C++中mutable与volatile的深入理解

前言

C++中修饰数据可变的关键字有三个：const、volatile和mutable。const比较好理解，表示其修饰的内容不可改变（至少编译期不可改变），而volatile和mutable恰好相反，指示数据总是可变的。mutable和volatile均可以和const搭配使用，但两者在使用上有比较大差别。

下面话不多说了，来一起看看详细的介绍吧

mutable

mutable只能作用在类成员上，指示其数据总是可变的。不能和const 同时修饰一个成员，但能配合使用：const修饰的方法中，mutable修饰的成员数据可以发生改变，除此之外不应该对类/对象带来副作用。

考虑一个mutable的使用场景：呼叫系统中存有司机(Driver)的信息，为了保护司机的隐私，司机对外展现的联系号码每隔五分钟从空闲号码池更新一次。根据需求，Driver类的实现如下伪代码：

class Driver {
private:
...
// real phone number
string phone;
// display phone number
mutable string displayPhone;

public:
string getDisplayPhone() const {
if (needUpdate()) {
lock.lock();
if (needUpdate()) {
updateDisplayPhone(); // displayPhone在这里被改变
}
lock.unlock();
}
return displayPhone;
}
};

在上述代码中，const方法中不允许对常规成员进行变动，但mutable成员不受此限制。对Driver类来说，其固有属性（姓名、年龄、真实手机号等）未发生改变，符合const修饰。mutable让一些随时可变的展示属性能发生改变，达到了灵活编程的目的。

volatile

volatile用于修饰成员或变量，指示其修饰对象可能随时变化，编译器不要对所修饰变量进行优化（缓存），每次取值应该直接读取内存。由于volatile的变化来自运行期，其可以与const一起使用。两者一起使用可能让人费解，如果考虑场景就容易许多：CPU和GPU通过映射公用内存中的同一块，GPU可能随时往共享内存中写数据。对CPU上的程序来说，const修饰变量一直是右值，所以编译通过。但其变量内存中的值在运行期间可能随时在改变，volatile修饰是正确做法。

在多线程环境下，volatile可用作内存同步手段。例如多线程爆破密码：

volatile bool found = false;

void run(string target) {
while (!found) {
// 计算字典口令的哈希
if (target == hash) {
found = true;
break;
}
}
}

在volatile的修饰下，每次循环都会检查内存中的值，达到同步的效果。

需要注意的是，volatile的值可能随时会变，期间会导致非预期的结果。例如下面的例子求平方和：

double square(volatile double a, volatile double b) {
return (a + b) * (a + b);
}

a和b都是随时可变的，所以上述代码中的第一个a + b可能和第二个不同，导致出现非预期的结果。这种情况下，正确做法是将值赋予常规变量，然后再相乘：

double square(volatile double a, volatile double b) {
double c = a + b;
return c * c;
}

一般说来，volatile用在如下的几个地方：

1. 中断服务程序中修改的供其它程序检测的变量需要加volatile；

2. 多任务环境下各任务间共享的标志应该加volatile；

3. 存储器映射的硬件寄存器通常也要加volatile说明，因为每次对它的读写都可能有不同意义；

总结

mutable只能用与类变量，不能与const同时使用；在const修饰的方法中，mutable变量数值可以发生改变；
volatile只是运行期变量的值随时可能改变，这种改变即可能来自其他线程，也可能来自外部系统。

好了，以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对猪先飞的支持。

参考

https://en.cppreference.com/w/cpp/language/cv

下面是其他网友的补充

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