目的

指定构造函数或转换函数为显示，即不能用于隐式转化；

语法

一般出现在类定义之内的构造函数中；
如：

class P
{explict P(){};
};

代码示例

#include <iostream>class Test
{
public:Test(int a):a_(a){std::cout << "Test()" << std::endl;  // 通过打印看调用几次构造函数}int Get_Data(){return a_;}Test operator + (Test t){return Test(a_ + t.a_);}
private:int a_;};int main()
{Test t1{3};Test t2 = t1 + 5;   // 5 会隐式的调用的Test构造函数std::cout << t2.Get_Data() << std::endl;  //输出 8
}

在上一个的例子的基础之上加上 explict

#include <iostream>class Test
{
public:explicit Test(int a):a_(a){std::cout << "Test()" << std::endl;  // 通过打印看调用几次构造函数}int Get_Data(){return a_;}Test operator + (Test t){return Test(a_ + t.a_);}
private:int a_;};int main()
{Test t1{3};Test t2 = t1 + 5;  //error   // error: no match for 'operator+' (operand types are 'Test' and 'int')//说明 5 还是个int类型， 不能隐式的转化为Test}

explicit用于多个参数

#include <iostream>
struct P
{P(int a, int b){std::cout << "P(int a, int b)" << std::endl;}explicit P(int a, int b, int c){std::cout << "explicit P(int a, int b, int c)" << std::endl;}
};int main()
{P p1(1,2);    //OK P(int a, int b)P p2{1,2};    //OK P(int a, int b)P p3 = {1,2}; //OK P(int a, int b)P p4(1,2,3);  //OK explicit P(int a, int b, int c)P p5{1,2,3};  //OK explicit P(int a, int b, int c)P p6 = {1,2,3}; //error:converting to 'P' from initializer list would use explicit constructor 'P::P(int, int, int)}