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Questions tagged [move-semantics]

Move semantics is a programming language feature that allows a copy operation to be replaced by a more efficient "move" when the source object is a temporary or an otherwise expiring object.

Use this tag for questions about move semantics, move constructors and move assignment.

Move semantics is a programming language feature that allows a copy operation to be replaced by a more efficient "move" when the source object is a temporary or an otherwise expiring object.

For more information on move semantics in C++, see Rvalue references and move constructors.

Related tags are , and .

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When to make a type non-movable in C++11?

I was surprised this didn't show up in my search results, I thought someone would've asked this before, given the usefulness of move semantics in C++11: When do I have to (or is it a good idea for me to) make a class non-movable in C++11? (Reasons…
user541686
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How does std::move() transfer values into RValues?

I just found myself not fully understanding the logic of std::move(). At first, I googled it but seems like there are only documents about how to use std::move(), not how its structure works. I mean, I know what the template member function is but…
Dean Seo
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initializer_list and move semantics

Am I allowed to move elements out of a std::initializer_list? #include #include template void foo(std::initializer_list list) { for (auto it = list.begin(); it != list.end(); ++it) { …
fredoverflow
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8 answers

Can I list-initialize a vector of move-only type?

If I pass the following code through my GCC 4.7 snapshot, it tries to copy the unique_ptrs into the vector. #include #include int main() { using move_only = std::unique_ptr; std::vector v { move_only(),…
R. Martinho Fernandes
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110
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How to enforce move semantics when a vector grows?

I have a std::vector of objects of a certain class A. The class is non-trivial and has copy constructors and move constructors defined. std::vector myvec; If I fill-up the vector with A objects (using e.g. myvec.push_back(a)), the vector will…
Bertwim van Beest
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108
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Efficiency of C++11 push_back() with std::move versus emplace_back() for already constructed objects

In C++11 emplace_back() is generally preferred (in terms of efficiency) to push_back() as it allows in-place construction, but is this still the case when using push_back(std::move()) with an already-constructed object? For instance, is…
Riot
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Reusing a moved container?

What is the correct way to reuse a moved container? std::vector container; container.push_back(1); auto container2 = std::move(container); // ver1: Do nothing //container2.clear(); // ver2: "Reset" container = std::vector() // ver3:…
ronag
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Why do we copy then move?

I saw code somewhere in which someone decided to copy an object and subsequently move it to a data member of a class. This left me in confusion in that I thought the whole point of moving was to avoid copying. Here is the example: struct S { …
user2030677
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Passing std::string by Value or Reference

Possible Duplicate: Are the days of passing const std::string & as a parameter over? Should I pass std::string by value or by reference (to a un-inlined function) if move semantics is supported? And what about implementations using small string…
Nordlöw
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Why no default move-assignment/move-constructor?

I'm a simple programmer. My class members variables most often consists of POD-types and STL-containers. Because of this I seldom have to write assignment operators or copy constructors, as these are implemented by default. Add to this, if I use…
Viktor Sehr
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Is the default Move constructor defined as noexcept?

It seems that a vector will check if the move constructor is labeled as noexcept before deciding on whether to move or copy elements when reallocating. Is the default move constructor defined as noexcept? I saw the following documentation but it…
bjackfly
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Is unique_ptr guaranteed to store nullptr after move?

Is unique_ptr guaranteed to store nullptr after move? std::unique_ptr p1{new int{23}}; std::unique_ptr p2{std::move(p1)}; assert(!p1); // is this always true?
lizarisk
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Is the pass-by-value-and-then-move construct a bad idiom?

Since we have move semantics in C++, nowadays it is usual to do void set_a(A a) { _a = std::move(a); } The reasoning is that if a is an rvalue, the copy will be elided and there will be just one move. But what happens if a is an lvalue? It seems…
jbgs
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Is there any case where a return of a RValue Reference (&&) is useful?

Is there a reason when a function should return a RValue Reference? A technique, or trick, or an idiom or pattern? MyClass&& func( ... ); I am aware of the danger of returning references in general, but sometimes we do it anyway, don't we? T&…
towi
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Why do some people use swap for move assignments?

For example, stdlibc++ has the following: unique_lock& operator=(unique_lock&& __u) { if(_M_owns) unlock(); unique_lock(std::move(__u)).swap(*this); __u._M_device = 0; __u._M_owns = false; return *this; } Why not just…
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