C++中关键字volatile和mutable用法

小咪咪 2023-07-02 11:29 9阅读 0赞

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 <iostream>
    #include <stdio.h>
    #include <time.h>
    #include <mutex>
    #include <string.h>
    
    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<int&>(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<int&>(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<std::mutex> lk(m);
    			return data;
    		}
    		void inc() {
    			std::lock_guard<std::mutex> 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/105014/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][https_github.com_fengbingchun_Messy_Test]

[https_github.com_fengbingchun_Messy_Test]: https://github.com/fengbingchun/Messy_Test