c ++ friend function - operator overloading istream>> [英] c++ friend function - operator overloading istream >>
问题描述
我的问题是关于朋友的功能，以及重载<<和>>。从我的理解，我认为朋友函数可以（应该）直接访问私有成员变量。但是在这种情况下，编译器只接受我的.cxx文件，当我使用get函数来获取每个私有变量。
这里是我的头文件
class BigNum
public：
// Constructors和DESTRUCTORS
BigNum ）;
BigNum（int num，size_t optional_base = 10）;
BigNum（const char strin []，size_t optional_base = 10）;
//会员函数
size_t get_digit（size_t index）const;
size_t get_used（）const;
size_t get_capacity（）const;
size_t get_base（）const;
bool get_sign（）const;
//友好函数
friend std :: ostream& operator<<（std :: ostream& os，const BigNum& bignum）;
friend std :: istream& operator>>（std :: istream& is，BigNum& bignum）;
private：
size_t base;
size_t * digits;
bool positive;
size_t used;
这是我的对应的.cxx文件的朋友函数的实现
#includefile.h
#include< cstdlib>
#include< iostream>
#include< string>
#include< cstring>
using namespace std;
std :: ostream&运算符<<<<（std :: ostream& os，const BigNum& bignum）
{
if（bignum.get_sign（）== false）
os< ' - ';
for（size_t i = 0; i
return os;
}
std :: istream& operator>>>（std :: istream& is，BigNum& bignum）
{
for（size_t i = 0; i
return is;
}
因此在这方面上面的friend运算符正确编译。但是为什么我的操作符>>可以直接访问一个私有变量（>> bignum.digits [i]），但其余的私有变量需要通过get functions来检索
下面，当我尝试写这方面的重载操作符（我认为朋友函数应该适当地调用私有变量）：
std :: ostream&运算符<<<<（std :: ostream& os，const BigNum& bignum）
{
if（bignum.positive == false）
os< ' - ';
for（size_t i = 0; i
return os;
}
std :: istream& operator>>（std :: istream& is，BigNum& bignum）
{
for（size_t i = 0; i
return is;
}
我获得以下错误。
BigNum2.cxx：在函数`std :: ostream&
csci2270_hw1B :: operator<<<（std :: ostream& const csci2270_hw1B :: BigNum&）'：
BigNum2.cxx：201：error：`used'undeclared（first use this function）
BigNum2.cxx：201：error：（对于
，每个未声明的标识符仅报告一次。）
BigNum2.cxx：在`std :: istream&
csci2270_hw1B :: operator>>（std :: istream& csci2270_hw1B :: BigNum&）'：
BigNum2.cxx：208：error： / code> 我使用的编译器是g ++（版本3.3.1）。
我更新了代码，所以bignum对象可以访问私有变量。我做了下面的朋友操作符重载<<编译好。感谢评论，这是一个菜鸟的错误。
std :: ostream&运算符<<<<（std :: ostream& os，const BigNum& bignum）
{
if（bignum.positive == false）
os< ' - ';
for（size_t i = 0; i
return os;但是编译器仍然会为>>运算符生成错误。
BigNum2.cxx：在函数 std :: istream&
csci2270_hw1B :: operator>>（std :: istream& csci2270_hw1B :: BigNum&）'：
BigNum2.cxx：208：error：语法错误令牌
>>应该读入一个数字，私有成员变量'used'应该记录数组的长度。我仍然有点困惑为什么编译器接受
std :: istream& operator>>（std :: istream& is，BigNum& bignum）
{
for（size_t i = 0; i
return is;
}
而不是：
std :: istream& operator>>（std :: istream& is，BigNum& bignum）
{
for（size_t i = 0; i
return is;
}
有任何想法吗？感谢。
解决方案朋友函数可以访问类的私有数据，得到一个这个指向使自动，所以每一次访问类数据（私人或其他）必须合格。例如：
os<< bignum.digits [used-i-1];
必须为：
os<< bignum.digits [bignum.used - i - 1];
My question is in regards to friend functions as well as overloading the << and >>. From my understanding I thought friend functions could (and should) access private member variables directly. However in the case I have here the compiler would only accept my .cxx file when I used "get" functions to obtain each private variable.
