【前言】

    今天遇到了個(gè)問題，被人問及 explicit 作用的時(shí)候，想不起來了。因此重新溫習(xí)了一下，詳細(xì)描述如下：

【正文】

    簡單的說，就是類的構(gòu)造函數(shù)禁止隱式類型轉(zhuǎn)換。先看一看微軟大哥是如何說的：

This keyword is a declaration specifier that can only be applied to inclass constructor declarations. Constructors declared explicit will not be considered for implicit conversions. For example:

class X {  public:     explicit X(int);      //legal     explicit X(double) {   //legal        // ...     }  };    explicit X::X(int) {}      //illegal

An explicit constructor cannot take part in implicit conversions. It can only be used to explicitly construct an object. For example, with the class declared above:

void f(X) {}  void g(int I) {     f(i);      // will cause error  }  void h() {     X x1(1);      // legal  }

The function call f(i) fails because there is no available implicit conversion from int to X.

Note   It is meaningless to apply explicit to constructors with multiple arguments, since such constructors cannot take part in implicit conversions.

總結(jié)一下，大致意思就是：

（1） explicit 只能在類的內(nèi)部使用，不能在外面聲明。

（2）聲明了explicit以后，就不允許隱式轉(zhuǎn)換了。

（3）explicit對于多個(gè)參數(shù)的構(gòu)造函數(shù)無任何意義。