獲取一個錯誤，但不明白爲什麼

Question

我有一個名爲Fraction的類，我試圖在屏幕上打印它們之前減小分數。我有一個名爲redue（）的函數試圖做到這一點，但不知何故，這不管用。下面是我有：

// Fraction.cpp 
// Implemenation for Fraction header file 
// 

#include <iostream> 
#include <iomanip> 
#include "Fraction.h" 
#include <stdexcept> 
using namespace std; 


// Constructor: empty 
Fraction:: Fraction(void) {} 

// Construcotr: Intializing numerator and denomenator 
Fraction:: Fraction(int a, int b) 
{ 
    if (b == 0) 
    { 
     throw invalid_argument("Error: division by zero"); 
    } 

    if(b != 0) 
    { 
     reduce(a,b); 
     setNumo(a); 
     setDeno(b); 
    } 
} 

// Overloading operators 
ostream &operator<<(ostream &output, const Fraction &fraction) 
{ 
    if(fraction.numo == fraction.deno) 
    { output<<1; 
     return output; } 

    if (fraction.deno == 1) 
    { output <<fraction.numo; 
     return output;} 

    if((fraction.numo >=0) && (fraction.deno > 0)) 
    { 
     output <<fraction.numo <<"/"<<fraction.deno; 
     return output; // prints a/b 
    } 

} 

// insertion operator overloading 
istream &operator>>(istream &input, Fraction &fraction) 
{ 
    int n1,n2; 
    input >> n1; 
    input >> fraction.ch; 
    input >> n2; 

    Fraction temp(n1, n2); 
    fraction.numo = temp.numo; 
    fraction.deno = temp.deno; 
    return input; 
} 

// = operator 
const Fraction& Fraction::operator=(const Fraction &f) 
{ 
    numo = f.numo; 
    deno = f.deno; 
    return *this; 
} 

Fraction Fraction::operator-(const Fraction &fraction) const { 
    Fraction temp; 
    int numo1, numo2; 
    temp.numo = numo * fraction.deno - fraction.numo * deno; 
    temp.deno = deno * fraction.deno; 
    Fraction::reduce(temp.numo, temp.deno); 
    return temp; 
} 

Fraction Fraction::operator+(const Fraction &fraction) const { 
    Fraction temp; 
    temp.numo = numo * fraction.deno + fraction.numo * deno; 
    temp.deno = deno * fraction.deno; 
    Fraction::reduce(temp.numo, temp.deno); 
    return temp; 
} 







void Fraction:: setNumo(int a) 
{ numo = a; } 

void Fraction:: setDeno(int b) 
{ deno = b; } 


static void Fraction::reduce(int &num1, int &num2) 
{ 
    int gcd; 
    gcd = find_gcd(num1, num2); 

    if(num2 != 0) 
    { 
     num1 = num1/gcd; 
     num2 = num2/gcd; 
    } 
} 



int Fraction::find_gcd(int num1, int num2) // returns gcd of two numbers 
{ 
    int gcd, remainder; 
    remainder = num1 % num2; 
    while(remainder !=0) 
    { 
     num1 = num2; 
     num2 = remainder; 
     remainder = num1 % num2; 
    } 
    gcd = num2; 
    return gcd; 
} 

這裏是我的錯誤，當我嘗試編譯該文件：

Fraction.cpp:102:50: error: cannot declare member function ‘static void Fraction::reduce(int&, int&)’ to have static linkage [-fpermissive] 
static void Fraction::reduce(int &num1, int &num2) 
               ^
Fraction.cpp: In static member function ‘static void Fraction::reduce(int&, int&)’: 
Fraction.cpp:105:30: error: cannot call member function ‘int Fraction::find_gcd(int, int)’ without object 
    gcd = find_gcd(num1, num2); 

有人能告訴我是什麼問題以及如何我可以修復這個問題？我真的需要幫助。先謝謝你。

Answer 1

static void Fraction::reduce(int &num1, int &num2) 
{ 
    int gcd; 
    gcd = find_gcd(num1, num2); <<<<<<<<<<This is a member function 

    if(num2 != 0) 
    { 
     num1 = num1/gcd; 
     num2 = num2/gcd; 
    } 
} 

當你聲明的功能的靜態編譯器不會插入該指針在函數調用，這意味着你不能直接調用任何其他成員函數，靜態函數中也不能使用類的數據成員。

Answer 2

我覺得你的兩個錯誤信息是很清楚的：

Fraction.cpp:102:50: error: cannot declare member function ‘static void Fraction::reduce(int&, int&)’ to have static linkage [-fpermissive] 
static void Fraction::reduce(int &num1, int &num2) 

這意味着你已經聲明Fraction::reduce(int, int)作爲一個成員函數（大概在Fraction.h），但現在，當你實現它，你」重新嘗試使其變爲靜態。刪除static，這將是罰款。

Fraction.cpp: In static member function ‘static void Fraction::reduce(int&, int&)’: 
Fraction.cpp:105:30: error: cannot call member function ‘int Fraction::find_gcd(int, int)’ without object 
    gcd = find_gcd(num1, num2); 

這從第一個錯誤開始流動。 C++中的成員函數是類型的函數，而不是該類型的實例。您已將reduce設爲靜態，但find_gcd爲非靜態。所以reduce正在作爲一個類函數執行，沒有this指針;當它調用find_gcd時，它不知道Fraction的哪個實例用作this指針。製作reduce非靜態也會解決這個問題。