c++ - Moving elements from std::vector to another one

C++ - Moving elements from std::vector to another one

To move elements from one std::vector to another in C++, you can use the std::move algorithm along with the std::vector::insert method. The std::move function is used to cast the elements to rvalue references, enabling move semantics, which transfers ownership of the elements rather than copying them.

Here's a step-by-step example:

#include <iostream> #include <vector> #include <utility> // For std::move int main() { // Create the source vector std::vector<int> source = {1, 2, 3, 4, 5}; // Create the destination vector std::vector<int> destination; // Move elements from source to destination destination.insert(destination.end(), std::make_move_iterator(source.begin()), std::make_move_iterator(source.end())); // Clear the source vector if necessary source.clear(); // Display the contents of the destination vector std::cout << "Destination vector: "; for (const auto& elem : destination) { std::cout << elem << " "; } std::cout << std::endl; // Display the contents of the source vector (should be empty) std::cout << "Source vector: "; for (const auto& elem : source) { std::cout << elem << " "; } std::cout << std::endl; return 0; }

Explanation:

  1. Create Source and Destination Vectors:

    • source is initialized with some values.
    • destination is empty initially.

  2. Move Elements:

    • std::make_move_iterator(source.begin()) and std::make_move_iterator(source.end()) are used to create move iterators for the source vector.
    • destination.insert(destination.end(), ...) inserts the elements at the end of the destination vector, moving them from the source.

  3. Clear the Source Vector:

    • After moving the elements, the source vector can be cleared if needed, though it's not strictly necessary if you don't plan to use it further.

  4. Display the Contents:

    • This shows the elements in both the destination and source vectors after the move operation.

This method is efficient because it avoids copying the elements, utilizing move semantics instead.

Examples

  1. How to move elements from one std::vector to another using std::move?

    Description: Use std::move to efficiently transfer elements from one vector to another, leaving the source vector in a valid but unspecified state.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; // Move elements from source to destination destination = std::move(source); // Output the contents of destination std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; // Source is now empty or in an unspecified state std::cout << "Source size after move: " << source.size() << std::endl; return 0; }

  2. How to move elements from std::vector to another one and maintain element order?

    Description: Use std::move with std::vector::insert to transfer elements while maintaining their order.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; // Move elements from source to destination while maintaining order destination.insert(destination.end(), std::make_move_iterator(source.begin()), std::make_move_iterator(source.end())); // Clear the source vector source.clear(); // Output the contents of destination std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; // Source is now empty std::cout << "Source size after move: " << source.size() << std::endl; return 0; }

  3. How to move a subset of elements from one std::vector to another?

    Description: Use std::move with std::vector::erase to move a subset of elements.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; // Move the first 3 elements from source to destination destination.insert(destination.end(), std::make_move_iterator(source.begin()), std::make_move_iterator(source.begin() + 3)); source.erase(source.begin(), source.begin() + 3); // Output the contents of destination and source std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; std::cout << "Source contains: "; for (int val : source) { std::cout << val << " "; } std::cout << std::endl; return 0; }

  4. How to move all elements from std::vector using a loop?

    Description: Use a loop with std::move to transfer elements one by one.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; // Move elements one by one while (!source.empty()) { destination.push_back(std::move(source.back())); source.pop_back(); } // Output the contents of destination std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; // Source is now empty std::cout << "Source size after move: " << source.size() << std::endl; return 0; }

  5. How to move elements from std::vector while preserving original order?

    Description: Use std::move and std::copy to preserve the order of elements.

    Code:

    #include <iostream> #include <vector> #include <algorithm> // for std::copy #include <utility> // for std::move int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; // Move elements while preserving order std::copy(std::make_move_iterator(source.begin()), std::make_move_iterator(source.end()), std::back_inserter(destination)); source.clear(); // Output the contents of destination std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; std::cout << "Source size after move: " << source.size() << std::endl; return 0; }

  6. How to move elements from a std::vector to another with a custom predicate?

    Description: Use std::remove_if and std::move to move elements based on a condition.

    Code:

    #include <iostream> #include <vector> #include <algorithm> // for std::remove_if #include <utility> // for std::move bool isOdd(int value) { return value % 2 != 0; } int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; auto end = std::remove_if(source.begin(), source.end(), [&](int value) { if (isOdd(value)) { destination.push_back(std::move(value)); return true; } return false; }); source.erase(end, source.end()); // Output the contents of destination and source std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; std::cout << "Source contains: "; for (int val : source) { std::cout << val << " "; } std::cout << std::endl; return 0; }

  7. How to move elements from std::vector to another with a specific starting position?

    Description: Use std::move with iterators to move elements starting from a specific position.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; // Move elements starting from position 2 destination.insert(destination.end(), std::make_move_iterator(source.begin() + 2), std::make_move_iterator(source.end())); source.erase(source.begin() + 2, source.end()); // Output the contents of destination and source std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; std::cout << "Source contains: "; for (int val : source) { std::cout << val << " "; } std::cout << std::endl; return 0; }

  8. How to move elements from std::vector with custom move operations?

    Description: Customize the move operation by defining a custom move constructor and assignment operator.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move class CustomClass { public: int value; CustomClass(int v) : value(v) {} CustomClass(CustomClass&& other) noexcept : value(other.value) { other.value = 0; // or other default state } CustomClass& operator=(CustomClass&& other) noexcept { if (this != &other) { value = other.value; other.value = 0; // or other default state } return *this; } }; int main() { std::vector<CustomClass> source = {1, 2, 3}; std::vector<CustomClass> destination; // Move elements from source to destination destination = std::move(source); // Output the contents of destination std::cout << "Destination contains: "; for (const auto& obj : destination) { std::cout << obj.value << " "; } std::cout << std::endl; // Source is now empty or in an unspecified state std::cout << "Source size after move: " << source.size() << std::endl; return 0; }

  9. How to move elements from std::vector to another vector with different element types?

    Description: Use std::move and type conversion if needed to transfer elements between vectors with different types.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move class SourceType { public: int value; SourceType(int v) : value(v) {} }; class DestinationType { public: int value; DestinationType(int v) : value(v) {} }; int main() { std::vector<SourceType> source = {1, 2, 3}; std::vector<DestinationType> destination; // Move and convert elements for (auto& item : source) { destination.push_back(DestinationType(std::move(item.value))); } source.clear(); // Output the contents of destination std::cout << "Destination contains: "; for (const auto& obj : destination) { std::cout << obj.value << " "; } std::cout << std::endl; std::cout << "Source size after move: " << source.size() << std::endl; return 0; }

  10. How to move elements from std::vector to another with indices?

    Description: Use std::move and indices to move a specific range of elements.

    Code:

    #include <iostream> #include <vector> #include <utility> // for std::move int main() { std::vector<int> source = {1, 2, 3, 4, 5}; std::vector<int> destination; // Move elements from index 1 to 3 destination.insert(destination.end(), std::make_move_iterator(source.begin() + 1), std::make_move_iterator(source.begin() + 4)); source.erase(source.begin() + 1, source.begin() + 4); // Output the contents of destination and source std::cout << "Destination contains: "; for (int val : destination) { std::cout << val << " "; } std::cout << std::endl; std::cout << "Source contains: "; for (int val : source) { std::cout << val << " "; } std::cout << std::endl; return 0; }

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