错误：预期为“；” 在“！”之前 代币

Question

// std:: iterator sample
#include <iostream>  // std::cout
#include <iterator>  // std::iterator, std::input_iterator_tag

class MyIterator:public std::iterator<std::input_iterator_tag, int>
{
int *p;
public:
MyIterator(int *x):p(x){}
MyIterator(const MyIterator& mit):p(mit.p){}
MyIterator& operator++(){++p; return *this;}
MyIterator operator++(int){MyIterator tmp(*this);operator++(); return tmp;}
bool operator==(const MyIterator& rhs){return p == rhs.p;}
bool operator!=(const MyIterator& rhs){return p!rhs.p;}
int& operator*(){return *p;}
};

int main(){
int numbers[] = {10, 20, 30, 40, 50};
MyIterator from(numbers);
MyIterator until(numbers+5);
for (MyIterator it=from; it!=until; it++)
std::cout << *it << '';
std::cout << '\n';

return 0;
};

When I tried to get a better understanding of what an "iterator" is. I copy such code to my compiler (codeBlock). There is an error: "expected ';' before '!' token". What's the matter with that?

Answer 1

You have a typo in operator!= :

p!rhs.p

should read

p != rhs.p

or, more generically,

!(*this == rhs)

You also have an invalid empty character constant in this line:

std::cout << *it << '';
                    ^^  // Get rid of that, or change it to something sensible

Answer 2

Looks like it's this line:

bool operator!=(const MyIterator& rhs){return p!rhs.p;}

Change it to this:

bool operator!=(const MyIterator& rhs){return p != rhs.p;}

Answer 3

I suggest defining != as

bool operator==(const MyIterator& rhs){return p == rhs.p;}
bool operator!=(const MyIterator& rhs){return !(*this == rhs);}

So if == becomes more complicated you don't have to duplicate code in !=

Likewise you can define < > <= >= in terms of just < and ==, minimising code duplication

bool operator == (const MyIterator& rhs) const {return p == rhs.p;}
bool operator <  (const MyIterator& rhs) const {return p < rhs.p;}

bool operator <= (const MyIterator& rhs) const {return (*this == rhs) || (*this < rhs);}
bool operator != (const MyIterator& rhs) const {return !(*this == rhs);}
bool operator >= (const MyIterator& rhs) const {return !(*this < rhs);}
bool operator >  (const MyIterator& rhs) const {return !(*this <= rhs);}

Answer 4

I believe it's in the overload for the != operator. The line should be:

bool operator!=(const MyIterator& rhs){return p!=rhs.p;}