This page assumes you have never seen an iterator, a lambda, or the notation [first, last). We build each symbol from a picture before the parent note is allowed to use it. Parent: STL algorithms — the topic.
Before any symbol, picture the thing we operate on.
Look at the figure: five boxes hold 10, 20, 30, 40, 50. The number under each box is its index — box 0 is the first. This numbering-from-zero is the seed of everything below.
The parent note says algorithms take "a pair of iterators." What is an iterator?
In the figure the cyan finger it points at box 2 (value 30). Writing *it gives 30. Writing ++it slides the finger to box 3. That is the whole interface an algorithm needs: read here, step forward. See Iterators and ranges for the full family.
The figure shows begin() on box 0 and end() hovering just after box 4 — on the imaginary "box 5" slot. end() never points at a real value. Never write *v.end(); the finger is over the cliff.
Two small pieces of notation the parent uses in examples.
If a finger sits on box 2, then it - v.begin() is 2−0=2. That is exactly the parent's size_t idx = it - v.begin(); trick — subtract the start-finger to convert a location into a number. This subtraction only works for random-access containers (rows in memory), which brings us to the next idea.
Algorithms like sort, find, all_of take a little function that decides things. What is that?
The figure shows a box entering a small machine labelled pred; out comes a light: green true or grey false. That machine is what all_of and any_of feed every box through.
There are three ways to write such a mini-function, from oldest to newest:
a plain named function,
a function object (functor) — an object with a () you can call,
a lambda — an inline throwaway function written right at the call site, [](int x){ return x % 2 == 0; }.
The parent uses lambdas everywhere: [](int a, int b){ return a > b; }. Read it as "given a and b, answer with a > b." The [] is where the lambda begins.
