c++containersrangeseparatorfmt

Control {fmt} formatting of nested containers/ranges

If I have a range I can print it with the fmt library, in this way: https://fmt.dev/8.1.0/api.html#ranges-api

#include <fmt/ranges.h>

std::vector<int> t = {1, 2, 3};
// Prints "[1, 2, 3]"
fmt::print("{}", t);

And if I want to control the separator:

// Prints "1| 2| 3"
fmt::print("{}", fmt::join(t, "| "));

What happens if I have a nested containers, for example std::vector<std::vector<int>>, how can I control the nested separator?

std::vector<std::vector<int>> v = ...
fmt::print("{}", fmt::join(t, "| "));

this will print [1, 2, 3]| [4, 5, 6]| [7, 8, 9].

Note that I can only control the top level format.

For context, I am trying to control formatting of multidimensional arrays: https://godbolt.org/z/M9cTEox7c

Solution

  • You can use ranges for that:

    #include <fmt/ranges.h>

#include <ranges>
#include <vector>

int main() {
    std::vector<std::vector<int>> v = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
    constexpr auto joinIntVec = [](auto& ve) { return fmt::join(ve, "~"); };

    fmt::print("{}", fmt::join(std::views::transform(v, joinIntVec), "| "));
}