Often, an individual will be trying to learn a new programming language, or a new feature of a programming language, and will find it extremely difficult to find information on their specific topic.

Or there will be information out there, but it will be complete, and you’ll have questions afterwards. I know I’ve come across this a lot, and this blog is here to catalog a number of issues I’ve come across.

One such, is lambdas.

What is a lambda?

In C++, a lambda is a function that you can assign to a variable. In modern C++, the simplest examples look like this:

auto myFunc = []() {
 printf("Hello World!\n");
};

In PHP (7+), a lambda can look like this:

$myFunc = function() {
 echo "Hello World!\n";
};

In Javascript:

const myFunc = function() {
  console.log("Hello World!");
};

Lambdas have some interesting properties, and I’m only going to cover them in C++ in detail, and PHP/Javascript will be covered lightly.

Let’s dissect the Lamda into two primary chunks, the declaration: auto myFunc = [] () and the body: { printf("Hello World!\n"); }.

auto tells the cpp compiler “you know what type this is, figure it out for me please.”, because lambda’s don’t actually have a type of their own.

[] tells the compiler “Whatever scope we’re in, any variables in here should be accessible inside this lambda.” This version is empty, so if the variable is not in global/namespaced scope, the lambda has no access to that variable.

() is like your standard-faire function declaration parenthesis.

The body of the lambda is your regular function. If the lambda is inside a templated function, things get complicated, and I won’t be covering those intricacies now.

A more complicated example:

//Try this out @ gcc.godbolt.org!
//x86-64 gcc 8.1 was my version, with the compiler flags: -std=c++17 -m32
#include <cstdio>
int main() { 

   short fib = 2;

   auto myFunc = [fib](int current) {
       printf("Hello World! Current: %d fib: %d\n", current, fib);
       fib + fib;
   };

   for(int i = 0; i < 6; i++) {
       myFunc(i);
   }
}

In order to replicate the capture argument (the []) in php, your lambda should look something like:

$someVar = 123;
$myFunc = function() use($someVar) {
 echo "Hello World!\n"0;
};

In Javascript, lambdas automatically have access to the variables inside the function they are defined and called from. This is the source of much heartache and pain for many users.