C/C++ Basics

This file shows some basic usages of C/C++

Initialize a std::unique_ptr with nullptr

See Why does my unique_ptr think is has a null function pointer deleter

class FooObj {
  FooObj() {}
  ~FooObj() { std::cout << "dtor!\n"; }
};

void freeFooObj(FooObj *obj) {
  if (obj) {
    std::cout << "Free func for FooObj\n";
    delete obj;
  }
}

class Foo {
  // Use nullptr and a free func to initialize it
  Foo::Foo() :  m_val(nullptr, freeFooObj) { }

  std::unique_ptr<FooObj, decltype(freeFooObj)> m_val;
};

static constexpr in class template

How to initialize a static constexpr const member of a class template in a header file?

#include <numeric>

// Template class definition here
template<typename T>
class Foo {
public:
	// constexpr must have an initializer
	static constexpr const int VAL_MAX = std::numeric_limits<T>::max();

public:
	Foo(int val) : m_val(val) {}
	~Foo() {}

private:
	int m_val = 0;
};

// Since static member needs definition, define it here
template<typename T> const int Foo<T>::VAL_MAX;

Why using typename？

C++ typedef & typename知识点总结 - 腾讯技术工程的文章 - 知乎

Why using template before a function when invoked?

static template function in template class

Quote from C++’03 Standard 14.2/4:

When the name of a member template specialization appears after . or -> in a postfix-expression, or after nested-name-specifier in a qualified-id, and the postfix-expression or qualified-id explicitly depends on a template-parameter (14.6.2), the member template name must be prefixed by the keyword template. Otherwise the name is assumed to name a non-template.

也就是说，如果调用一个class template中的function temple，就要在function名字的前面加上template关键字，防止有时编译器不能resolve的情况（是的，有时候编译器可以识别，但有时又不行）。例如下面

template<typename T>
class Foo {
	template<bool HoV, bool AscOrder>
	static int32_t check(int32_t i) {
		if (HoV) {
			return AscOrder ? i + 1 : i - 1;
		}
		return i + 3;
	}
	// data members...
	int k = 0;
}

template<typename T>
class Foo2 {
	bool set_ok(int32_t k);
	// data members...
	int m = 0;
}

template<typename T>
bool Foo2<T>::set_ok(int32_t k) {
	// Invoke
	return Foo<T>::template check<true, false>(8) > 0;
}

What is placement new?

What uses are there for “placement new”?

简而言之，就是先分配一块”raw”内存，然后在这个内存上构造一个object，这样就把内存分配和对象构造这两步分开了，比如有时候只需要先分配一大块内存，然后在需要的时候再构造，这样就可以提高性能，memory pool中可以用到这项技术。

Locate the path of STL headers used by g++

You can display the full search paths with

g++ -print-search-dirs

or you can find a specific header without writing a source file with something along the lines of

echo '#include <vector>' | g++ -x c++ -E - | grep '/vector'

Link: Locate the path of STL headers used by g++

C++ unordered_map under the hood

C++ Unordered Map Under the Hood

C++ Variadic Macros

See page on 3.6 Variadic Macros