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Why Does `enable_if_t` Fail in Function Definitions?

Nov 18, 2024 am 03:21 AM

Why Does `enable_if_t` Fail in Function Definitions?

Clash of Template Arguments: Why enable_if_t Fails in Definitions

In template programming, it's not uncommon to encounter situations where we need to conditionally enable or disable template functions based on a specific condition. In such cases, the std::enable_if template is often used to achieve this. However, a recent attempt to employ the newer syntax of std::enable_if_t has led to unexpected errors.

In the code snippet below, we define two template functions f() using std::enable_if:

template&lt;typename T,
         typename std::enable_if&lt;std::is_same&lt;int, T&gt;::value&gt;::type* = nullptr&gt;
void f() { }

template&lt;typename T,
         typename std::enable_if&lt;std::is_same&lt;double, T&gt;::value&gt;::type* = nullptr&gt;
void f() { }

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When porting this code to use the new std::enable_if_t syntax, we encounter an issue:

template&lt;typename T,
         typename = std::enable_if_t&lt;std::is_same&lt;int, T&gt;::value&gt;&gt;
void g() { }

template&lt;typename T,
         typename = std::enable_if_t&lt;std::is_same&lt;double, T&gt;::value&gt;&gt;
void g() { }

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GCC complains with the following error:

error: redefinition of 'template&lt;class T, class&gt; void g()'
       void g() { }

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To understand why this error occurs, we need to examine the syntax of template arguments. Removing unnecessary code, we have the following simplified representation of our functions:

template&lt;
  class T,
  class U/* = std::enable_if_t&lt;std::is_same&lt;int, T&gt;::value&gt;*/
&gt;
void g() { }

template&lt;
  class T,
  class U/* = std::enable_if_t&lt;std::is_same&lt;double, T&gt;::value&gt;*/
&gt;
void g() { }

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It becomes clear that both templates are of the same type template<class,class>void(), regardless of their different default values for the second type argument. This conflicts with the C requirement that two template functions of the same template type need to have different signatures.

To solve this issue, we can change the second argument to a pointer type with a dependent type:

template&lt;
  class T,
  std::enable_if_t&lt;std::is_same&lt;int, T&gt;::value, int&gt;* = nullptr
&gt;
void f() { }

template&lt;
  class T,
  std::enable_if_t&lt;std::is_same&lt;double, T&gt;::value, int&gt;* = nullptr
&gt;
void f() { }

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In this modified code, the signature of the template functions is now unique, eliminating the conflict and allowing the correct template matching.

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