Thread: Can I sum a std::vector at compile time?

Hi everyone. I have adapted this from page 9 of "A Tour of C++". I can't seem to sum the values in a std::vector<double> at compile time. Is it possible?

Code:

#include <iostream>
#include <vector>

using std::vector;

constexpr double sum(const vector<double>& v)
{
    double s = 0.0;

/* Will not compile:

    for (auto& x : v)
        s += x;

*/

/* Will not compile:

    for (vector<double>::size_type i = 0; i < v.size(); ++i)
        s += v[i];

*/
    return s;
}

int main()
{
    vector<double> v {1.2, 3.4, 4.5};

    const double s1 = sum(v);

    std::cout << s1 << std::endl;
}

What exactly is the point of the exercise?

Do you realize that you can't modify a const, and that a constexpr is a compile time constant expression (every thing must be computable at compile time.

I think the only real wrinkle in something like this is that I'm not sure vectors can be initialized at compile time.

It is definitely possible to achieve something similar. Here's a template meta-programming version.

Code:

C:\Users\jk\Desktop>more sum.cpp
#include <iostream>
#include <cstddef>

static constexpr double arr[3] = {1.2, 3.4, 4.5};

template <typename T, const T *arr, const size_t idx> struct sum
{
    static constexpr T value = arr[idx-1] + sum<T,arr,idx-1>::value;
};

template<typename T, const T *arr> struct sum<T, arr, 1>
{
    static constexpr T value = arr[0];
};


int main()
{
    std::cout << sum<double, arr, 3>::value << std::endl;
}

C:\Users\jk\Desktop>g++ -std=c++11 -c sum.cpp

C:\Users\jk\Desktop>g++ sum.o -o sum

C:\Users\jk\Desktop>sum
9.1