C++ basics (9.3)-namespace (namespace)

Namespace (namespace)

• Declaration region (declaration region) A
  declaration region is a region in which declarations can be made. For example, global variables can be declared outside the function. For such variables, the declaration area is the file where they are declared. For a variable declared in a function, its declaration area is the code block where it is declared.
• Potential scope (potential scope)
  The potential scope of a variable starts at the point of declaration and ends at the end of its declaration area. Therefore, the potential scope is smaller than the declaration area, because the variable must be defined before it can be used.
  + Namespace features

/* Ok 1. 名称空间可以是全局的　*/
namespace Jack {
    
    
	double pail;
	void fetch();
	int pal;
}

/* Ok */
namespace Jill {
    
    
	double bucket(double n) {
    
    ...}
	double fetch;
	int pail;
}

/* 2. 名称空间是开放的(open)，可以把名称加入到已有的名称空间中　*/
namespace Jack {
    
    
	int b;
}

/* 3. 名称空间可以在另一个名称空间中　*/
namespace A {
    
    
	namespace B {
    
    
		int a;
		int b;
	}
}
int main()
{
    
    
	/* 4. 名称空间不可以位于代码块中，默认情况下，
		在名称空间中声明的名称的链接性为外部的(除非它引用了常量) */
	// namespace hello {} No
	
	/* use namespace */
	// 通过作用域解析运算符::,使用名称空间来限定该名称
	Jack::Pail = 12.34;
	/* 未被修饰的名称(如pail)称为未限定的名称(unqualified name); 包含名称空间的名称(如Jack::pail)限定的名称　*/
}

• Using declaration and using compiler directive
  C++ provides using declaration and using compiler directive to simplify the use of names in the namespace. The using declaration makes specific identifiers available, and the using compilation makes the entire namespace available.

/* using 声明　*/
using Jack::pail; // a using declaration

/* using编译指令与using声明比较　*/
int main() 
{
    
    
	/* using声明　*/
	double pail;
	using Jack::pail; // not allow，当某个名称已经在函数中存在，不可以使用using声明导入相同的名称

	/* 2. using编译指令　*/
	/* 使用using编译指令导入一个名称空间与使用using声明是不同的，更像是使用大量的作用域解析运算符，*/
	using namespace Jack; // ok 但是相同的局部名称将隐藏名称空间名，就像隐藏同名的全局变量一样
	pail = 3;
	cout << Jack::pail << endl;  // 0
	return 0;
}

• Other features of the namespace

/* 1. 可以将名称空间声明进行嵌套　*/
namespace elements {
    
    
	namespace fire {
    
    
	};
}

/* 2. 可以在名称空间中使用using编译指令和using声明　*/
namespace myth {
    
    
	using Jack::pail;
	using namespace elements;
}

/* 3. 可以给名称空间创建别名来简化对嵌套空间的声明　*/
//namespace myth = myt;
namespace MEF = myth::elements::fire;
//using MEF::flame;

/* 4. 未命名的名称空间　*/
/* 未命名名称空间中声明的名称的潜在作用域为:从声明点到该声明末尾,这个方面与全局变量相似
由于没有名称，所以不能显式的使用using编译指令或using声明来使其在其他位置可用。不可以在其
所属文件之外使用，这提供了链接性为内部的静态变量的替代品(对于没有声明为static的符号具有外部链接性，所以它们的名字在整个程序中必须唯一。典型的Ｃ解决方法为将这些符号声明为static,使它们的链接性由外部变为内部)
*/
// 例如:
static int counts; // static storage, internal linkage
int other();
int func(){
    
     ... }
int other(){
    
     ... }
// 采用名称空间的方法
namespace {
    
    
	int counts; // static storage, internal linkage
}
int func();
int other();

int main()
{
    
    
	/* 如果要访问jill::fetch */
	cin >> myth::fetch;
	/* 若没有与之冲突的变量　*/
	using namespace myth;
	cin >> fetch;

	using MEF::flame;
	return 0;
}

• Namespace guidelines
  1. Use variables declared in the named namespace instead of using external global variables
  2. Use variables declared in unnamed namespaces instead of static global variables
  3. If you develop a function library or class library, put it in a namespace. C++ advocates putting the standard function library in the namespace std to expand functions from the C language. But not all compilers have completed this excessive
  4. Only use the compiler directive using as a stopgap measure to convert old code to use namespace
  5. Do not use the using compiler directive in the header file. First of all, this obscures the need to make those names available; in addition, the order in which the header files are included may affect the behavior of the program. If you must use the using compiler directive using, it should be placed after #include
  6. When importing names, the preferred method of using scope resolution operators or using declarations
  7. For the using statement, it is preferred to set its scope to be local rather than global.