When Should You Use `std::move` for Function Return Values?

When Is std::move Appropriate for Function Return Values?

In programming, the std::move function is used to transfer ownership of a resource from one variable to another. It effectively prevents the copy constructor from being called, allowing the resource to be moved instead of copied. While std::move is commonly used in certain situations, it's crucial to understand when it's truly necessary.

One instance where std::move is deemed unnecessary occurs when a function returns an object that was created as an xvalue. In the code snippet below, the meh function returns an instance of Foo using the move constructor. However, the move is redundant since the object being returned is already an xvalue.

struct Foo {};
Foo meh() {
  return std::move(Foo());
}

However, there are scenarios where using std::move becomes essential. Consider the following code:

struct Foo {};
Foo meh() {
  Foo foo;
  // Perform operations on foo
  return std::move(foo);
}

In this case, std::move is required because the foo object is an lvalue, and its lifetime is uncertain beyond the scope of the function. By explicitly calling std::move, the object's resources are transferred to the return value, ensuring proper resource management.

The reasoning behind this behavior is detailed in section 12.8/32 of the C Standard. This section outlines the conditions under which a copy operation from an lvalue can be replaced by a move. According to the standard, the compiler is only permitted to perform this substitution in specific situations, and it cannot make this change on its own.

Therefore, std::move should be used on a function return value in the following circumstances:

• You intend to move the object, and
• It is an lvalue, and
• It is not eligible for copy elision, and
• It is not a by-value function parameter.

In simplified terms, for non-template code, std::move can be used if the object is an lvalue and you want it to be moved, regardless of other conditions.

It's worth noting that using std::move unnecessarily can prevent move elision, reducing optimization opportunities. However, this is usually not a significant concern for cheap-to-move types like std::vector. For expensive-to-move types or templates where move cost is unknown, careful consideration should be taken before using std::move.

The above is the detailed content of When Should You Use `std::move` for Function Return Values?. For more information, please follow other related articles on the PHP Chinese website!