std::move does not actually move anything, it just converts the object to an rvalue reference, telling the compiler that the object can be used for a move operation. For example, when string assignment, if the class supports moving semantics, the target object can take over the source object resource without copying. Should be used in scenarios where resources need to be transferred and performance-sensitive, such as returning local objects, inserting containers, or exchanging ownership. However, it should not be abused, because it will degenerate into a copy without a moving structure, and the original object status is not specified after the movement. Appropriate use when passing or returning an object can avoid unnecessary copies, but if the function returns a local variable, RVO optimization may already occur, adding std::move may affect the optimization. Prone to errors include misuse on objects that still need to be used, unnecessary movement, and use of immovable types. In short, std::move is a prompt rather than a movement, and it needs to be understood and used reasonably.
std::move doesn't actually move anything. It's just a way to tell the compiler, “Hey, you can treat this object as temporary, feel free to steal its resources if needed.” Under the hood, it's a cast — nothing more.
What does std::move really do?
It casts an object to an rvalue reference. That's it. Once something is cast to an rvalue reference (like T&& ), the compiler considers it eligible for move operations.
For example:
std::string a = "hello"; std::string b = std::move(a);
Here, a is treated as a temporary, and b can take over a 's internal buffer without copying — if the string class supports move semantics.
So, std::move enables moves but doesn't perform them directly. The actual move happens in the move constructor or move assignment operator of the type you're working with.
When should I use std::move ?
You should use it when you're done with an object and want to transfer its resources somewhere else — especially when performance matters.
Common scenarios:
- Returning a local object from a function.
- Inserting a temporary into a container.
- Swapping or transferring ownership between objects.
But don't overuse it. If a type doesn't have a proper move constructor, std::move will fall back to a copy. Also, after using std::move , the original object is still valid but in an unspecified state — don't rely on its value afterward.
How does it interact with containers and functions?
When passing or returning objects, using std::move can avoid unnecessary copies — but only when appropriate.
For example, returning a local vector:
std::vector<int> make_big_vector() {
std::vector<int> temp(1000000);
return std::move(temp); // Not strictly needed here
} In this case, the compiler might already apply Return Value Optimization (RVO), so std::move isn't necessary and could even prevent RVO in some cases.
Another example: inserting into a vector:
std::vector<std::string> vs; std::string s = "abc"; vs.push_back(std::move(s)); // Now s is moved, not copied
If you don't std::move , push_back will call the copy constructor. Using std::move tells it to use the move constructor instead.
What's easy to get wrong?
- Moving from something you still need: After moving, the object is still around but in a valid but unspecified state. Don't assume it's empty or has any specific value.
- Using
std::moveunnecessarily: Like in return statements where RVO applications, addingstd::movecan hurt performance. - Trying to move non-movable types: If a class doesn't define move operations,
std::movewill just copy.
Also, be careful with generic code. Sometimes templates may deduce types incorrectly if you mix lvalues ??and rvalues ??unexpectedly.
So that's how std::move works — it's a signal, not an action. Use it where appropriate, understand what it enables, and let the compiler handle the rest.
Basically that's it.
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