std::assign

Function: std::assign

std::assign is a member function of several standard C++ containers, including std::vector, std::list, and std::forward_list. It provides a convenient way to replace the contents of a container with a new set of elements. This function is particularly useful when you want to overwrite the existing elements of a container or initialize it with a specific set of values.

Example 1: Basic Usage with std::vector

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> vec = {1, 2, 3, 4, 5};

    // Assign new values to the vector
    vec.assign({10, 20, 30, 40, 50});

    std::cout << "Vector after assign: ";
    std::for_each(vec.begin(), vec.end(), [](int n) { std::cout << n << " "; });
    std::cout << std::endl;

    return 0;
}

Explanation

In this example, we start with a vector containing the values 1 to 5. We then use assign to replace its contents with a new set of values (10 to 50). The assign function overwrites the existing elements and adjusts the size of the vector if necessary.

Example 2: Assigning a Range of Elements

#include <iostream>
#include <vector>
#include <list>
#include <algorithm>

int main() {
    std::list<double> source = {1.1, 2.2, 3.3, 4.4, 5.5};
    std::vector<double> destination;

    // Assign elements from the list to the vector
    destination.assign(source.begin(), source.end());

    std::cout << "Vector after assign: ";
    std::for_each(destination.begin(), destination.end(), [](double n) { std::cout << n << " "; });
    std::cout << std::endl;

    return 0;
}

Explanation

• This example demonstrates how to use assign with iterators.
• We start with a std::list of doubles and an empty std::vector.
• The assign function is used to copy all elements from the list to the vector.
• This showcases the flexibility of assign, as it can work with different container types and transfer elements between them.

Example 3: Assigning Multiple Copies of a Value

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<std::string> vec;

    // Assign 5 copies of "Hello" to the vector
    vec.assign(5, "Hello");

    std::cout << "Vector after assign: ";
    std::for_each(vec.begin(), vec.end(), [](const std::string& s) { std::cout << s << " "; });
    std::cout << std::endl;

    return 0;
}

Explanation

• This example shows another form of assign that takes a count and a value.
• It assigns 5 copies of the string "Hello" to the vector.
• This is useful when you need to initialize a container with multiple copies of the same value.

Example 4: Using assign with std::list

#include <iostream>
#include <list>
#include <algorithm>

int main() {
    std::list<char> charList;

    // Assign characters from an array
    char chars[] = {'a', 'b', 'c', 'd', 'e'};
    charList.assign(std::begin(chars), std::end(chars));

    std::cout << "List after assign: ";
    std::for_each(charList.begin(), charList.end(), [](char c) { std::cout << c << " "; });
    std::cout << std::endl;

    // Reassign with multiple copies
    charList.assign(3, 'X');

    std::cout << "List after reassign: ";
    std::for_each(charList.begin(), charList.end(), [](char c) { std::cout << c << " "; });
    std::cout << std::endl;

    return 0;
}

Explanation

• This example demonstrates the use of assign with std::list.
• First, we assign characters from an array to the list using iterators.
• Then, we reassign the list with multiple copies of a single character.
• This shows that assign works similarly across different container types that support it.

Performance Considerations

When using assign, it's important to note:

1. assign replaces all elements in the container, which can be more efficient than clearing the container and then inserting new elements one by one.
2. For vectors, if the new size is larger than the current capacity, reallocation will occur.
3. For lists and forward lists, no reallocation occurs, but all existing nodes are destroyed and new ones are created.

Summary

std::assign is a powerful and flexible function for replacing the contents of containers in C++. It can be used with initializer lists, ranges specified by iterators, or to fill a container with multiple copies of a value. This function is supported by several standard containers including std::vector, std::list, and std::forward_list.

Key points to remember: - It replaces all existing elements in the container. - It can change the size of the container. - It provides a convenient way to initialize or reset a container's contents. - It works with different types of input (initializer lists, ranges, count and value).

By using std::assign, you can write more concise and efficient code when you need to replace the entire contents of a container, making it a valuable tool in C++ programming.

Related

• What is the purpose of std::assign
• How does std::assign differ from other assignment operators
• Can std::assign be used with custom data types
• Are there any performance considerations when using std::assign
• Is std::assign part of the C++ standard library