std::bind1st

#include <iostream>
#include <algorithm>
#include <functional>
#include <cmath>
#include <vector>
 
int main()
{
    std::vector<double> a= {0, 30, 45, 60, 90, 180};
    std::vector<double> r(a.size());
    double pi = std::acos(-1);
 
    std::transform(a.begin(), a.end(), r.begin(),
        std::bind1st(std::multiplies<double>(), pi / 180.));
// equivalent lambda: [pi](double a){ return a*pi/180.; });
 
    for(size_t n = 0; n < a.size(); ++n)
        std::cout << a[n] << " deg = " << r[n] << " rad\n";
}

0 deg = 0 rad
30 deg = 0.523599 rad
45 deg = 0.785398 rad
60 deg = 1.0472 rad
90 deg = 1.5708 rad
180 deg = 3.14159 rad

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