How to Use std::copy in C++: Syntax and Examples

In C++, you use std::copy from the <algorithm> header to copy elements from one range to another. It requires three iterators: the start and end of the source range, and the start of the destination range. The function copies elements in order until the source range is exhausted.

Syntax

The std::copy function has this syntax:

• InputIt first: Iterator to the start of the source range.
• InputIt last: Iterator to the end of the source range (one past the last element).
• OutputIt d_first: Iterator to the start of the destination range where elements will be copied.

The function copies elements from first up to but not including last into the destination starting at d_first.

cpp

template<class InputIt, class OutputIt>
OutputIt copy(InputIt first, InputIt last, OutputIt d_first);

Example

This example copies elements from one vector to another using std::copy. It shows how to copy all elements from the source vector to the destination vector.

cpp

#include <iostream>
#include <vector>
#include <algorithm> // for std::copy

int main() {
    std::vector<int> source = {10, 20, 30, 40, 50};
    std::vector<int> destination(source.size()); // allocate space

    std::copy(source.begin(), source.end(), destination.begin());

    std::cout << "Copied elements: ";
    for (int num : destination) {
        std::cout << num << ' ';
    }
    std::cout << '\n';
    return 0;
}

Output

Copied elements: 10 20 30 40 50 

Common Pitfalls

Common mistakes when using std::copy include:

• Not allocating enough space in the destination container before copying, which causes undefined behavior.
• Using overlapping ranges, which std::copy does not handle safely (use std::copy_backward instead).
• Passing incorrect iterators that do not represent valid ranges.

cpp

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

int main() {
    std::vector<int> src = {1, 2, 3};
    std::vector<int> dest; // no space allocated

    // Wrong: destination has no space, this causes undefined behavior
    // std::copy(src.begin(), src.end(), dest.begin());

    // Correct: allocate space first
    dest.resize(src.size());
    std::copy(src.begin(), src.end(), dest.begin());

    for (int n : dest) std::cout << n << ' ';
    std::cout << '\n';
    return 0;
}

Output

1 2 3 

Quick Reference

Tips for using std::copy:

• Include <algorithm> header.
• Ensure destination has enough space before copying.
• Use iterators to specify source and destination ranges.
• For overlapping ranges, use std::copy_backward.

Key Takeaways

Use std::copy with three iterators: source start, source end, and destination start.

Always allocate enough space in the destination container before copying.

std::copy does not handle overlapping ranges safely; use std::copy_backward for that.

Include to access std::copy.

Check iterator validity to avoid undefined behavior.