Introduce C++ Lambda

Introduction

I will show three concepts of C++ Lambda in the blog post.

• Subroutine
• Capture by value
• Capture by reference

Subroutine

We cannot define a routine in any existing routine.

void f(void) {
  int sum(int x, int y) {
    return x + y;
  }
}

So, we need the lambda expression. Let's rewrite the code as following.

void f(void) {
  auto sum = [](int x, int y) -> int {
    return x + y;
  };
}

Capture by value

One of the C++ Lambda features calls Capture, here is the example usage.

void f(void) {
  auto sum = [s = 1](int x, int y) -> int {
    return (x + y) << s;
  };
}

As you can see it likes function parameters, it's the capture by value.

Capture by reference

Typically, we have the call-by-value and call-by-reference in Computer Science. Just like that, we have the capture by value and capture by reference in C++.

void f(void) {
  auto s   = int(1);
  auto sum = [&](int x, int y) -> int {
    return (x + y) << s;
  };
}

The benefit of the capture by reference is that can avoid the coping overhead.

Thank you for reading, see you next time.