Skip to main content

Really short STL notes

My STL notes after reading Effective STL by Scott Meyer. For most of the points below, there are subtle important reasons for that. The reasons are not mentioned here as it would be too long. Please read Effective C++ by Scott Meyer.

1. Use member insert function instead of copy algorithm and inserter.

2. Minimize capacity: string (s).swap (s)
Clear and minimize capacity : string ().swap (s)

3. Associative containers use equivalence (strict weak ordering) and not equality.
Equivalence: !(w1 < w2) && !(w2 > w1)
In strict weak ordering, equal values are not equivalent!
Therefore, Comparison function should return false for equal values when equivalence is expected.

4. Use map::operator [] to modify/retrieve existing elements in the map
Use map::insert to add new elements

5. Iterator can be implicitely converted to reverse_iterator.
reverse_iterator to iterator: use member base() function.
const_cast works for conversion from const_iterator to iterator of strings and vector because they are char* and T* pointers. For others to convert from const_iterator to iterator use std::advance (i, distance (i,ci))

6. istream_iterator is used for formatted input. (does not read spaces)
istreambuf_iterator also reads spaces between strings.

7. v.reserve(size) does not invoke constructor (even default). It simply allocates more memory, if any, to hold size objects. Insertion of value at v.end () is wrong in general, use back_inserter (v) instead. Insertion is important and not assignment even after reserve.

8. v.remove () may not erase elements from the container, therefore, v.size() may not change after v.remove.

9. binary_search should receive same comparison function as the sort function did.

10. Custom function objects should be adaptable. To make them adaptable inherit from unary_function or binary_function. Adaptable function objects can be used with not1.
ptr_fun, mem_fun and mem_fun_ref are used to adapt member functions for algorithms like for_each.


Popular posts from this blog

Multi-dimensional arrays in C++11

What new can be said about multi-dimensional arrays in C++? As it turns out, quite a bit! With the advent of C++11, we get new standard library class std::array. We also get new language features, such as template aliases and variadic templates. So I'll talk about interesting ways in which they come together.

It all started with a simple question of how to define a multi-dimensional std::array. It is a great example of deceptively simple things. Are the following the two arrays identical except that one is native and the other one is std::array?

int native[3][4];
std::array<std::array<int, 3>, 4> arr;

No! They are not. In fact, arr is more like an int[4][3]. Note the difference in the array subscripts. The native array is an array of 3 elements where every element is itself an array of 4 integers. 3 rows and 4 columns. If you want a std::array with the same layout, what you really need is:

std::array<std::array<int, 4>, 3> arr;

That's quite annoying for two r…

Understanding Fold Expressions

C++17 has an interesting new feature called fold expressions. Fold expressions offer a compact syntax to apply a binary operation to the elements of a parameter pack. Here’s an example. template <typename... Args> auto addall(Args... args) { return (... + args); } addall(1,2,3,4,5); // returns 15. This particular example is a unary left fold. It's equivalent to ((((1+2)+3)+4)+5). It reduces/folds the parameter pack of integers into a single integer by applying the binary operator successively. It's unary because it does not explicitly specify an init (a.k.a. identity) argument. So, let add it. template <typename... Args> auto addall(Args... args) { return (0 + ... + args); } addall(1,2,3,4,5); // returns 15. This version of addall is a binary left fold. The init argument is 0 and it's redundant (in this case). That's because this fold expression is equivalent to (((((0+1)+2)+3)+4)+5). Explicit identity elements will come in handy a little la…

Folding Monadic Functions

In the previous two blog posts (Understanding Fold Expressions and Folding Functions) we looked at the basic usage of C++17 fold expressions and how simple functions can be folded to create a composite one. We’ll continue our stride and see how "embellished" functions may be composed in fold expressions.

First, let me define what I mean by embellished functions. Instead of just returning a simple value, these functions are going to return a generic container of the desired value. The choice of container is very broad but not arbitrary. There are some constraints on the container and once you select a generic container, all functions must return values of the same container. Let's begin with std::vector.
// Hide the allocator template argument of std::vector. // It causes problems and is irrelevant here. template <class T> struct Vector : std::vector<T> {}; struct Continent { }; struct Country { }; struct State { }; struct City { }; auto get_countries…