Lambda Expressions in C++

A brief talk given at HSV.cpp meetup on 3/15/17 concerning Lambdas in C++

Video is here: https://youtu.be/5splHkTJ2LY

1. [](){}(); Or How I Learned To Stop Worrying and Love the Lambda Pat Viafore HSV.cpp Meetup 3/15/2017

2. Lambda Expressions

3. Lambda Expressions Lambda Calculus Church-Turing Thesis Conversion and Reduction Complicated Mathematics Functional Programming Underpinnings of Computer Science Lisp?

4. Anonymous Functions

5. [](){}

6. [](){}

7. []() {} Function Body

8. void print(std::function<bool()> func) { std::string str = func() ? "true" : "false"; std::cout << str << “\n”; } auto isFiveGreaterThanThree = []() { return 5 > 3; } ; print(isFiveGreaterThanThree);

9. [] () {} Parameters Function Body

10. void print(int param, std::function<bool( int )> func) { std::string str = func( param ) ? "true" : "false"; std::cout << str << “\n”; } auto isGreaterThanThree = [] (int num) { return num > 3; }; print(5, isGreaterThanThree);

11. [] () {} Capture List Parameters Function Body

12. void print(int num, std::function<bool(int)> func) { std::string str = func(num) ? "true" : "false"; std::cout << str << “\n”; } i nt target = 3; auto isGreaterThan = [ target ](int n){ return n > target ; }; target = 6; print(5, isGreaterThan);

13. Capture List By Value By Reference [this] [a,b] [=] [&a,&b] [&] Prefer Explicit Captures Over Default Captures

14. class A { public: int x; A(): x(10) {} std::function<bool()> getLambda() { return [=] (){ return x+2; }; } }; int main() { A* a = new A(); auto lambda = a->getLambda(); delete a; lambda(); } This is implicitly this->x. The this parameter is what gets captured, not x.

15. class A { public: int x; A(): x(10) {} std::function<bool()> getLambda() { return [copy=x] (){ return copy+2; }; } }; int main() { A* a = new A(); auto lambda = a->getLambda(); delete a; lambda(); } C++14 gives us init captures, where we can initialize the variables we capture (useful for any non-local captures)

16. Why? Dependency Injection Pass a function to inject a dependency Functional Programming Styles Mapping and Filtering just got a whole lot easier Algorithm Library auto isSpecial = [](MacAddress& mac){ return MacAddress::ItuAddress == mac; }; any_of(macs.begin(), macs.end(), isSpecial ); count_if(macs.begin(), macs.end(), isSpecial ); replace_if(macs.begin(), macs.end(), isSpecial , MacAddress::BlankMacAddress); sort(ips.begin(), ips.end(), [](IpAddressV4& ip1, IpAddressV4& ip2) { return ip1.getNetworkByteOrder() < ip2.getNetworkByteOrder()); });

17. [] () {} Capture List Parameters Function Body ();

18. Twitter: @PatViaforever