c++ - How can we use std::transform, if we don't want to transform each element into one transformed element, but two? -

how can use std::transform, if don't want transform each element one transformed element, two?

the following pseudo code illustrates want achieve

std::transform(a.cbegin(), a.cend(), std::back_inserter(b), [](t const& x) {     return f(x) , g(x); }); 

of course, invoke std::transform 2 times, annoying. maybe need provide custom inserter. other option?

transform doing one-to-one transformation. custom inserter wouldn't anyway since transform implemented this:

while (first1 != last1) {     *d_first++ = unary_op(*first1++); // have no way write                                       // more 1 element } return d_first; 

you have write custom iterator a iterate on each element twice, , keep state in functor know if you're on f state or g state. can see how complicated getting.

anything outside of simple 1-1 transformations, should use for loop:

for (const auto& x : a) {     b.push_back(f(x));     b.push_back(g(x)); } 

and simple 1-1 transformations, think simple range-for expression wins too.

you additionally write own transform takes arbitrary number of functors:

template <typename init, typename outit, typename... functors> void transform(init first, init last, outit d_first, functors... fs) {     while (first != last) {         apply(*first, d_first, fs...);         first++;     } } 

with;

template <typename in, typename outit> void apply(const in&, outit ) { }  template <typename in, typename outit, typename f, typename... functors> void apply(const in& in, outit& out, f f, functors... fs) {     *out++ = f(in);     apply(in, out, fs...); } 

used (example):

transform(a.begin(), a.end(), back_inserter(b), f, g);