C++ 将迭代器作为函数参数传递
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Pass iterator as a function parameter
提问by Iter Ator
I try to write a function, which will sum the elements of a container. This container can be Vector, List, Queue, etc... That's why I tried templates.
我尝试编写一个函数，它将对容器的元素求和。这个容器可以是 Vector、List、Queue 等……这就是我尝试模板的原因。
Unfortunately I get this error:
不幸的是，我收到此错误：
'C' is not a template
'C' 不是模板
Source:
来源：
#include <iostream>
#include <vector>
using namespace std;
template<class C, typename T>
T sum( C<T>::iterator begin, C<T>::iterator end ) {
T s = null;
for (C<T>::iterator it = begin; it != end; it++) {
s += *it;
}
return s;
}
int main()
{
vector<int> v = {5, 9, 0, 11};
cout << sum(v.begin(), v.end()) << endl;
return 0;
}
What do I wrong? How should I fix it?
我怎么了？我该如何解决？
采纳答案by Sebastian Mach
The particular error you get is because you'd need a template template argument:
你得到的特定错误是因为你需要一个模板模板参数：
template<template <typename> class C, typename T>
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^
T sum( C<T>::iterator begin, C<T>::iterator end )
However, the standard containers typically have more than just one template argument:
然而，标准容器通常不止一个模板参数：
template < class T, class Alloc = allocator<T> > class vector
and it is a bit non-trivial to write such function correctly. You could use variadic template arguments, or you could do like the standard library does, and only specialize as much as you really need:
正确编写这样的函数有点不简单。您可以使用可变参数模板参数，或者您可以像标准库那样做，并且只根据您的实际需要进行专门化：
// <algorithm>
namespace std {
template <class RandomAccessIterator>
void sort (RandomAccessIterator first, RandomAccessIterator last);
}
In your case (pretending that your need is not covered by the standard algorithms library already):
在您的情况下（假装您的需求尚未包含在标准算法库中）：
template <typename Iterator>
auto sum(Iterator begin, Iterator end)
-> decltype(*begin+*begin) // the type of summing two of them
{
if (begin == end) throw std::logic_error("....");
auto s = *begin;
++begin;
for (; begin != end; ++begin) {
s += *begin;
}
return s;
}
There are some more differences from your original code:
与您的原始代码还有一些不同之处：
- the new code does not assume a null or a default constructor defined (
T s = null;) - does not introduce additional iterator (
it) - uses pre-increment
- throws an exception when begin==end
- 新代码不假定定义为 null 或默认构造函数 (
T s = null;) - 不引入额外的迭代器 (
it) - 使用预增量
- 开始==结束时抛出异常
If you add an initparameter, you can make it almost noexcept:
如果你添加一个init参数，你几乎可以做到noexcept：
template <typename Iterator, typename T>
T sum(Iterator begin, Iterator end, T init)
{
for (; begin!=end; ++begin)
init += *begin;
return init;
}
But only almost, because init += *begincould still throw.
但也只是差不多，因为init += *begin还是会扔。
If you have such signature, you've by the way reproduced the signature of std::accumulate.
如果你有这样的签名，你顺便复制了std::accumulate.
回答by juanchopanza
You could express the whole thing in terms of a iterator type, and use iterator_traitsto get the value_type:
您可以用迭代器类型来表达整个事情，并使用它iterator_traits来获取 value_type：
#include <iterator>
template<typename Iterator>
typename std::iterator_traits<Iterator>::value_type
sum(Iterator begin, Iterator end)
{
using value_type = typename std::iterator_traits<Iterator>::value_type;
value_type s = value_type();
for (Iterator it = begin; it != end; it++) {
s += *it;
}
return s;
}
In real life, use std::accumulate:
在现实生活中，使用std::accumulate：
int sum = std::accumulate(v.begin(), v.end(), 0);
