CODEWITHSUNDEEP
c++copy-constructorc++11rvalue-reference
Potatoswatter
Potatoswatter

Reputation: 137890

Best implementation of the missing unary std::copy

C++11 introduces semantics to avoid unnecessarily copying objects, and std::move to apply those semantics when otherwise a copy would occur. However, there are now also some cases where a copy is required, but not the default.

Consider this naive implementation of reverse, for example. Because range-based for uses perfect forwarding, modification to the container within the loop amounts to corruption.

auto out_iter = container.rbegin();
for ( auto value : container ) {
    * out_iter ++ = value;
}

The goal is to fix this using

for ( auto value : copy( container ) ) {

It seems simple enough… accept any argument, get the underlying type and return a temporary copy.

Upvotes: 4

Views: 214

Answers (1)

Luc Danton
Luc Danton

Reputation: 35459

The return type you're computing is better known as typename std::decay<T>::type. Other than that, use std::forward in the body to reap the full benefits of perfect forwarding:

template<typename T>
typename std::decay<T>::type
val(T&& t)
{ return std::forward<T>(t); }

Upvotes: 8

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