Comparison stdfor_each vs. Range‑based for Loop

Comparison: std::for_each vs. Range‑based for Loop

Both std::for_each (from <algorithm>) and the range‑based for loop (C++11 and later) iterate over every element of a container. However, they have different strengths, syntax, and ideal use cases. Since your article covers <functional>, std::for_each is especially relevant because it shines when you already have a callable object (like a functor from <functional>) that you want to apply.

1. Basic Syntax

2. When to Use Each

3. Detailed Comparison Table

4. Examples Side‑by‑Side

Simple transformation (print each element)

std::vector<int> values = {1, 2, 3, 4, 5};
// Range‑based for
for (int x : values) {
    std::cout << x << ' ';
}
// std::for_each
std::for_each(values.begin(), values.end(),
              [](int x) { std::cout << x << ' '; });

Using a predefined functor from <functional>

void increment(int& n) { ++n; }
// Range‑based for
for (int& x : values) {
    increment(x);
}
// std::for_each – cleaner when you already have the function
std::for_each(values.begin(), values.end(), increment);

Using std::mem_fn with std::for_each

struct Person { void greet() const; };
std::vector<Person> people;
// Range‑based for
for (const auto& p : people) {
    p.greet();
}
// std::for_each – very concise
std::for_each(people.begin(), people.end(), std::mem_fn(&Person::greet));

Parallel execution (C++17)

#include <execution>
// Range‑based for cannot be parallelised directly.
// std::for_each can:
std::for_each(std::execution::par, values.begin(), values.end(),
              [](int& x) { x *= 2; });

5. Returning a Stateful Function Object

std::for_each returns the callable after processing. This is useful for accumulating state.

struct Sum {
    int total = 0;
    void operator()(int x) { total += x; }
};
Sum s = std::for_each(values.begin(), values.end(), Sum());
std::cout << "Sum = " << s.total << '\n';

With a range‑based for, you would need an external accumulator variable:

int total = 0;
for (int x : values) total += x;
// total holds the sum

Both work, but std::for_each encapsulates the state inside the functor.

6. Performance

Both constructs compile to identical assembly for simple operations. There is no performance penalty for using std::for_each – it’s just a template that inlines the loop. The range‑based for is also syntactic sugar for a traditional iterator loop.

Example compiled with -O2 (both produce the same loop):

// Range‑based for
for (int x : values) sum += x;
// std::for_each
std::for_each(values.begin(), values.end(), [&](int x){ sum += x; });

7. Recommendation Summary

• Prefer range‑based for for most day‑to‑day loops – it’s simpler, supports break, and is very readable.

• Use std::for_each when:

  • You already have a callable object (e.g., from <functional>) and want to apply it directly.

  • You need parallel execution.

  • You are writing generic code that works with arbitrary iterator pairs.

  • You want to use the return value (stateful functor).

  • You are working in a “functional” style (pipeline of algorithms).

8. Integration with <functional>

std::for_each is a natural partner for <functional> because it directly accepts:

• std::function objects,

• results of std::bind,

• std::mem_fn wrappers,

• built‑in functors like std::plus, std::greater (though those are unary/binary, so you’d need std::bind or a lambda to adapt them).

Example:

std::vector<int> v = {1,2,3};
std::for_each(v.begin(), v.end(), std::bind(std::multiplies<int>(), std::placeholders::_1, 2));
// multiplies each element by 2

But in modern C++, a lambda is often clearer than std::bind. Still, std::for_each remains a valid choice.

Final verdict: Both are excellent tools. Know both and choose the one that makes your code most expressive and maintainable. For teaching <functional>, std::for_each provides a perfect stage to demonstrate callable objects.