explicit.md

C++ explicit

• explicit关键字用来修饰类的构造函数，被修饰的构造函数的类，不能发生相应的隐式类型转换，只能以显示的方式进行类型转换。

• 示例

    #include<cstring>
    #include<string>
    #include<iostream>
  
    class Explicit
    {
        private:
  
        public:
            Explicit(int size)
            {
                std::cout << " the size is " << size << std::endl;
            }
            Explicit(const char* str)
            {
                std::string _str = str;
                std::cout << " the str is " << _str << std::endl;
            }
  
            Explicit(const Explicit& ins)
            {
                std::cout << " The Explicit is ins" << std::endl;
            }
  
            Explicit(int a,int b)
            {
                std::cout << " the a is " << a  << " the b is " << b << std::endl;
            }
    };
  
    int main()
    {
        Explicit test0(15);
        Explicit test1 = 10;// 隐式调用Explicit(int size)
  
        Explicit test2("RIGHTRIGHT");
        Explicit test3 = "BUGBUGBUG";// 隐式调用Explicit(const char* str)
  
        Explicit test4(1, 10);
        Explicit test5 = test1;
    }
  

• 原因 上面的程序虽然没有错误，但是对于Explicit test1 = 10;和Explicit test2 = "BUGBUGBUG";这样的句子，把一个int类型或者const char*类型的变量赋值给Explicit类型的变量看起来总归不是很好，并且当程序很大的时候出错之后也不容易排查。所以为了禁止上面那种隐式转换可能带来的风险，一般都把类的单参构造函数声明的显示调用的，就是在构造函数加关键字explicit。如下：

    #include<cstring>
    #include<string>
    #include<iostream>
  
    class Explicit
    {
        private:
  
        public:
            explicit Explicit(int size)
            {
                std::cout << " the size is " << size << std::endl;
            }
            explicit Explicit(const char* str)
            {
                std::string _str = str;
                std::cout << " the str is " << _str << std::endl;
            }
  
            Explicit(const Explicit& ins)
            {
                std::cout << " The Explicit is ins" << std::endl;
            }
  
            Explicit(int a,int b)
            {
                std::cout << " the a is " << a  << " the b is " << b << std::endl;
            }
    };
  
    int main()
    {
        Explicit test0(15);
        Explicit test1 = 10;// 无法调用
  
        Explicit test2("RIGHTRIGHT");
        Explicit test3 = "BUGBUGBUG"; // 无法调用
  
        Explicit test4(1, 10);
        Explicit test5 = test0;
    }