做一些计数器操作，我该如何做，如果 setCounter() 没有 arguments，计数器将为零？

Question

Given the following C++ program:

#include "counterType.h"
#include <iostream>
#include <string>

using namespace std;

counterType::counterType()
{
    counter = 0;
}

counterType::counterType(int c)
{
    setCounter(c);
}

void counterType::setCounter(int ck)
{
    counter = ck;
}

int counterType::getCounter()
{
    return counter;
}

void counterType::incrementCounter()
{
    ++counter;
}

void counterType::decrementCounter()
{
    --counter;
}

void counterType::print()
{
    cout << "Counter = "<< counter << endl;
}

The code seems to only work when setCounter() has an argument in it. The only tests that fail are when is has no argument. So how do I check it in a way that if there is no argument, then the counter would be 0?

Answer 1

This is the perfect place for a default function argument. Since this is a class member function that means you need to change your function declaration to be

void setCounter(int ck = 0);

To tell the compiler that if a value is not provided for ck that it can use 0 as the default value. That means your function definition stays the same since it "pulls in" the default value from he declaration.

Answer 2

Two approaches:

1) Supply a default argument in the function declaration so you can call the function without supplying it.

void setCounter(int ck = 0);

2) Overload the function

void counterType::setCounter()
{
    counter = 0;
}

Some software houses (particularly those where the code standard guys are Java folk) disallow the first style. A less facetious reason for not liking the first way is that the default argument value can be run-time evaluable (a member variable perhaps) and that can harm program stability.

Answer 3

For starters you can initialize the data member counter in the declaration of the data member within the class definition. For example

class counterType
{
    //...
    int counter = 0;
};

In this case the constructors can look like

counterType::counterType()
{
}

counterType::counterType(int c) : counter( c )
{
}

Or you could define the constructors the following way

counterType::counterType() : counterType( 0 ) 
{
}

counterType::counterType(int c) : counter( c )
{
}

The function setCounter can have a default argument

void counterType::setCounter(int ck = 0 )
{
    counter = ck;
}

It is better to declare the function getCounter within the class definition like

int getCounter() const;

because the function does not change the object itself. And define it like

int counterType::getCounter() const
{
    return counter;
}

And this function

void counterType::print()
{
    cout << "Counter = "<< counter << endl;
}

is better to declare and define like

std::ostream & print( std::ostream &os = std::cout ) const;

and define it like

std::ostream & print( std::ostream &os = std::cout ) const;
{
    return os << "Counter = "<< counter << endl;
}

It also does not change the object itself so it should be declared with the qualifier const .

As it seems your class has only one data member then you could define the operator << for the whole class. For example

class counterType
{
    //...
    friend std::ostream & operator <<( std::ostream &os, const counterType &c );
};

std::ostream & operator <<( std::ostream &os, const counterType &c )
{
    return os << "Counter = "<< c.counter << endl;
}

Answer 4

Do you mean something like that?

void counterType::setCounter()
{
    counter = 0;
}

Answer 5

What you're trying to achieve is called function overloading : given a different list of arguments, C++ lets you define a different "version" of the same function.

void counterType::setCounter()           // No argument in there
{
    counter = 0;
}