How to utilize "format" in C++?

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Greetings Kind Regards I am completely befuddled re/ C++ format. The code below generates the compiler errors shown. Kindly Advise Thank You

#include <format>
#include <iostream>

void foo(std::string_view _format)
{
	std::cout << std::format(_format);
}

int main()
{
	std::string_view _format = "Hello World\n";
	std::cout << std::format(_format);
	foo(_format);
}

Build started...
1 > ------Build started : Project: ONLY_stl, Configuration : Debug x64------
1 > ONLY_stl.C++
1 > C:\Users\Bernard\source\repos\ONLY_stl\ONLY_stl.C++(6, 27) : error C7595 : 'std::_Basic_format_string<char>::_Basic_format_string' : call to immediate function is not a constant expression
1 > C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.32.31302\include\format(2959, 63) : message: failure was caused by a read of a variable outside its lifetime
1 > C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.32.31302\include\format(2959, 63) : message: see usage of '_format'
1 > C:\Users\Bernard\source\repos\ONLY_stl\ONLY_stl.C++(12, 27) : error C7595 : 'std::_Basic_format_string<char>::_Basic_format_string' : call to immediate function is not a constant expression
1 > C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.32.31302\include\format(2959, 63) : message: failure was caused by a read of a variable outside its lifetime
1 > C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.32.31302\include\format(2959, 63) : message: see usage of '_format'
1 > Done building project "ONLY_stl.vcxproj" --FAILED.
========== Build: 0 succeeded, 1 failed, 0 up - to - date, 0 skipped ==========

What I have tried:

I tried every possible type declaration for the format argument passed to foo() i.e. to wit e.g. basic_string, basic_string&, const basic_string, const basic_string&, constexpr basic_string, constexpr basic_string&, string_view, string_view&, const string_view, const string_view&, constexpr string_view, constexpr string_view&,

and of course searched internet to no avail, as surprisingly little information re/ same, in particular all demonstrations found utilize a call to format w/ an argument initialized within the call, rather than as obtained via function argument, as I prefer as shown above.

Posted 17-Mar-22 13:29pm

BernardIE5317

Updated 9-Jun-22 7:17am

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k5054 17-Mar-22 22:23pm

The compiler explorer (https://godbolt.org/) has no issues compiling your code. But I did need to add the /std:c++latest flag. I tested with X64 MSVC v19.29 VS 16.10 through X64 MSVC v19.latest. Maybe there's an issue with the 2022 Preview tools?

Richard MacCutchan 18-Mar-22 5:15am

I did the same as k5054, and received the same results. I also added the following include directives:

#include <format>
#include <string_view>

Member 14138220 9-Jun-22 12:56pm

I also got these error messages recently. I have no solution to this yet. Compiler Explorer also fails to compile this code now (with x64 MSVC v19.32, https://godbolt.org/z/o8bWjPsxP).

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Member 14138220 · Answer 1 · 2022-06-09T07:17:00

The problem seems to be consteval _Basic_format_string constructor that cannot convert runtime strings like that.

std::_Basic_format_string<char> s = _format;

I went with using the underlying vformat() method that doesn't enforce this additional safety, and accepts the usual string classes just fine:

#include <iostream>
#include <format>
#include <string_view>

void foo(std::string_view _format)
{
	std::cout << std::vformat(_format, std::make_format_args());
}

int main()
{
	std::string_view _format = "Hello World\n";
	std::cout << std::vformat(_format, std::make_format_args());
	foo(_format);
}