Why can't for_each modify its functor argument?

Question

http://www.cplusplus.com/reference/algorithm/for_each/
Unary function taking an element in the range as argument. This can either be a pointer to a

Answer 1

The function object is taken by value. for_each returns the function object, so if you change it to:

multiply = std::for_each(vec.begin(),vec.end(),multiply);

you get the expected output.

Answer 2

The semantics of For_each dont fit into what you are trying to do. accumulate does exactly what you are trying, use that instead.

Answer 3

While James is correct, using std::accumulate with std::multiplies would be more correct, probably:

#include <iostream>
#include <functional>
#include <numeric>
#include <vector>

int main(void)
{
    std::vector<double> vec;
    vec.push_back(1);
    vec.push_back(2);
    vec.push_back(3);

    double result = std::accumulate(vec.begin(), vec.end(),
                                    1.0, std::multiplies<double>());

    std::cout << "\nResult: " << result << std::endl;

}

With your for_each version, you don't really need to copy the functor again, rather:

double result = std::for_each(vec.begin(), vec.end(), multiply).result();

Or C++0x, for fun:

double result = 1;
std::for_each(vec.begin(), vec.end(), [&](double pX){ result *= pX; });