C++：禁止隐式构造

如果没有明确的禁止，C++默认是开启了隐式构造：

#include <iostream>
using namespace std;

class A{
public:
	A(int a = 0)
	{
		cout<<"Construct A, this addr:"<<this<<endl;
	}
	A(const A& a)
	{
		cout<<"Copy Construct A, origin a addr:"<<&a<<", this addr:"<<this<<endl;
	}
};

void func(A a)
{
	cout<<"func a addr:"<<&a<<endl;
}

int main(){

	A a1(1);    //显示构造
	cout<<"a1 addr:"<<&a1<<endl;
	A a2 = 2;   //隐式构造
	cout<<"a2 addr:"<<&a2<<endl;
	func(A(3)); //显示构造
	func(4);    //隐式构造
	return 0;
}

运行程序输出：
Construct A, this addr:0x63fe1a        //通过默认构造函数，显示构造a1
a1 addr:0x63fe1a

Construct A, this addr:0x63fe1b        //通过隐式构造函数，先构造一个临时对象,记作tmpA1
Copy Construct A, origin a addr:0x63fe1b, this addr:0x63fe19   //拷贝构造函数再通过临时对象tmpA1构造对象a2        
a2 addr:0x63fe19

Construct A, this addr:0x63fe1d       //通过默认构造函数，显示构造匿名对象，记作tmpA3
Copy Construct A, origin a addr:0x63fe1d, this addr:0x63fe1c    //拷贝构造函数通过匿名对象tmpA3构造函数参数a    
func a addr:0x63fe1c

Construct A, this addr:0x63fe1f        //通过默认构造函数，隐式构造一个临时对象记作tmpA4
Copy Construct A, origin a addr:0x63fe1f, this addr:0x63fe1e    //拷贝构造函数通过临时对象tmpA4构造函数参数a    
func a addr:0x63fe1e

通过将构造函数声明explicit，可以禁止隐式构造：

#include <iostream>
using namespace std;

class A{
public:
	explicit A(int a = 0)
	{
		cout<<"Construct A, this addr:"<<this<<endl;
	}
	A(const A& a)
	{
		cout<<"Copy Construct A, origin a addr:"<<&a<<", this addr:"<<this<<endl;
	}
};

void func(A a)
{
	cout<<"func a addr:"<<&a<<endl;
}

int main(){

	A a1(1);        //显示构造，可以通过编译
	A a2 = 2;       //隐式构造，不能通过编译，error: conversion from 'int' to non-scalar type 'A' requested
	func(A(3));     //显示构造，可以通过编译
	func(4);        //隐式构造，不能通过编译，error: could not convert '4' from 'int' to 'A'
	return 0;
}