SUCCESS
using namespace std;
class sequence
{
public:
    // TYPEDEFS and MEMBER CONSTANTS
    typedef double value_type;
    typedef std::size_t size_type;
    static const size_type CAPACITY = 30;
    // CONSTRUCTOR
    sequence( );
    // MODIFICATION MEMBER FUNCTIONS
    void start( );
    void advance( );
    void insert(const value_type& entry);
    void attach(const value_type& entry);
    void remove_current( );
    // CONSTANT MEMBER FUNCTIONS
    size_type size( ) const;
    bool is_item( ) const;
    value_type current( ) const;
private:
    value_type data[CAPACITY];
    size_type used;
    size_type current_index;
};

// PROTOTYPES for functions used by this test program:
void print_menu( );
// Postcondition: A menu of choices for this program has been written to     cout.
char get_user_command( );
// Postcondition: The user has been prompted to enter a one character command.
// The next character has been read (skipping blanks and newline characters), 
// and this character has been returned.
void show_sequence(sequence display);
// Postcondition: The items on display have been printed to cout (one per line).
double get_number( );
// Postcondition: The user has been prompted to enter a real number. The
// number has been read, echoed to the screen, and returned by the function.
sequence::size_type sequence::size( ) const
{
return (used);
}
bool sequence::is_item( ) const
{
if (used == 0)
{
    cout << "You have reached the end of the sequence" << endl;
    return (1);
}
else
return (0);
}
sequence::value_type sequence::current( ) const
{
if (is_item() == 0)
{
return (current_index); 
}
return (0);
}

void sequence::start( )
{
    current_index = data[0];
return; 
}
void sequence::advance( )
{
unsigned int i;
while (is_item() == 0)
{
    for (i = 1, current_index = data[0]; used >= i ; i++ )
    {
        current_index = data[i];
    }
}
return;
}
void sequence::insert(const value_type& entry)
{
unsigned int i;
while (size( ) < CAPACITY)
{
    for (i = 1, data[i] = current_index; used>= i ; i++)
    {
        data[i] = data[i+1];
    }
    current_index = entry;
}
return;
}
void sequence::attach(const value_type& entry)
{
    unsigned int i;
while (size( ) < CAPACITY)
{
    for (i = 1, data[i] = current_index + 1; used>= i ; i++)
    {
        data[i] = data[i+1];
    }
    current_index = entry;
}
return;
}
void sequence::remove_current( )
{
unsigned int i;
while (is_item() == 0)
{ int temp;
    temp = current_index + 1;
    --current_index;
    current_index = temp;
    for (i = 1, data[i] = current_index; used>= i ; i++)
    {
        data[i] = data[i-1];
    }
}
return;
}

int main( )
{
sequence test; // A sequence that we’ll perform tests on
char choice;   // A command character entered by the user
cout << "I have initialized an empty sequence of real numbers." << endl;
do
{
    print_menu( );
    choice = toupper(get_user_command( ));
    switch (choice)
    {
        case '!': test.start( );
                  break;
        case '+': test.advance( );
                  break;
        case '?': if (test.is_item( ))
                      cout << "There is an item." << endl;
                  else 
                      cout << "There is no current item." << endl;
                  break;
        case 'C': if (test.is_item( ))
                       cout << "Current item is: " << test.current( ) <<     endl;
                  else
                      cout << "There is no current item." << endl;
                  break;
        case 'P': show_sequence(test);
                  break;
        case 'S': cout << "Size is " << test.size( ) << '.' << endl;
                  break;
        case 'I': test.insert(get_number( ));
                  break;
        case 'A': test.attach(get_number( ));
                  break;
        case 'R': test.remove_current( );
                  cout << "The current item has been removed." << endl;
                  break;     
        case 'Q': cout << "Ridicule is the best test of truth." << endl;
                  break;
        default:  cout << choice << " is invalid." << endl;
    }
}
while ((choice != 'Q'));
return EXIT_SUCCESS;
}
void print_menu( )
// Library facilities used: iostream.h
{
cout << endl; // Print blank line before the menu
cout << "The following choices are available: " << endl;
cout << " !   Activate the start( ) function" << endl;
cout << " +   Activate the advance( ) function" << endl;
cout << " ?   Print the result from the is_item( ) function" << endl;
cout << " C   Print the result from the current( ) function" << endl;
cout << " P   Print a copy of the entire sequence" << endl;
cout << " S   Print the result from the size( ) function" << endl;
cout << " I   Insert a new number with the insert(...) function" << endl;
cout << " A   Attach a new number with the attach(...) function" << endl;
cout << " R   Activate the remove_current( ) function" << endl;
cout << " Q   Quit this test program" << endl;
}
char get_user_command( )
// Library facilities used: iostream
{
char command;
cout << "Enter choice: ";
cin >> command; // Input of characters skips blanks and newline character
return command;
}
void show_sequence(sequence display)
// Library facilities used: iostream
{
for (display.start( ); display.is_item( ); display.advance( ))
    cout << display.current( ) << endl;
}
double get_number( )
// Library facilities used: iostream
{
double result;
cout << "Please enter a real number for the sequence: ";
cin  >> result;
cout << result << " has been read." << endl;
return result;
}

Answer 1

问题是您已经声明了 sequence的默认构造函数:

class sequence
{
    ...
    // CONSTRUCTOR
    sequence( );

但是还没有定义。要定义，需要编写如下代码:

sequence::sequence( )
{
    ...
}

或者，您可以删除声明，让编译器自动为您生成默认构造函数。(只要您没有其他构造函数，并且只要编译器提供的默认行为适合您的目的，这就可以工作。)