C++中关键字volatile和mutable用法

C/C++中的volatile关键字和const对应，用来修饰变量，用于告诉编译器该变量值是不稳定的，可能被更改。使用volatile注意事项：

(1). 编译器会对带有volatile关键字的变量禁用优化(A volatile specifier is a hint to a compiler that an object may change its value in ways not specified by the language so that aggressive optimizations must be avoided)。

(2). 当多个线程都要用到某一个变量且该变量的值会被改变时应该用volatile声明，该关键字的作用是防止编译器优化把变量从内存装入CPU寄存器中。如果变量被装入寄存器，那么多个线程有可能有的使用内存中的变量，有的使用寄存器中的变量，这会造成程序的错误执行。volatile的意思是让编译器每次操作该变量时一定要从内存中取出，而不是使用已经存在寄存器中的值(It cannot cache the variables in register)。

(3). 中断服务程序中访问到的变量最好带上volatile。

(4). 并行设备的硬件寄存器的变量最好带上volatile。

(5). 声明的变量可以同时带有const和volatile关键字。

(6). 多个volatile变量间的操作，是不会被编译器交换顺序的，能够保证volatile变量间的顺序性，编译器不会进行乱序优化(The value cannot change in order of assignment)。但volatile变量和非volatile变量之间的顺序，编译器不保证顺序，可能会进行乱序优化。

C++中的mutable关键字使用场景：

(1). 允许即使包含它的对象被声明为const时仍可修改声明为mutable的类成员(sometimes there is requirement to modify one or more data members of class/struct through const function even though you don’t want the function to update other members of class/struct. This task can be easily performed by using mutable keyword)。

(2). 应用在C++11 lambda表达式来表示按值捕获的值是可修改的，默认情况下是不可修改的，但修改仅在lambda式内有效(since c++11 mutable can be used on a lambda to denote that things captured by value are modifiable (they aren’t by default))。

详细用法见下面的测试代码，下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：

#include "volatile_mutable.hpp" #include 
  
    #include 
   
     #include 
    
      #include 
     
       #include 
      
        namespace volatile_mutable_ { /// int test_volatile_1() { volatile int i1 = 0; // correct int volatile i2 = 0; // correct return 0; } /// // reference: https://en.cppreference.com/w/c/language/volatile int test_volatile_2() { { // Any attempt to read or write to an object whose type is volatile-qualified through a non-volatile lvalue results in undefined behavior volatile int n = 1; // object of volatile-qualified type int* p = (int*)&n; int val = *p; // undefined behavior in C, Note: link does not report an error under C++ fprintf(stdout, "val: %d\n", val); } { // A member of a volatile-qualified structure or union type acquires the qualification of the type it belongs to typedef struct ss { int i; const int ci; } s; // the type of s.i is int, the type of s.ci is const int volatile s vs = { 1, 2 }; // the types of vs.i and vs.ci are volatile int and const volatile int } { // If an array type is declared with the volatile type qualifier (through the use of typedef), the array type is not volatile-qualified, but its element type is typedef int A[2][3]; volatile A a = { {4, 5, 6}, {7, 8, 9} }; // array of array of volatile int //int* pi = a[0]; // Error: a[0] has type volatile int* volatile int* pi = a[0]; } { // A pointer to a non-volatile type can be implicitly converted to a pointer to the volatile-qualified version of the same or compatible type. The reverse conversion can be performed with a cast expression int* p = nullptr; volatile int* vp = p; // OK: adds qualifiers (int to volatile int) //p = vp; // Error: discards qualifiers (volatile int to int) p = (int*)vp; // OK: cast } { // volatile disable optimizations clock_t t = clock(); double d = 0.0; for (int n = 0; n < 10000; ++n) for (int m = 0; m < 10000; ++m) d += d * n*m; // reads and writes to a non-volatile fprintf(stdout, "Modified a non-volatile variable 100m times. Time used: %.2f seconds\n", (double)(clock() - t) / CLOCKS_PER_SEC); t = clock(); volatile double vd = 0.0; for (int n = 0; n < 10000; ++n) for (int m = 0; m < 10000; ++m) vd += vd * n*m; // reads and writes to a volatile fprintf(stdout, "Modified a volatile variable 100m times. Time used: %.2f seconds\n", (double)(clock() - t) / CLOCKS_PER_SEC); } return 0; } /// // reference: https://en.cppreference.com/w/cpp/language/cv int test_volatile_3() { int n1 = 0; // non-const object const int n2 = 0; // const object int const n3 = 0; // const object (same as n2) volatile int n4 = 0; // volatile object const struct { int n1; mutable int n2; } x = { 0, 0 }; // const object with mutable member n1 = 1; // ok, modifiable object //n2 = 2; // error: non-modifiable object n4 = 3; // ok, treated as a side-effect //x.n1 = 4; // error: member of a const object is const x.n2 = 4; // ok, mutable member of a const object isn't const const int& r1 = n1; // reference to const bound to non-const object //r1 = 2; // error: attempt to modify through reference to const const_cast 
       
