c – 应用于运算符而不是构造函数的显式关键字

I thought that explicit was to prevent implicit calling of
constructors?

从C 11开始,它也适用于user-defined conversions(也就是演员操作符).

Why would you make the operators explicit

在此上下文中使用,explicit关键字使转换仅适用于直接初始化和显式转换.见[class.conv.fct?2]：

A conversion function may be explicit ([dcl.fct.spec]),in which case
it is only considered as a user-defined conversion for
direct-initialization ([dcl.init]). Otherwise,user-defined
conversions are not restricted to use in assignments and
initializations.

这有助于您确保编译器不会针对您的意图尝试转换,因此您必须自己明确地进行转换,从而减少出错的空间.例：

struct Foo
{
    explicit operator int() {return 0;}
    operator int*() {return nullptr;}
};

int main()
{
    Foo foo;

    //int xi = foo; // Error,conversion must be explicit
    int i = static_cast<int>(foo); // OK,conversion is explicit
    int* i_ptr = foo; // OK,implicit conversion to `int*` is allowed

    int i_direct(foo); // OK,direct initialization allowed
    int* i_ptr_direct(foo); // OK,direct initialization after implicit conversion

    return 0;
}

它还可以帮助解决多个转换选项适用的情况下的歧义,使编译器没有标准来决定选择哪一个：

struct Bar
{
    operator int() {return 1;}
    operator char() {return '1';}
};

int main()
{
    Bar bar;    
    //double d = bar; // Error,implicit conversion is ambiguous    
    return 0;
}

添加明确：

struct Bar
{
    operator int() {return 1;}
    explicit operator char() {return '1';}
};

int main()
{
    Bar bar;    
    double d = bar; // OK,implicit conversion to `int` is the only option    
    return 0;
}