Category: C++

Coroutines Coroutines are special functions that can have their execution suspended and resumed. To define a coroutine, the co_return, co_await, or co_yield keywords must be present in the function’s body. C++20’s coroutines are stackless; unless optimized out by the compiler, their state is allocated on the heap. Designated initializers Formatting library Combine the simplicity of printf with the type-safety of iostream. Uses braces…

std::string_view std::optional A common use case for optional is the return value of a function that may fail. std::filesystem

Unless you really need to expose a read-only reference to clients (rare). Thread safety is also a problem

If a function takes a std::unique_ptr<>, that means it takes ownership of the argument. Callers need to use std::move() to indicate that they’re passing ownership if the object being passed is not a temporary: If a function returns a std::unique_ptr<>, that means the caller takes ownership of the returned object. Usage of std::move() while returning an object is only needed if the return type of…

Summary In this post, I will introduce the scoping in nested classes and how friend classes work here. Conclusion The rules are, As any member of its outer class, nested classes have access to all names (private, protected, etc) to which the outer class has access; however, outer classes have no special access to inner…

Summary In this post, I will introduce custom deleters when using C++ std::shared_ptr and std::unique_ptr. Conclusion Use the following snippets. Note the fourth case, where a unique_ptr with custom deleter will not pass the deleter to the shared_ptr when using release(). class OldThing { public: ~OldThing() { if (allocated_) { std::cout << "MEMORY LEAK!" <<…

Summary In the previous post, I introduced the rules of using directives (using namespace std) and using declarations (using std::vector). Generally speaking, the rules are good if there is no confliction, in which two using-declarations are the same. As a quick recap, using directives are forbidden. Since this directive makes all names from a namespace…

Summary In this post, I will introduce the different between #include "file" and #include <file> in C++. Conclusion #include <file> This variant is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I option. #include "file"…

Summary In this post, I will introduce some common compiling errors I used to see related to in-class initialization: Error: in-class initialization of static data member xxx of non-literal type; Error: ISO C++ forbids in-class initialization of non-const static member xxx; Conclusion As usual, I present the correct ways to do in-class initialization at the…

Summary In this post, I will introduce the difference between include guard and #pragma directives in C++. Conclusion As usual, I will present the conclusion first. Include guard is the standard way to prevent multiple inclusion of the same header; #pragma directive is not required by C++ standard but now widely supported by most of…

Summary In this post, I will introduce some methods to overload the default order in C++’s STL. Please refer to my other post if you are not familiar with Ordered Containers in C++. This post is a helpful reference when solving coding challenges. Conclusion You are giving a sequence of pairs, {{5, “five”}, {1, “one”},…

Summary In this post, I will introduce function objects, also known as functors. Conclusion I provid the following snippets as a quick reference. See meeting-rooms-II for the problem details. /** * Definition for an interval. * struct Interval { * int start; * int end; * Interval() : start(0), end(0) {} * Interval(int s, int…

Summary In this post, I will introduce how to use "using" in C++, in other words, how to use an existing namespace. Conclusion As usual, I present the conclusion first. The followings are recommended ways which minimize name collision possibilities and maximize code clarity and writing convenience. See this post as well. For Headers Do…

Summary In this post, I will introduce const, * and & and their combinations. I also have similar posts discussed auto and &. Conclusion As usual, I present the golden rule first: “read from right to left”. In other words, the const-on-the-right style always puts the const on the right of what it constifies”. When…

Summary In this post, I will discuss when to use References and when to use Pointers in C++. Conclusion As usual, I present the conclusion first. 1. If the varible can be nullptr, declare the varible as a pointer; 2. If the you want to re-assign the varible, declare the varible as a pointer; 3….

Summary I come across a compiling error, in which it says class A was not declared. However, I actually include A.h. Details After some research, I find out the bugs. In utils.h, I want to use some static member varible in class A and thus I include A.h. However, since utils.h acts as its name,…

Summary In this post, we will discuss rvalue references and move semantics in C++11. Details Rvalue references and move semantics can be very complex, see this article. But the truth is that: the simplest is the best. It starts with three examples of returning values from functions. Read them and ask yourself which of them…

Summary In this post, I will introduce how to get a vector of files in a directory in C++. Instead of filesystem in boost or C++ 17, this method is purely based on C++ 11 and Unix platforms. Details The following codes are self-explained. // safe path join in unix-based system string path_join(const string& p1,…

Summary It is another post related to String in C++. I will introduce some useful codes which implement APIs in Java String. Content String.startsWith() String.endsWith() String.toLowerCase() String.toUpperCase() String.trim() Detials 1. String.startsWith() We implement a.startsWith(b) of java using C++. bool starts_with(const string & a, const string & b) { return b == a.substr(0, b.size()); } int…

Summary In this post, I will introduce lvalues and rvalues in C/C++. But I leave rvalue references and move semantics to the next post. Details Definition Lvalues: (locator value) represents an object that occupies some unique location in memory (has an address in some scope). Rvalues: not lvalues. Important points and examples 1. Rvalues can’t…

Summary In this post, I will show delegating constructors introduced in C++ 11. Details I come across the codes below, and get confused about the systax. class Iterator { struct Data; Data* data; public: Iterator(const vector<int>& nums); Iterator(const Iterator& iter); virtual ~Iterator(); int next(); bool hasNext() const; }; class PeekingIterator : public Iterator { private:…

Summary In this post we will discuss how to change object while iterating it. Details Say we have a map. unordered_map<char, int> char_counter ({{‘1’, 1}, {‘a’, 1}, {‘b’, 2}, {‘c’, 3}}); Now, we want to delete all elements whose value is 1. To delete an element using iterator we will use erase() function. But the…

Summary In this post, I will introduce some syntax sugar in C++. Content 1. Initializer List Constructor 2. Range in Container Construtor 3. Copy Constructor Details 1. Initializer List Constructor vector<int> vec({1,2,3,4,5}); unordered_map<char, int> char_counter ({{‘a’, 1}, {‘b’, 2}, {‘c’, 3}}); The syntax below is also legal. // use {} in vector vector<int> nums =…

Summary In this post, I will introduce the string class in c++ STL. It covers useful APIs and some of my recommendations. 1. Split 2. Access 3. Comparison 4. Numeric-Conversions Details Split Thanks to this post, we present the conlusions for split first. 1. If you have access to boost, please do Solution 1. 2….

Summary In this post, I will introduce auto, const auto, auto&, const auto&, auto&&, and const auto&&. Conclusion We provide the conclusion first. When you want a read-only access (generic or non-generic codes), use auto const &, which will give you a reference rather than a copy. When you want a read-write access in generic…

Summary This post will be updated from time to time. The post contains some mistakes that I will make in work and study when coding in C++. Contents container.size() – 1 when the container is empty negative % positive ‘+’ does work in “C_String” remember to delete after new to_string(‘a’) signed integers compared to unsinged…

Summary In this post, I will introduce auto and decltype in C++ 11. Why Auto In some situations, auto makes codes more readable. It will tell the compiler to deduce the type of a declared variable from the initialization expression. The details are complex. At present, we want to make codes correct, clean, and simple….

Summary In this post, I will introduce the map and priority_queue in c++ STL. Why Ordered Containers Unlike the easy version unordered_map, in map, key values are generally used to sort. Thanks to the binary search tree, map has good performance in search and erase elements $(log(n))$. Constructor The following codes are self-explained. // Example…