Beispiellösung

#ifndef DIM_HPP
#define DIM_HPP

#include <cstdlib>

std::size_t dim(const char* const* argv) {
   std::size_t count = 0;
   while (*argv++) ++count;
   return count;
}

template<typename T, std::size_t N>
constexpr std::size_t dim(const T (&)[N]) {
   return N;
}

template<typename T>
auto dim(const T& container) -> decltype(container.size()) {
   return container.size();
}

#endif

#ifndef DOTPROD_HPP
#define DOTPROD_HPP

#include <cassert>
#include <iterator>
#include "dim.hpp"

template<typename A, typename B>
auto dotprod(const A& a, const B& b) ->
      decltype(
	 dim(a), dim(b),
	 std::begin(a) + 3,
	 std::begin(b) + 3,
	 *std::begin(a) + *std::begin(a) * *std::begin(b)
      ) {
   using T = decltype(*std::begin(a) * *std::begin(b));
   auto len = dim(a);
   using Index = decltype(len);
   assert(len = dim(b));
   T sum{};
   for (Index i = 0; i < len; ++i) {
      sum += *(std::begin(a) + i) * *(std::begin(b) + i);
   }
   return sum;
}

#endif

#include <iostream>
#include <vector>
#include "dotprod.hpp"

int main() {
   std::vector<double> a = {2.0, 4.5, 7.0};
   int b[] = {4, 5, 6};
   std::cout << "a dot b = " << dotprod(a, b) << std::endl;
}

#ifndef DOTPROD_HPP
#define DOTPROD_HPP

#include <cassert>
#include <iterator>

template<typename A, typename B>
auto dotprod(const A& a, const B& b) ->
      decltype(
	 std::end(a), std::end(b),
	 *std::begin(a) + *std::begin(a) * *std::begin(b)
      ) {
   using T = decltype(*std::begin(a) * *std::begin(b));
   auto it1 = std::begin(a);
   auto it2 = std::begin(b);
   T sum{};
   for(;;) {
      bool end1 = it1 == std::end(a);
      bool end2 = it2 == std::end(b);
      assert(end1 == end2);
      if (end1) break;
      auto val1 = *it1++;
      auto val2 = *it2++;
      sum += val1 * val2;
   }
   return sum;
}

#endif

#include <iostream>
#include <vector>
#include "dotprod2.hpp"

int main() {
   std::vector<double> a = {2.0, 4.5, 7.0};
   int b[] = {4, 5, 6};
   std::cout << "a dot b = " << dotprod(a, b) << std::endl;
}