Using {fmt} to format containers

Question

I am trying to use {fmt} to print a std::vector or std::array like this:

std::vector<double> vec = {1., 2., 3.};
fmt::print("{:0.3f}\n", fmt::join(vec, ","));

Thing is, I hope to print a transformed vector:

std::vector<double> vec = {1., 2., 3.};
std::vector<double> vec_dup;
std::transform(vec.begin(), vec.end(), std::back_inserter(vec_dup), [](auto x){return x * M_PI;});
fmt::print("{:0.3f}\n", fmt::join(vec_dup, ","));

Is there a way to do this in C++17 without the need to create a new container?

Answer 1

Let's first see how in C++20 it can be easily achieved using the standard <ranges> library (Godbolt):

    std::vector<double> vec = {1., 2., 3.};

    auto func = [](auto x){ return x * M_PI; };
    auto&& new_range = vec | std::views::transform(func);
    
    fmt::print("{:0.3f}\n", fmt::join(new_range, ","));

If you are limited to C++17 you have two options:

1. Use the 3rd party ranges-v3 library implementing the same functionality. For it, just replace in the above code:

   #include <range>       --> <range/v3/view/transform.hpp>
   std::views::transform  --> ranges::views::transform
   

   Godbolt demonstrating this.

2. Implement such a generator class yourself. The class should get the source range and generator function as parameters, implement begin() and end() functions returning a helper iterator class. The helper iterator class should keep iterator to the underlying range, and call the generator function on de-reference.