用 range-v3 替换数据

Question

TL;DR

How is it possible to obtain the same results of

std::copy(std::begin(a), std::end(a), std::begin(b));

using range-v3, and possibly its nice syntax?

EDIT

The reason I was having troubles is the misunderstanding of how to correctly use ranges::copy : the second argument must be an iterator , not a range object. My fault ;)

Nevertheless, I am still asking if some sort of syntactic sugar is available to perform a ranged assignment , like the following:

ranges::???(b) = a | op1 | op2 | ... ; 

The problem

I have got two fixed size vectors (at runtime). I need to perform some complex transformation of the data in the first vector and store the results in the second vector. I need to keep the first vector and I do not want to create a new temporary vector.

Common code

using namespace std;
vector<double> a;
...
vector<double> b(a.size());

With std

transform(begin(a), end(a), begin(b), complexFun);

Hybrid std-rangev3

auto transformation = a | ranges::view::transform(complexFun);
copy(begin(transformation), end(transformation), begin(b));

In this simple case, it is a bit unnecessary to do so. However, if more than one operation is involved, creating the range view then using std::copy is particularly useful.

What I would like to write

ranges::???(b) = a | ranges::view::transform(complexFun);

What I am expecting is that this feature already exists, and I am not able to find it.

Answer 1

How about:

ranges::transform(a, b.begin(), complexfun);

?

EDIT: ... or maybe

ranges::copy( a | ranges::views::transform(complexFun), begin(b) );

?

Answer 2

There are a couple of good ways to do this. First, if you don't yet have the destination vector and you want to create it:

auto b = a | ranges::view::transform(complexFun) | ranges::to_vector;

Second, if you already have a destination vector whose capacity you want to reuse:

b.clear(); // Assuming b already contains junk
b |= ranges::action::push_back(a | ranges::view::transform(complexFun));

In both cases, range-v3 is smart enough to reserve capacity in the destination vector for ranges::size(a | ranges::view::transform(complexFun)) elements to avoid copies due to reallocation.