[英]Custom inserter for std::copy
Given a std::vector
which holds objects of MyClass. 给定一个
std::vector
来保存MyClass的对象。 How can I create another vector which holds just data of one member of MyClass using std::copy
? 如何使用
std::copy
创建另一个只保存MyClass成员数据的向量? I guess I would have to implement a custom back_inserter
but I could not figure out how to do this so far. 我想我必须实现一个自定义的
back_inserter
但到目前为止我无法弄清楚如何做到这一点。
struct MyClass {
int a;
}
std::vector<MyClass> vec1;
// I could copy that to another vector of type MyClass using std::copy.
std::copy(vec1.begin(), vec1.end(); std::back_inserter(someOtherVec)
// However I want just the data of the member a, how can I do that using std::copy?
std::vector<int> vec2;
Use std::transform
for that. 使用
std::transform
。
std::transform(vec1.begin(), vec1.end(), std::back_inserter(vec2),
[](const MyClass& cls) { return cls.a; });
(If you can't use C++11, you could make a function object yourself: (如果你不能使用C ++ 11,你可以自己创建一个函数对象:
struct AGetter { int operator()(const MyClass& cls) const { return cls.a; } };
std::transform(vec1.begin(), vec1.end(), std::back_inserter(vec2), AGetter());
or use std::tr1::bind
if you can use TR1: 或者如果你可以使用TR1,请使用
std::tr1::bind
:
std::transform(vec1.begin(), vec1.end(), std::back_inserter(vec2),
std::tr1::bind(&MyClass::a, std::tr1::placeholders::_1));
BTW, as @Nawaz commented below, do a .reserve()
to prevent unnecessary reallocation during the copy. BTW,正如@Nawaz在下面评论的那样,做一个
.reserve()
来防止在复制过程中不必要的重新分配。
vec2.reserve(vec1.size());
std::transform(...);
You want to use std::transform
not std::copy
and std::bind
to bind to a pointer to a member variable: 您希望使用
std::transform
而不是std::copy
和std::bind
来绑定到成员变量的指针:
#include <algorithm>
#include <iterator>
#include <vector>
#include <iostream>
#include <functional>
struct foo {
int a;
};
int main() {
const std::vector<foo> f = {{0},{1},{2}};
std::vector<int> out;
out.reserve(f.size());
std::transform(f.begin(), f.end(), std::back_inserter(out),
std::bind(&foo::a, std::placeholders::_1));
// Print to prove it worked:
std::copy(out.begin(), out.end(), std::ostream_iterator<int>(std::cout, "\n"));
}
My example is C++11, but if you skip the handy vector initalization and use boost::bind
instead this works just as well without C++11. 我的例子是C ++ 11,但是如果你跳过方便的矢量初始化并使用
boost::bind
,那么没有C ++ 11也能正常工作。
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