Tag dispatch versus static methods on partially specialised classes

Question

Suppose I want to write a generic function void f(), which does one thing if T is a POD type and another thing if T is non-PO

Answer 1

A readable alternative to [boost|std]::enable_if, tags and partial specialization for simple compile-time dispatch that I like is the following:

[Remember that booleans have conversion to integers, that zero-length arrays are invalid and that offending templates are discarded (SFINAE). Also, char (*)[n] is a pointer to an array of n elements.]

template  
void foo(T, char (*)[is_pod::value] = 0)
{
    // POD
}

template  
void foo(T, char (*)[!is_pod::value] = 0)
{
    // Non POD
}

It also has the advantage of not needing external classes which pollute the namespace. Now, if you want to externalize the predicate like in your question, you can do:

template 
void foo(T, char (*)[what] = 0)
{
    // taken when what is true
}

template 
void foo(T, char (*)[!what] = 0)
{
    // taken when what is false
}

Usage:

foo::value>(some_variable);