std :: enable_if如何工作? [英] How Does std::enable_if work?
问题描述
我刚刚问了一个问题: std :: numeric_limits作为条件
I just asked this question: std::numeric_limits as a Condition
我了解 std :: enable_if
将有条件地定义方法的返回类型的用法,导致该方法无法执行
I understand the usage where std::enable_if
will define the return type of a method conditionally causing the method to fail to compile.
template<typename T>
typename std::enable_if<std::numeric_limits<T>::is_integer, void>::type foo(const T &bar) { isInt(bar); }
我不明白的是第二个论点和对<$ c $的看似毫无意义的赋值c> std :: enable_if 作为模板语句的一部分声明时,例如 Rapptz 答案。
What I don't understand is the second argument and the seemingly meaningless assignment to std::enable_if
when it's declared as part of the template statement, as in Rapptz answer.
template<typename T, typename std::enable_if<std::is_integral<T>::value, int>::type = 0>
void foo(const T& bar) { isInt(); }
推荐答案
正如40two ,对替换失败不是错误是理解 std :: enable_if
的先决条件。
As is mentioned in comment by 40two, understanding of Substitution Failure Is Not An Error is a prerequisite for understanding std::enable_if
.
std :: enable_if
是一个专用模板,定义为:
std::enable_if
is a specialized template defined as:
template<bool Cond, class T = void> struct enable_if {};
template<class T> struct enable_if<true, T> { typedef T type; };
此处的关键在于 typedef T type $ c仅当
bool Cond
为 true
时定义$ c。
The key here is in the fact that typedef T type
is only defined when bool Cond
is true
.
现在有了对 std :: enable_if
的理解,很明显 void foo(const T& bar){isInt(bar); }
的定义如下:
Now armed with that understanding of std::enable_if
it's clear that void foo(const T &bar) { isInt(bar); }
is defined by:
template<typename T>
typename std::enable_if<std::numeric_limits<T>::is_integer, void>::type foo(const T &bar) { isInt(bar); }
如 firda的答案, = 0
是第二个模板参数的默认值。 template< typename T,typename std :: enable_if< std :: is_integral< T> :: value,int> :: type = 0>
默认的原因是这样两个选项都可以使用 foo< int>(1);
。如果未默认 std :: enable_if
模板参数,则调用 foo
将需要两个模板参数,而不仅仅是 int
。
As mentioned in firda's answer, the = 0
is a defaulting of the second template parameter. The reason for the defaulting in template<typename T, typename std::enable_if<std::is_integral<T>::value, int>::type = 0>
is so that both options can be called with foo< int >( 1 );
. If the std::enable_if
template parameter was not defaulted, calling foo
would require two template parameters, not just the int
.
一般说明,通过显式使此答案更清晰输入 typename std :: enable_if< std :: numeric_limits< T> :: is_integer,void> :: type
但 void
是 std :: enable_if
的默认第二个参数,如果您具有 c ++ 14 enable_if_t
是已定义的类型,应使用。因此,返回类型应压缩为: std :: enable_if_t< std :: numeric_limits< T> :: is_integer>
General note, this answer is made clearer by explicitly typing out typename std::enable_if<std::numeric_limits<T>::is_integer, void>::type
but void
is the default second parameter to std::enable_if
, and if you have c++14 enable_if_t
is a defined type and should be used. So the return type should condense to: std::enable_if_t<std::numeric_limits<T>::is_integer>
针对visual-studio 在视觉-studio-2013 :不支持默认模板参数,因此您只能在函数返回值上使用 enable_if
:std::numeric_limits作为条件
A special note for users of visual-studio prior to visual-studio-2013: Default template parameters aren't supported, so you'll only be able to use the enable_if
on the function return: std::numeric_limits as a Condition
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