C++ mutable关键字

1、mutable关键字作用

今天在看代码的时候，发现有个关键字mutable好像没见过，特此记录一下其使用方法。mutable是用来修饰在可以在const函数中改变的成员变量。其使用比较简单。

2、mutable示例

如下面代码和编译结果所示，如果在const函数中修改数据，编译器会提示报错，从而无法通过编译。

#include <iostream>
#include <string.h>
#include <vector>
#include <stdio.h>
#include <stdlib.h>
#include <sys/io.h>
#include <unistd.h>
#include <fstream>
using namespace std;

class Test {
public:
    Test();
    int getIntData() const;
    std::string getStringData() const;
    float getFloatData() const;
private:
    float mFloatData;
    int mIntData;
    std::string mStringData;
};

Test::Test() {
    mFloatData = 15.0;
    mIntData = 10;
    mStringData = "[Test] init string data.";
}

int Test::getIntData() const {
    mIntData = mIntData + 10;
    return mIntData;
}

std::string Test::getStringData() const {
    mStringData = mStringData + std::string("this is a string test.");
    return mStringData;
}

float Test::getFloatData() const {
    mFloatData = mFloatData + 13.3;
    return mFloatData;
}

int main() {
    class Test test;
    std::cout << "int:" << test.getIntData() << std::endl;
    std::cout << "float:" << test.getFloatData() << std::endl;
    std::cout << "string:" << test.getStringData() << std::endl;
    
}

增加metable关键字修饰的成员变量，则可以通过编译，并且执行，符合预期结果。

#include <iostream>
#include <string.h>
#include <vector>
#include <stdio.h>
#include <stdlib.h>
#include <sys/io.h>
#include <unistd.h>
#include <fstream>
using namespace std;

class Test {
public:
    Test();
    int getIntData() const;
    std::string getStringData() const;
    float getFloatData() const;
private:
    mutable float mFloatData;
    mutable int mIntData;
    mutable std::string mStringData;
};

Test::Test() {
    mFloatData = 15.0;
    mIntData = 10;
    mStringData = "[Test] init string data.";
}

int Test::getIntData() const {
    mIntData = mIntData + 10;
    return mIntData;
}

std::string Test::getStringData() const {
    mStringData = mStringData + std::string("this is a string test.");
    return mStringData;
}

float Test::getFloatData() const {
    mFloatData = mFloatData + 13.3;
    return mFloatData;
}

int main() {
    class Test test;
    std::cout << "int:" << test.getIntData() << std::endl;
    std::cout << "float:" << test.getFloatData() << std::endl;
    std::cout << "string:" << test.getStringData() << std::endl;
    
}

执行结果如下：