C std::move example
The purpose of std::move is to convert the lvalue to an rvalue reference, triggering the move semantics rather than copying. 1. In the vector example, std::move(source) makes dest obtain the source's resources through the move constructor, and the source is empty to avoid deep copy overhead. 2. In the custom class Person, the mobile constructor realizes resource transfer by taking over the pointer and juxtaposing the original pointer to nullptr, improving efficiency and preventing memory leakage. 3. After using std::move, the original object can still be destructed but should not be used again. Its state is valid but undefined. 4. Common uses include inserting large objects into containers, function return value optimization (although often replaced by RVO), and member variable transfer. Proper use of std::move can significantly improve performance.
std::move is an important tool introduced in C 11 to implement moving semantics , which can "move" objects instead of "copying", thereby improving performance, especially when dealing with large objects (such as std::vector , std::string ).
Here is a simple and intuitive std::move example to help you understand its role.
? Basic concept: What is std::move ?
std::move does not really "move" anything, it just converts an lvalue into an rvalue reference , allowing the call to the move constructor or the move assignment operator.
✅ Example: Use std::move mobile vector
#include <iostream>
#include <vector>
#include <utility> // std::move
int main() {
std::vector<int> source = {1, 2, 3, 4, 5};
std::cout << "source size before move: " << source.size() << std::endl; // Output 5
// Use std::move to move source to dest
std::vector<int> dest = std::move(source);
std::cout << "source size after move: " << source.size() << std::endl; // Usually 0
std::cout << "dest size after move: " << dest.size() << std::endl; // Output 5
return 0;
}? Output result:
source size before move: 5 source size after move: 0 dest size after move: 5
? illustrate:
-
std::move(source)convertssourceto rvalue, so the move constructor ofdestis called. - The original
sourceis "empty" (resources are transferred) and no longer has the original memory data. - This is much more efficient than deep copying, avoiding copying 5 integers and memory allocations.
?️ Example of mobile semantics in custom classes
#include <iostream>
#include <string>
#include <utility>
class Person {
public:
std::string* name;
// Constructor Person(const std::string& n) {
name = new std::string(n);
std::cout << "Constructed: " << *name << std::endl;
}
// Destructor ~Person() {
delete name;
std::cout << "Destroyed" << std::endl;
}
// Copy constructor Person(const Person& other) {
name = new std::string(*other.name);
std::cout << "Copied: " << *name << std::endl;
}
// Move constructor Person(Person&& other) noexcept {
name = other.name; // Transfer pointer other.name = nullptr; // The original object no longer owns the resource std::cout << "Moved: " << *name << std::endl;
}
};
int main() {
Person p1("Alice");
// Use std::move to trigger the movement construct Person p2 = std::move(p1);
// At this time p1.name is nullptr, and an error occurs when accessing it (p1.name) {
std::cout << "p1 name: " << *p1.name << std::endl;
} else {
std::cout << "p1's name is null after move" << std::endl;
}
std::cout << "p2 name: " << *p2.name << std::endl;
return 0;
}✅ Output:
Constructed: Alice Moved: Alice p1's name is null after move p2 name: Alice Destroyed Destroyed
Note: After
p1is moved, itsnamepointer is set tonullptr, which is good practice to prevent repeated releases.
⚠️ Notes on using std::move
- The original object after
std::movecan still be destructed , but its resources should not be used anymore. - The object after the move is in a valid but undefined state (usually empty or null), and do not rely on its value.
- Move operations are usually used in scenarios such as temporary objects, function return values, container element transfers, etc.
? Summary of common uses
std::move is commonly used for:
Add large objects to the container:
std::vector<std::string> vec; std::string str = "Hello"; vec.push_back(std::move(str)); // Avoid copying
Automatically moves when the function returns (implicitly or explicitly):
std::vector<int> createVec() { std::vector<int> v = {1,2,3}; return std::move(v); // Usually not required, RVO is optimized, but sometimes useful}Member variable exchange or transfer:
class Container { std::vector<int> data; public: void setData(std::vector<int> input) { data = std::move(input); // Transfer instead of copy} };
Basically that's it.
std::moveis not magic, but it can significantly improve performance, especially in scenarios where large objects are created/destroyed frequently. Understanding it will allow you to write more efficient C code.The above is the detailed content of C std::move example. For more information, please follow other related articles on the PHP Chinese website!
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