explicit

#include<iostream>
using namespace std;
class Test1
{
public:
    Test1(int n){
    num = n;
    }
private:
    int num;
};
class Test2
{
public:
    explicit Test2(int n){num = n;}
private:
    int num;
};

int main()
{
    Test1 t1 = 12;
    cout<<"abc"<<endl;
   // Test2 t2 = 12;//error: conversion from ‘int’ to non-scalar type ‘Test2’ requested
}