我想教自己C++,並且我正在經歷一個關於構造函數的基本練習。我有一個程序,它被意外地表現:C++:構造函數的麻煩
Fraction.h:
#include <iostream>
#ifndef FRACTION_H
#define FRACTION_H
using namespace std;
class Fraction
{
private:
int num;
int denom;
static int gcd(int a, int b);
void reduce();
public:
Fraction(int n=0, int d=1);
Fraction(Fraction& f);
~Fraction();
Fraction& operator=(const Fraction& f);
friend Fraction operator+(const Fraction& f1, const Fraction& f2);
friend ostream& operator<<(ostream& out, const Fraction& f);
};
#endif // FRACTION_H
Fraction.cpp(一些實施方式中被省略):
#include "../include/Fraction.h"
#include <cassert>
#include <iostream>
using namespace std;
int Fraction::gcd(int a, int b) {
// implementation omitted
}
void Fraction::reduce() {
// implementation omitted
// this just reduces fractions to lowest terms, like 3/6 to 1/2
}
Fraction::Fraction(int n, int d) {
cout << "new fraction, n=" << n << ", d=" << d << endl;
assert(d != 0);
if (d < 0) {
num = -n;
denom = -d;
} else {
num = n;
denom = d;
}
reduce();
}
Fraction::Fraction(Fraction& f) {
cout << "copy fraction " << f << " at " << &f << endl;
num = f.num;
denom = f.denom;
}
Fraction::~Fraction() {
}
Fraction& Fraction::operator=(const Fraction& f) {
cout << "assign fraction to " << f << " at " << &f << endl;
if (this == &f)
return *this;
num = f.num;
denom = f.denom;
return *this;
}
Fraction operator+(const Fraction& f1, const Fraction& f2) {
cout << "adding " << f1 << " and " << f2 << endl;
return Fraction(f1.num * f2.denom + f2.num * f1.denom,
f1.denom * f2.denom);
}
ostream& operator<<(ostream& out, const Fraction& f) {
out << f.num << "/" << f.denom;
return out;
}
main.cpp中:
#include "include/Fraction.h"
#include <iostream>
using namespace std;
int main()
{
Fraction f1(1, 3);
Fraction f2(1, 2);
cout << f1 << endl;
cout << f2 << endl;
cout << (f1 + f2) << endl;
return 0;
}
當我運行這個時,前兩個打印語句輸出1/3
和1/2
如預期,但第三個打印0/1
而不是5/6
。從我的調試語句中,我通過Fraction(int, int)
構造函數創建了5/6,但由於某種原因,它隨後被調用了0/1。當我刪除複製構造函數時,代碼打印出5/6
。這裏發生了什麼,以及如何在不刪除複製構造函數的情況下修復它?
太多的代碼負責該問題缺失。你可以發佈代碼的其餘部分,還是創建一個可以展示問題的完整的可編譯示例? – 2012-04-08 23:47:35
嘿,出於好奇 - 你使用哪種編譯器?我很感興趣的是如何讓它首先允許缺少'const'。 – 2012-04-09 02:27:28