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Some template metaprogramming techniques that employ "substitution failure is not an error" (SFINAE) use variadic functions to implement compile-time type queries, as in:

typedef char True;
typedef struct { char a[2]; } False;

template <typename T>
True isPtr(T *);

False isPtr(...);

#define is_ptr(e) (sizeof(isPtr(e)) == sizeof(True))

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This example uses a variadic function to concatenate an arbitrary number of null-terminated character sequences (NTCS) in a single NTCS. Each call to the function must use a null pointer value to mark the end of the argument list.

#include <cstdarg>

char *concatenate(char const *s, ...)
    {
    // code to actually concatenate the strings
    }

char *separator = /* some reasonable value */;

char *t = concatenate("hello", separator, "world", NULL);

Calling this function without the trailing null pointer, or with an argument of any type other than "pointer to possibly-CV-qualified char" yields undefined behavior:

char *u = concatenate("hello", separator, "world"); // undefined behavior

char *v = concatenate("hello", ' ', "world", NULL); // undefined behavior

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Rather than use a variadic function, you can use a chain of binary operations:

#include <string>

string separator = /* some reasonable value */;

string s = "hello" + separator + "world";

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