Skip to main content

STL short / one liners

A very nice collection of wrappers is available on Stackoverflow.

1. Copy everything in the a container to std::cout (e.g. std::set<std::string> c; )
std::copy (c.begin(), c.end(), std::ostream_iterator <std::string> (std::cout, "\n"));

2. Clear vector v and minimize its capacity (potential number of elements it can hold without resizing).
vector <std::string>().swap (v); Yes! It is a swap with an empty temporary!!

3. Remove all integers of value = 0xDeadBeef;
std::erase (std::remove (c.begin(), c.end(), value), c.end());
This is known as erase-remove idiom.

4. Invoke a member function on all the elements of the container.
std::for_each (c.begin(), c.end(), std::mem_fun (&Class::function));

5. Copy one container to other
std::copy(V.begin(), V.end(), L.begin());

6. Fill an array with random numbers
std::generate(V.begin(), V.end(), rand);

7. Read a file of integers in a set
std::istream_iterator <int> data_begin (std::cin), data_end;
std::set <int> S (data_begin, data_end);

OR
std::set <int> data ((istream_iterator <int> (datafile)),istream_iterator <int> ());

OR
istream_iterator <int> start (cin), end;
copy (start, end, std::back_inserter (v));

8. Reading entire file in one go.

Solution 1:
std::istreambuf_iterator < char > begin(std::cin), end;
std::string str(begin, end);

Solution 2:
std::ostringstream temp;
std::ifstream infile ("file.txt");
temp << infile.rdbuf();
std::string str = temp.str();

Comments

rduht said…
Your Best Choice! wow power leveling and wow gold wow gold

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…

Covariance and Contravariance in C++ Standard Library

Covariance and Contravariance are concepts that come up often as you go deeper into generic programming. While designing a language that supports parametric polymorphism (e.g., templates in C++, generics in Java, C#), the language designer has a choice between Invariance, Covariance, and Contravariance when dealing with generic types. C++'s choice is "invariance". Let's look at an example.
struct Vehicle {}; struct Car : Vehicle {}; std::vector<Vehicle *> vehicles; std::vector<Car *> cars; vehicles = cars; // Does not compile The above program does not compile because C++ templates are invariant. Of course, each time a C++ template is instantiated, the compiler creates a brand new type that uniquely represents that instantiation. Any other type to the same template creates another unique type that has nothing to do with the earlier one. Any two unrelated user-defined types in C++ can't be assigned to each-other by default. You have to provide a c…

Inheritance vs std::variant

C++17 added std::variant and std::visit in its repertoire. They are worth a close examination. I've been wondering about whether they are always better than inheritance for modeling sum-types (fancy name for discriminated unions) and if not, under what circumstances they are not. We'll compare the two approaches in this blog post. So here it goes.

Inheritancestd::variantNeed not know all the derived types upfront (open-world assumption)Must know all the cases upfront (closed-world assumption)Dynamic Allocation (usually)No dynamic allocationIntrusive (must inherit from the base class)Non-intrusive (third-party classes can participate)Reference semantics (think how you copy a vector of pointers to base class?)Value semantics (copying is trivial)Algorithm scattered into classesAlgorithm in one placeLanguage supported (Clear errors if pure-virtual is not implemented)Library supported (poor error messages)Creates a first-class abstractionIt’s just a containerKeeps fluent interfaces…