cpp——类——explicit构造函数

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概述

explicit只能修饰构造函数，构造函数默认implicit，implicit不是keyword

implicit构造函数：可用于直接初始化，也可用于复制初始化
explicit构造函数：可用于直接初始化，不可用于复制初始化

implicit

class CAnimal{public:    CAnimal(int group) : mGroup(group)    {        cout << "CAnimal(" << group << ")" << endl;    }        CAnimal(const CAnimal &other) : mGroup(other.mGroup)    {        cout << "CAnimal(const CAnimal &other) " << mGroup << endl;    }        CAnimal& operator=(const CAnimal &other)    {        mGroup = other.mGroup;        cout << "CAnimal operator= " << mGroup << endl;        return *this;    }    private:    int mGroup;};

void implicit_construct(){    {        cout << "-----init-----" << endl;        CAnimal animal1(1);        CAnimal animal2 = 2;        CAnimal animal3(animal1);        CAnimal animal4 = animal2;    }        {        cout << "-----assign-----" << endl;        CAnimal animal1(1);        CAnimal animal2(2);        animal2 = animal1;        animal2 = 3;    }}

output：

-----init-----CAnimal(1)CAnimal(2)CAnimal(const CAnimal &other) 1CAnimal(const CAnimal &other) 2-----assign-----CAnimal(1)CAnimal(2)CAnimal operator= 1CAnimal(3)CAnimal operator= 3

explicit

class CAnimal{public:    explicit CAnimal(int group) : mGroup(group)    {        cout << "CAnimal(" << group << ")" << endl;    }        explicit CAnimal(const CAnimal &other) : mGroup(other.mGroup)    {        cout << "CAnimal(const CAnimal &other) " << mGroup << endl;    }        CAnimal& operator=(const CAnimal &other)    {        mGroup = other.mGroup;        cout << "CAnimal operator= " << mGroup << endl;        return *this;    }    private:    int mGroup;};

void explicit_construct(){    {        cout << "-----init-----" << endl;        CAnimal animal1(1);        //CAnimal animal2 = 2;        CAnimal animal3(animal1);        //CAnimal animal4 = animal1;    }        {        cout << "-----assign-----" << endl;        CAnimal animal1(1);        CAnimal animal2(2);        animal2 = animal1;        //animal2 = 3;    }}

output：

-----init-----CAnimal(1)CAnimal(const CAnimal &other) 1-----assign-----CAnimal(1)CAnimal(2)CAnimal operator= 1

总结

CAnimal animal = value，复制初始化，如果构造函数CAnimal(value)为implicit，正确初始化，如果为explicit，编译error
CAnimal &other = value，初始化式期望右值value为CAnimal类型，若value为非CAnimal类型，如果构造函数CAnimal(value)为implicit，生成匿名CAnimal对象，如果为explicit，编译error
implicit构造函数可用于直接初始化，也可用于复制初始化
explicit构造函数可用于直接初始化，不可用于复制初始化
复制初始化只允许传递1个参数

直接初始化VS复制初始化

非类类型

非类类型直接初始化和复制初始化无区别，值复制

类类型

直接初始化可使用implicit构造函数，也可使用explicit构造函数
复制初始化可使用implicit构造函数，不可使用explicit构造函数
复制初始化只允许传递1个参数

class CAnimal{public:    CAnimal(int group, int subGroup = 1) : mGroup(group), mSubGroup(subGroup)    {        cout << "CAnimal(" << group << ", " << subGroup << ")" << endl;    }        CAnimal(const CAnimal &other) : mGroup(other.mGroup)    {        cout << "CAnimal(const CAnimal &other) " << mGroup << endl;    }        CAnimal& operator=(const CAnimal &other)    {        mGroup = other.mGroup;        cout << "CAnimal operator= " << mGroup << endl;        return *this;    }    private:    int mGroup;    int mSubGroup;};

void construct(){    CAnimal animal1(5);    CAnimal animal2(5, 8);    CAnimal animal3 = 5;    CAnimal animal4 = (5, 8);}

output：

CAnimal(5, 1)CAnimal(5, 8)CAnimal(5, 1)CAnimal(8, 1)

注：(5, 8)为逗号表达式，值为8，因此CAnimal animal4 = (5, 8)等价于CAnimal animal4 = 8，最终转换为CAnimal animal4(8)

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