C++ static compilation of function with a variable number or arguments

Question

I found myself writing code like this:

template <class T>
inline bool allequal(T a,T b,T c) {return a==b&&a==c;}
template <class T>
inline bool allequal(T a,T b,T c,T d) {return a==b&&a==c&&a==d;}
template <class T>
inline bool allequal(T a,T b,T c,T d,T e) {return a==b&&a==c&&a==d&&a==e;}

I was wondering if there is an automated way of doing that, without using vectors or variadic arguments, as speed is important in this context.

Answer 1

You could try this:

#include <iostream>

template< typename T >
inline bool allequal( const T& v )
{
  return true;
}

template< typename T, typename U, typename... Ts >
inline bool allequal( const T& v, const U& v1, const Ts&... Vs)
{
  return (v == v1) && allequal( v1, Vs... );
}

int main()
{
    std::cout << std::boolalpha;
    std::cout << allequal(1, 1, 1, 1) << std::endl;
    std::cout << allequal(1, 1, 2, 2) << std::endl;
    return 0;
}

AFAIK there is a proposal for C++17 to include custom expansion of variadic templates with operators that could avoid this kind of recursion and this ugly (IMO) termination with return true for one argument.

Note: this could possibly not be inlined for many arguments.

Answer 2

#include <algorithm>
#include <iterator>

template <typename T, typename... Args>
bool allequal(T a, Args... rest)
{
    std::initializer_list<T> values = {a, rest...};
    return std::adjacent_find(begin(values), end(values), std::not_equal_to<T>()) == end(values);
}

Working demo

Answer 3

template<typename First, typename Second>
bool AllOf(First a, Second b)
{
    return a == b;
}

template<typename First, typename Second, typename... Rest>
bool AllOf(First a, Second b, Rest... r)
{
    return  a == b && AllOf(b, r...);
}


std::cout << AllOf(1, 1,1,1) << "\n";
std::cout << AllOf(1, 1,2) << "\n";
std::cout << AllOf(1,2) << "\n";
std::cout << AllOf(1,1) << "\n";