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C++类的 explicit 关键字和隐式转换
explicit 关键字用于 C++的类定义中,其作用是禁止隐式转换和复制初始化操作：
The explicit specifier specifies that a constructor or conversion
function (since C++11) doesn’t allow implicit conversions or
copy-initialization. It may only appear within the decl-specifier-seq
of the declaration of such a function within its class definition.
这里介绍一个概念:转换构造函数, 它的英文定义如下:
A constructor with a single non-default parameter (until C++11) that
is declared without the function specifier explicit is called a
converting constructor.
大意就是说不用 explicit 关键字声明, 只包含一个非默认参数的构造函数.
具体使用用一个例子来说明:
#include <iostream>using std::cout;struct A
{A(int) { cout << "A(int)\n"; } // converting constructorA(int, int) { cout << "A(int, int)\n"; } // converting constructor (C++11)A(const A&) {cout << "A(const A&)\n";}operator bool() const { return true; }
};struct B
{explicit B(int) { }explicit B(int, int) { }explicit operator bool() const { return true; }
};int main()
{A a1 = 1; // copy-initialization, call A(int)A a2(2); // direction-initialization, call A(int)A a3 {4, 5}; //direction-list-initialization, call A(int int)A a4 = {4, 5};//copy-list-initialization, call A(int int)A a5 = (A)1; //explicit castif(a1) //copy-initializat, call operator boolcout << "true\n";bool na1 = a1;cout << std::boolalpha << na1 << '\n';bool na2 = static_cast<bool>(a1); cout << std::boolalpha << na2 << '\n';//B b1 = 1; //no viable conversion from 'int' to 'B'B b2 = (B)1; B b3(2);B b4 {4, 5};//B b5 = {4, 5};//chosen constructor is explicit in copy-initializationif(b2)cout << "true\n";//bool nb1 = b2;//no viable conversion from 'B' to 'bool'bool nb1 = b2.operator bool();bool nb2 = static_cast<bool>(b2);
}
上例中选用了两个 struct, 这里出于使用方便, 你完全可以可用 class, 不过需要使用 public 声明成员函数. A 中没有使用 explicit 关键字, 所以对于 A 隐式转换和复制初始化都可以顺利进行, 但是与之相对 B 由于使用了 explicit 声明, 隐式转换和复制初始化被禁止.
B b1 = 1;
//no viable conversion from 'int' to 'B'B b2 = (B)1; //explicit conversion
B b5 = {4, 5};
//chosen constructor is explicit in copy-initializationbool nb1 = b2;//no viable conversion from 'B' to 'bool'bool nb1 = b2.operator bool();
这时, 我们或可以强制进行类型转换, 也可显示调用转换函数
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