为什么使用 Iterator 和 Range 的 Fibonacci 系列会抛出错误

Question

我正在练习CPP速成教程（第8章，练习1）中的C++代码片段，Link下面的代码无法编译，下面提供了程序的错误。如果提供任何有助于理解的建议或链接，将会很有帮助。 :).

#include <cstdio>
struct FibonacciIterator {
    bool operator!=(int x) const {
    return x >= current; 
    }
    FibonacciIterator& operator++() {
    const auto tmp = current; 
    current += last; 
    last = tmp; 
    return *this;
    }
    int operator*() const {
    return current;
    }
private:
    int current{ 1 }, last{ 1 };
};

struct FibonacciRange {
    explicit FibonacciRange(int max) : max{ max } { }
    FibonacciIterator begin() const { 
    return FibonacciIterator{};
    }
    int end() const { 
    return max;
    }
private:
    const int max;
};

int main() {
    for (const auto i : FibonacciRange{ 50 }) {
    printf("%d ", i); 
    }
}

错误：

user@stretch:/tmp$ g++ test.cpp -o test
test.cpp: In function ‘int main()’:
test.cpp:32:46: error: inconsistent begin/end types in range-based ‘for’ statement: ‘FibonacciIterator’ and ‘int’
     for (const auto i : FibonacciRange{ 5000 }) {
                                              ^
test.cpp:32:46: error: conversion from ‘int’ to non-scalar type ‘FibonacciIterator’ requested
test.cpp:32:46: error: no match for ‘operator!=’ (operand types are ‘FibonacciIterator’ and ‘FibonacciIterator’)
test.cpp:3:10: note: candidate: bool FibonacciIterator::operator!=(int) const
     bool operator!=(int x) const {
          ^~~~~~~~
test.cpp:3:10: note:   no known conversion for argument 1 from ‘FibonacciIterator’ to ‘int’

Answer 1

您的 begin 和 end 成员函数 return 不同类型。 Pre-C++17，这些类型需要相同，如错误信息所述

error: inconsistent begin/end types in range-based ‘for’ statement

从 C++17 开始，放宽了对 range-for 循环的限制

As of C++17, the types of the begin_expr and the end_expr do not have to be the same, and in fact the type of the end_expr does not have to be an iterator: it just needs to be able to be compared for inequality with one. This makes it possible to delimit a range by a predicate (e.g. "the iterator points at a null character").

因此，如果您使用 C++17 编译您的代码，通过传递 -std=c++17 标志，您的代码将编译，假设它满足所有其他约束，它确实如此。

为什么使用 Iterator 和 Range 的 Fibonacci 系列会抛出错误

Why does the Fibonacci Series using Iterator and Range throw error

c++

logic

c++11

c++17