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c/c++ const 关键字详解
首先讲讲c语言中const关键字的作用，很多书把const修饰的关键字叫做常量，但我认为还是叫做只读变量恰当点，毕竟它和真正的右值常量还是有区别的，除了只读（即只能定义时初始化，其他任何时候不能再修改）这个限制外，和普通变量没有什么区别。
基本使用方法，就是在变量声明的加上const修饰即可。如const int N = 0;即声明了一个只读变量N。这个N是不允许修改的。
关键在于涉及指针时需要弄清楚const修饰的是指针本身还是修饰指针指向的对象，这点很重要！
可以这样简单记忆，如果const在 * 前面则const修饰的是指针指向的对象，即指向的对象是只读的。如果const 在 *后面则修饰的是指针本身，即该指针变量不可指向其他对象，而指向的对象并不受const约束。如
int x = 5, y = 6; const int *p = &x; int * const q = &x; // q = &y; // *p = 6; p = &y; *q = 7;
p指针不能修改指向对象的值（注意：只能说不能通过p来修改所指向对象的值，不能说指向的内容不能修改，完全可以通过其他方式修改其值，比如q指针），而q指针可以修改指向对象的值，但本身是一个只读变量，即不能再指向其他对象。另外，注意一点的是，const变量必须在声明时初始化，这和java中的final修饰符不同，final可以声明不进行初始化，可以在任何时候进行初始化。
c++的引用类型也基本类似,看 & 位置决定const修饰的内容。另外注意以下代码：
typedef int* Pointer; int x = 5; const Pointer p = &x;
很容易把const Pointer p = &x, 展开为 const int *p = &x，引起错误！ 注意typedef并不是简单的展开，它和宏有本质的区别。上面定义的p应该相对于int * const p = &x; 即p本身是可读变量，而不是其指向的对象不可修改。
const修饰符有什么作用，也许第一反应就是声明一个常量（为什么要声明常量？与宏#define的区别？）。其实更确切地说，const修饰符的功能是为了保护数据安全。
这在函数传参中更容易理解，假如我有一个char *s 字符串， 我要一个查找某个字符的位置的函数find，返回第一次出现该字符的索引，但我不希望由于传入参数后意外的被修改了原来的字符串，关键是这个函数还不一定是我自己写的，有可能是其他人写的。我们可以声明一个函数为 int find(const char *s)来达到这样的目的。 这样我就不用担心我的字符串被恶意修改了。因此：只要是不需要修改原来的内容，则尽量声明为const参数，减少出错率。比如在写qsort的cmp函数时，应该写成int cmp(const void *a, const void *b)，因为不希望在比较的时候就把原来的值篡改了。不过一般情况下，const只对指针或者引用参数有效，int foo(const int n)纯属胡闹，因此该函数是传值，每次调用函数时，都会重新创建该函数所有的形参，此时所传递的实参将会初始化对应的形参，普通的非引用类型的参数通过复制对应的实参实现初始化，函数并没有访问调用所传递的实参本身，因此不会修改实参的值，而只是操作实参的一个副本。因此foo函数形参n没有必要const。当然也有时这样使用，比如int op(const int *a, const int n)，a表示一个动态数组，n表示数组大小，我不希望在函数体内意外的修改了n的值，因此声明n为const形参是有用的。另外需要注意：如果是在c++中，应该尽量将不需要修改的参数定义为const引用，而不应该使用传值的方法！这样能够避免复制实参，减少开销。虽然op(string s1) 和 op(const string &s1)都不会修改s1,但前者需要复制s1,而后者不需要，如果一个类型的复制代价很大时，这效率更明显！
另外需要注意的是const 指针和非const指针、const引用和非const引用是可以重载的，即int foo(const int *x) 和 int foo(int *x)可以重载，int foo(const int &x) 和 foo(int &x)可以重载，所以说重载与否只看参数类型和个数是不确切的，还需要考虑是否const指针或者引用。int foo(const int &x)可以传入右值， 而foo(int &x)不可以传入右值，因为右值是不可修改的。（注意c++11标准中右值引用 && 可以修改右值，因此int foo(int &&x)可以传入右值). 在决定调用哪个版本时，由传入的参数是否const决定！
在c++类中可以声明const成员函数，即 int foo() const {} ,这样声明说明这个函数不能修改对象成员，即this指针是const的，但注意，const成员函数仍然可以修改mutable修饰的数据成员！
总结一下：const的最主要功能是保护数据， 另外使用const引用还能提高函数运行效率，减少复制实参开销！
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