c++ - How to print or format a std::filesystem::path? - Stack Overflow

I'm trying to write the following C++ function, however I cannot figure out how to format a std::filesystem::path object.

const auto load_test_file(std::filesystem::path filename) {

    std::ifstream ifile(filename, std::ios::in | std::ios::binary);

    if(!ifile.is_open()) {

        const auto error_message = std::format("failed to open file {}", filename.string());

        throw std::runtime_error(error_message.c_str());
    }
    
    // do some stuff here
}

I am using std::filesystem::path as the type of the argument to this function, because a std::string_view is not compatiable with the constructor of std::ifstream. (Or std::ofstream, for that matter.)

This reason for this is (AFAIK) that the underlying C API requires a null-terminated string, which a std::string_view does not guarantee.

I don't want to pass a std::string&, because this is inefficient if a char* is used in place of an already constructed std::string object. (Although to be honest in this context, where I am writing a unit test, this is really irrelevant.)

Therefore my understanding is a std::filesystem::path is the most correct type to use in this situation.

Is there a way to format a std::filesystem::path?

I'm trying to write the following C++ function, however I cannot figure out how to format a std::filesystem::path object.

const auto load_test_file(std::filesystem::path filename) {

    std::ifstream ifile(filename, std::ios::in | std::ios::binary);

    if(!ifile.is_open()) {

        const auto error_message = std::format("failed to open file {}", filename.string());

        throw std::runtime_error(error_message.c_str());
    }
    
    // do some stuff here
}

I am using std::filesystem::path as the type of the argument to this function, because a std::string_view is not compatiable with the constructor of std::ifstream. (Or std::ofstream, for that matter.)

This reason for this is (AFAIK) that the underlying C API requires a null-terminated string, which a std::string_view does not guarantee.

I don't want to pass a std::string&, because this is inefficient if a char* is used in place of an already constructed std::string object. (Although to be honest in this context, where I am writing a unit test, this is really irrelevant.)

Therefore my understanding is a std::filesystem::path is the most correct type to use in this situation.

Is there a way to format a std::filesystem::path?

• c++
• c++23
• stdformat

Share Improve this question Follow asked Mar 6 at 14:08 user2138149user2138149 17.7k3030 gold badges150150 silver badges296296 bronze badges 9

• It has a constructor from string (same with assignment). – wohlstad Commented Mar 6 at 14:10
• @wohlstad I'm unsure what you intend me to do with it? The point here being that using a std::string as the function argument type is probably the wrong choice, no? – user2138149 Commented Mar 6 at 14:14
• I thought you wanted to initialize a std::filesysyem::path. The link I posted has some examples for that. If it's something else then I missunderstood the issue. – wohlstad Commented Mar 6 at 14:16
• 1 while path is the correct type to use if that is what the string represents, if there is not a built in formatter for path then you might as well fall back to using a string. Both path and string are going to copy a char* so you are not gaining any performance by using path instead of string – NathanOliver Commented Mar 6 at 14:17
• 7 std::format support for std::filesystem::path is only available in C++26 which is not yet implemented in all three compilers. – 康桓瑋 Commented Mar 6 at 14:28

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1 Answer 1

Sorted by: Reset to default 1

According to https://en.cppreference/w/cpp/compiler_support via Ctrl+f 'std::formatter<std::filesystem::path>' it seems it's not supported by any compiler at the moment.

However, if you can use external libraries, consider using fmt:

#include <filesystem>
#include <fmt/std.h>

int main()
{
    std::filesystem::path path{"/home/user/a_path"};
    fmt::print("fmt: {}\n", path);
    return 0;
}

https://godbolt./z/9rbsv1oGc