         (r1) = 2; // ok, modifies non-const object n1 fprintf(stdout, "n1: %d\n", n1); // 2 const int& r2 = n2; // reference to const bound to const object //r2 = 2; // error: attempt to modify through reference to const const_cast 
        
          (r2) = 2; // undefined behavior: attempt to modify const object n2, Note: link does not report an error under C++ fprintf(stdout, "n2: %d\n", n2); // 0 return 0; } /// // reference: https://www.geeksforgeeks.org/understanding-volatile-qualifier-in-c/ int test_volatile_4() { { const int local = 10; int *ptr = (int*)&local; fprintf(stdout, "Initial value of local : %d \n", local); // 10 *ptr = 100; fprintf(stdout, "Modified value of local: %d \n", local); // 10 } { const volatile int local = 10; int *ptr = (int*)&local; fprintf(stdout, "Initial value of local : %d \n", local); // 10 *ptr = 100; fprintf(stdout, "Modified value of local: %d \n", local); // 100 } return 0; } /// // reference: https://en.cppreference.com/w/cpp/language/cv int test_mutable_1() { // Mutable is used to specify that the member does not affect the externally visible state of the class (as often used for mutexes, // memo caches, lazy evaluation, and access instrumentation) class ThreadsafeCounter { public: int get() const { std::lock_guard 
         
           lk(m); return data; } void inc() { std::lock_guard 
          
            lk(m); ++data; } private: mutable std::mutex m; // The "M&M rule": mutable and mutex go together int data = 0; }; return 0; } /// // reference: https://www.tutorialspoint.com/cplusplus-mutable-keyword int test_mutable_2() { class Test { public: Test(int x = 0, int y = 0) : a(x), b(y) {} void seta(int x = 0) { a = x; } void setb(int y = 0) { b = y; } void disp() { fprintf(stdout, "a: %d, b: %d\n", a, b); } public: int a; mutable int b; }; const Test t(10, 20); fprintf(stdout, "t.a: %d, t.b: %d \n", t.a, t.b); // 10, 20 //t.a=30; // Error occurs because a can not be changed, because object is constant. t.b = 100; // b still can be changed, because b is mutable. fprintf(stdout, "t.a: %d, t.b: %d \n", t.a, t.b); // 10, 100 return 0; } /// // reference: https://www.geeksforgeeks.org/c-mutable-keyword/ int test_mutable_3() { using std::cout; using std::endl; class Customer { public: Customer(char* s, char* m, int a, int p) { strcpy(name, s); strcpy(placedorder, m); tableno = a; bill = p; } void changePlacedOrder(char* p) const { strcpy(placedorder, p); } void changeBill(int s) const { bill = s; } void display() const { cout << "Customer name is: " << name << endl; cout << "Food ordered by customer is: " << placedorder << endl; cout << "table no is: " << tableno << endl; cout << "Total payable amount: " << bill << endl; } private: char name[25]; mutable char placedorder[50]; int tableno; mutable int bill; }; const Customer c1("Pravasi Meet", "Ice Cream", 3, 100); c1.display(); c1.changePlacedOrder("GulabJammuns"); c1.changeBill(150); c1.display(); return 0; } /// // reference: https://stackoverflow.com/questions//does-the-mutable-keyword-have-any-purpose-other-than-allowing-the-variable-to int test_mutable_4() { int x = 0; auto f1 = [=]() mutable { x = 42; }; // OK //auto f2 = [=]() { x = 42; }; // Error: a by-value capture cannot be modified in a non-mutable lambda fprintf(stdout, "x: %d\n", x); // 0 return 0; } } // namespace volatile_mutable_ 
           
          
         
        
       
      
     
    
  

GitHub：https://github.com/fengbingchun/Messy_Test