


How Does `std::enable_if` Work: Unraveling the Mysteries of Its Implementation and Usage?
Understanding std::enable_if: Deciphering Its Purpose and Implementation
While the nature of std::enable_if is grasped in certain contexts, its intricacies, particularly the second argument and the assignment to std::enable_if within the template statement, remain enigmatic. Delving deeper into its workings will unravel these mysteries.
The Essentials of std::enable_if
std::enable_if is a specialized template defined as follows:
<code class="cpp">template<bool cond class t="void"> struct enable_if {}; template<class t> struct enable_if<true t> { typedef T type; };</true></class></bool></code>
Crucially, the type alias typedef T type is only defined when Cond is true.
Unveiling the Usage
Consider the following declaration:
<code class="cpp">template<typename t> typename std::enable_if<:numeric_limits>::is_integer, void>::type foo(const T &bar) { isInt(bar); }</:numeric_limits></typename></code>
Here, the return type of foo is defined by std::enable_if<:numeric_limits>::is_integer, void>::type. Since std::numeric_limits
Clarifying the Second Argument
In the notation:
<code class="cpp">template<typename t typename std::enable_if>::value, int>::type = 0> void foo(const T& bar) { isInt(); }</typename></code>
The = 0 is utilized to default the second template parameter. This allows both options to be invoked using foo
Noteworthy Details
- Explicitly typing out typename std::enable_if<:condition t>::type enhances clarity.
- In C 14, enable_if_t is an established type that should be employed for the return type, simplifying it to std::enable_if_t
. - For Visual Studio versions prior to 2013, only the return type can employ enable_if.
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