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How can I create a format string using std functions (not the external fmt library), where the format arguments are contained within a std::vector<std::string>.

Something like the following (but this example version doesn't compile):

std::string format(std::string fmt, const std::vector<std::string>& list) {
    return std::vformat(fmt, std::make_format_args(list.begin(), list.end()));
}

How can I create a format string using std functions (not the external fmt library), where the format arguments are contained within a std::vector<std::string>.

Something like the following (but this example version doesn't compile):

std::string format(std::string fmt, const std::vector<std::string>& list) {
    return std::vformat(fmt, std::make_format_args(list.begin(), list.end()));
}

• c++

Share Improve this question Follow edited Mar 30 at 23:43 David G 96.9k4141 gold badges172172 silver badges258258 bronze badges asked Mar 30 at 23:28 DessDess 2,6872727 silver badges4141 bronze badges 3

• The library is no longer external as of C++20. – PaulMcKenzie Commented Mar 30 at 23:53
• @PaulMcKenzie: I'm talking about the actual 'fmt' external library, not the C++ format facilities. I obviously showed that in the example code. – Dess Commented Mar 30 at 23:58
• You can't. If you can at least change to a container with compile-time size, you can do something like this: godbolt./z/earGYsWvT – Intelligent Shade of Blue Commented Mar 31 at 0:58

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1 Answer 1

Sorted by: Reset to default 0

I don't think what you are attempting is possible with standard formatting functions. std::format() and friends utilize template parameter packs for their input arguments, and parameter packs can only be constructed at compile-time, not dynamically at runtime.

To do what you want, you will need to manually pull apart the fmt string into replacement and non-replacement substrings, and then build up the output std::string concatenation those substrings, making replacements as needed.

For example, if you have a fmt string "{} {} my name is {}" and a vector {"Hello", "World", "Dess"}, then you would extract the following substrings and then concatenate them together:

"{}" -> "Hello"
" "
"{}" -> "World"
" my name is "
"{}" -> "Dess"

However, note that prior to C++26, std::format() requires a string literal for its fmt parameter. So you can't use std::format() in this case. C++26 will introduce std::runtime_format so you can pass in a std::string/std::string_view constructed at runtime, but in the meantime, you will have to use std::vformat() and std::make_format_args() with single-argument replacements.

Try something like this:

std::string format(const std::string_view &fmt, const std::vector<std::string>& list) {
    std::string result;
    std::string::size_type start = 0, pos, end;
    std::vector<std::string>::size_type idx = 0;
    while ((pos = fmt.find('{', start)) != std::string::npos) {
        if (pos > start) result += fmt.substr(start, pos-start);
        if ((end = fmt.find('}', pos+1)) == std::string::npos) break;
        ++end;
        result += std::vformat(fmt.substr(pos, end-pos), std::make_format_args(list.at(idx++)));
        start = end;
    }
    if (start < fmt.size()) result += fmt.substr(start);
    return result;
}

Live Demo

Obviously, you'll have to make this more robust to support the full syntax that the fmt parameter supports, but this should give you an idea to start with.

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