C++ essentials 之 explicit constructor

这篇博客的源起是下面的一段代码

#include <bits/stdc++.h>
using namespace std;

int main(){
    priority_queue<long long> que;
    // some operations on que
    que = {};
    // some operations on que
    return 0;
}

其中 que = {} 相当于 que.clear();（std::priority_queue 并没有 clear() 方法）。以前我清空 priority_queue 用的是

while(!que.empty()){
    que.pop();
}

然而编译器对这个语句给出了一个警告：
（g++ 6.3.0：g++ -Wall -std=c++14）

In function 'int main()':
7:12: warning: converting to 'std::priority_queue<long long int>' from initializer list would use explicit constructor 'std::priority_queue<_Tp, _Sequence, _Compare>::priority_queue(const _Compare&, _Sequence&&) [with _Tp = long long int; _Sequence = std::vector<long long int, std::allocator<long long int> >; _Compare = std::less<long long int>]'
    que = {};
           ^
7:12: note: in C++11 and above a default constructor can be explicit

概念

implicit class-type conversion

Every constructor that can be called with a single argument defines an implicit conversion to a class type. Such constructors are sometimes referred to as conversion constructors.

explicit constructors

We can prevent the use of a constructor in a context that requires an implicit conversion by declaring the constructor as explicit.

The explicit keyword is meaningful only on constructors that can be called with a single argument. Constructors that require more arguments are not used to perform a conversion, so there is no need to designate such constructors as explicit. The explicit keyword is used only on the constructor declaration inside the class. It is not repeated on a definition made outside the class body.

One context in which implicit conversions happen is when we use the copy form of initialization (with an =). An explicit constructor cannot be used with the copy form of initialization; it can be used only with the direct form of initialization. Moreover, the compiler will not use this constructor in an automatic conversion.