How does std::remove in C function, and what is the difference between std::remove and std::erase?

Understanding the Functionality of std::remove

In C , the std::remove algorithm operates on an arbitrary sequence represented by two forward iterators. Its purpose is to rearrange elements within the sequence, moving non-matching elements forward. Unlike std::erase, remove does not physically delete elements from the container. Instead, it reorganizes them.

Example

Consider the following code snippet:

std::vector<int> a;
a.push_back(1);
a.push_back(2);

std::remove(a.begin(), a.end(), 1);

std::vector<int>::iterator iter = a.begin();
std::vector<int>::iterator endIter = a.end();

std::cout << "Using iter...\n";
for (; iter != endIter; ++iter)
{
    std::cout << *iter << "\n";
}

std::cout << "Using size...\n";
for (int i = 0; i < a.size(); ++i)
{
    std::cout << a[i] << "\n";
}

The output will be:

Using iter...
2
2

Using size...
2
2

Although remove has effectively moved the element 2 forward, the vector's size remains unchanged at 2 because the uninitialized memory has not been removed.

Erase-Remove Idiom

The erase-remove idiom combines remove and erase to physically remove unwanted elements. The code:

a.erase(std::remove(a.begin(), a.end(), 1), a.end());

would remove the element 1 from the vector and reduce its size to 1.

Use Cases of std::remove

While the primary use of remove is in the erase-remove idiom, it can also be utilized in other situations, such as:

• Identifying the last occurrence of an element: By removing all occurrences except the last one, you can use the iterator returned by remove to pinpoint the final appearance.
• Creating a new sequence with unique elements: By removing duplicate elements, you can generate a new sequence containing only the distinct elements.
• Partitioning a sequence into two parts: By using remove, you can partition a sequence into two segments, one containing matching elements and the other containing non-matching elements.

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