[英]How to reimplement this with std algorithms?
I have a container of pointers to objects. 我有一个指向对象的容器容器。 The pointers are a base class, and the hierarchy implements a virtual function
count()
. 指针是基类,层次结构实现虚函数
count()
。 I want to calculate a sum of count()
in the container. 我想计算容器中
count()
的总和。
I currently do this with for_each
and a lambda function: 我目前使用
for_each
和lambda函数执行此操作:
size_t sum = 0;
std::for_each(ptrs_.begin(), ptrs_.end(), [&sum](ptr const *p) {
expr += p->count();
});
return sum;
Can anyone help me reimplement this with boost::bind
and std::accumulate
or other std algorithms? 任何人都可以帮我用
boost::bind
和std::accumulate
或其他std算法重新实现这个吗?
auto getcount = std::mem_fun(&Base::count); // nothing to bind, we just need a functor
size_t sum = std::accumulate(
boost::make_transform_iterator(ptrs_.begin(), getcount),
boost::make_transform_iterator(ptrs_.end(), getcount),
(size_t)0
);
If you don't like auto
, or more likely if your compiler doesn't, then of course you can paste the thing twice, or go looking for the return type of mem_fun
, or capture it using a function template: 如果你不喜欢
auto
,或者你的编译器不喜欢,那么当然你可以粘贴两次,或者去寻找mem_fun
的返回类型,或者使用函数模板捕获它:
template <typename IT, typename FUNC, typename T>
T transform_accumulate(IT first, IT last, T init, FUNC func) {
return std::accumulate(
boost::make_transform_iterator(first, func),
boost::make_transform_iterator(last, func),
init
);
}
Then call it as: 然后将其称为:
transform_accumulate(ptrs_.begin(), ptrs_.end(), size_t(), std::mem_fun(&Base::count));
Alternately, use the form of std::accumulate
that takes a binary functor: 或者,使用带有二元仿函数的
std::accumulate
形式:
struct AddCount {
size_t operator()(size_t result, Base *p) const {
return result + p->count();
}
};
size_t sum = std::accumulate(ptrs_.begin(), ptrs_.end(), size_t(), AddCount());
Instead of writing AddCount
, you could of course use a lambda expression. 您可以使用lambda表达式,而不是编写
AddCount
。 I expect you can construct it using the stuff in <functional>
too, but I'm not going to. 我希望你也可以使用
<functional>
的东西构建它,但我不会这样做。
I haven't tested any of this code, so let the error-spotting begin! 我没有测试过这些代码,所以让错误发现!
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