Here is my header file
class BigNum
public:
// CONSTRUCTORS and DESTRUCTORS
BigNum();
BigNum(int num, size_t optional_base = 10);
BigNum(const char strin[], size_t optional_base = 10);
// MEMBER FUNCTIONS
size_t get_digit(size_t index) const;
size_t get_used() const;
size_t get_capacity() const;
size_t get_base() const;
bool get_sign() const;
// FRIEND FUNCTIONS
friend std::ostream& operator<<(std::ostream &os, const BigNum &bignum);
friend std::istream& operator>>(std::istream &is, BigNum &bignum);
private:
size_t base;
size_t *digits;
bool positive;
size_t used;
Here is my corresponding .cxx file with the implementations for the friend functions
#include "file.h"
#include <cstdlib>
#include <iostream>
#include <string>
#include <cstring>
using namespace std;
std::ostream& operator <<(std::ostream &os, const BigNum &bignum)
{
if (bignum.get_sign() == false)
os << '-';
for (size_t i = 0; i < bignum.get_used(); ++i)
os << bignum.get_digit(bignum.get_used() - i - 1);
return os;
}
std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.get_used(); ++i)
is >> bignum.digits[i];
return is;
}
So in this regard the above friend operators compiled correctly. However why is it that my operator >> can access one private variable directly (is >> bignum.digits[i]) but the rest of the private variables need to be retrieved by 'get functions'
Below, when I try to write the overload operators in this regard (how I thought friend functions should properly call private variables):
std::ostream& operator <<(std::ostream &os, const BigNum &bignum)
{
if (bignum.positive == false)
os << '-';
for (size_t i = 0; i < bignum.used; ++i)
os << bignum.digits[used - i - 1];
return os;
}
std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.used); ++i)
is >> bignum.digits[i];
return is;
}
I obtain the following errors.
BigNum2.cxx: In function `std::ostream&
csci2270_hw1B::operator<<(std::ostream&, const csci2270_hw1B::BigNum&)':
BigNum2.cxx:201: error: `used' undeclared (first use this function)
BigNum2.cxx:201: error: (Each undeclared identifier is reported only once for
each function it appears in.)
BigNum2.cxx: In function `std::istream&
csci2270_hw1B::operator>>(std::istream&, csci2270_hw1B::BigNum&)':
BigNum2.cxx:208: error: syntax error before `)' token
The compiler I am using is g++ (Version 3.3.1). Any help is appreciated, thank you.
Revised:
I updated the code so the bignum object could access the private variables. I did the following to the friend operator overloading << and it compiled fine. Thanks for the comments, that was a rookie mistake.
std::ostream& operator <<(std::ostream &os, const BigNum &bignum)
{
if (bignum.positive == false)
os << '-';
for (size_t i = 0; i < bignum.used; ++i)
os << bignum.digits[bignum.used - i - 1];
return os;
}
However the compiler is still producing errors for the >> operator
BigNum2.cxx: In function std::istream&
csci2270_hw1B::operator>>(std::istream&, csci2270_hw1B::BigNum&)':
BigNum2.cxx:208: error: syntax error before)' token
The >> is supposed to read in a number and the private member variable 'used' is supposed to record the length of the array. I am still somewhat confused on why the compiler accepts
std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.get_used()); ++i)
is >> bignum.digits[i];
return is;
}
as opposed to:
std::istream& operator >>(std::istream &is, BigNum &bignum)
{
for (size_t i = 0; i < bignum.used); ++i)
is >> bignum.digits[i];
return is;
}
Any thoughts? thanks.
解决方案 A friend function has access to the class' private data, but it does not get a this point to make that automatic, so every access to the class data (private or otherwise) has to be qualified. For example this:
os << bignum.digits[used - i - 1];
needs to be:
os << bignum.digits[bignum.used - i - 1];
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