undefined reference to `fmt::v7::detail::vformat[abi:cxx11](fmt::v7::basic_string_view<char>, fmt::v7::format_args)

[MrClyde07]

I have been running into a compiling issue with GCC 10.2.0. Where I tried to compile my code in MVSC with the header "fmt\format.h" and it would tell me that vformat is undefined. I'm very new to programming and still learning. I need this header for the book I'm reading and I don't really want to skip anything.

`#include
#include "fmt/format.h"

int main()
{

std::cout << fmt::format("Hello your number is {}", 42) << std::endl;
return 0;`

}`

undefined reference to `fmt::v7::detail::vformat[abi:cxx11](fmt::v7::basic_string_view, fmt::v7::format_args)'
collect2.exe: error: ld returned 1 exit status

[MrClyde07]

Nevermind I figured it out...

[Blazin64]

Nevermind I figured it out...

Would you mind sharing what the solution was? I have been experiencing the same error.

Oh, I missed the last entry in the features section of the README. 😅

For anybody else who encounters this error: You need to insert #define FMT_HEADER_ONLY above #include "fmt/format.h" so your code will compile.

[ElectricRCAircraftGuy]

Wow, what a pain in the butt! If you do not include the define like this:

#define FMT_HEADER_ONLY
#include <fmt/format.h>

...then what's the solution?

UPDATE 4 May 2022: here's the answer: #2157 (comment).

In short: run cmake, make, and make install, then build with -lfmt or "/usr/local/lib/libfmt.a" as part of your g++ build command to link against the fmt library. See the link just above for full instructions.

[vitaut]

To enable the header-only mode you should define FMT_HEADER_ONLY in your build system or use fmt::fmt-header-only CMake target instead of doing it before includes.

[ElectricRCAircraftGuy]

@vitaut , thanks. How does one use the not header-only mode? I mean: what's the other way to build and use the lib if we choose to use the mode which is not header only?

[vitaut]

Please see https://fmt.dev/latest/usage.html.

[ElectricRCAircraftGuy]

@vitaut , thank you very much for responding so quickly! What I really needed, unfortunately, wasn't to be found in that documentation link, but I just figured out the following. Perhaps you can update the documentation at that link with this additional information? I had already followed those instructions to make and install the library prior to my first comment here: #2157 (comment). I had already run:

git clone https://github.com/fmtlib/fmt.git
cd fmt
mkdir -p build 
cd build
time cmake ..  # takes ~2 sec
time make      # takes ~3~5 minutes
time sudo make install  # takes ~7 sec

Then I tried to build and run with:

time g++ -Wall -Wextra -Werror -O3 -std=c++17 fmt_lib_demo.cpp -o bin/a && bin/a

and I got this error:

/tmp/ccYuiFgR.o: In function `main':
fmt_lib_demo.cpp:(.text.startup+0x121): undefined reference to `fmt::v8::vformat[abi:cxx11](fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char> >)'
fmt_lib_demo.cpp:(.text.startup+0x174): undefined reference to `fmt::v8::vprint(fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char> >)'
fmt_lib_demo.cpp:(.text.startup+0x19b): undefined reference to `fmt::v8::vprint(fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char> >)'
collect2: error: ld returned 1 exit status

When I ran sudo make install previously, however, I remembered this important output line:

-- Installing: /usr/local/lib/libfmt.a

So, I linked to that /usr/local/lib/libfmt.a library by adding either -lfmt OR "/usr/local/lib/libfmt.a" to the build command, as follows. Either of those approaches now work! Please add that info. to your documentation here (https://fmt.dev/latest/usage.html) for those, like me, who are using raw gcc/g++ to build and test, rather than a build system.

Here are the 2 solutions to link to the libfmt.a static library produced and installed by cmake and make. Both work. Neither are documented in your documentation:

# Option 1: add `-lfmt`
time g++ -Wall -Wextra -Werror -O3 -std=c++17 fmt_lib_demo.cpp -o bin/a -lfmt && bin/a

# OR: Option 2: add `"/usr/local/lib/libfmt.a"`
time g++ -Wall -Wextra -Werror -O3 -std=c++17 fmt_lib_demo.cpp "/usr/local/lib/libfmt.a" -o bin/a && bin/a

Thanks!

I'm now also enforcing compile-time error checks by adding the FMT_ENFORCE_COMPILE_STRING define to my build command, and using FMT_STRING() around all format strings, as explained here: https://fmt.dev/latest/api.html#compile-time-format-string-checks.

Here is my final build and run command:

time g++ -Wall -Wextra -Werror -O3 -std=c++17 -DFMT_ENFORCE_COMPILE_STRING fmt_lib_demo.cpp -o bin/a -lfmt && bin/a

This will be going into my eRCaGuy_hello_world demonstration code in this file here: eRCaGuy_hello_world/cpp/fmt_lib_demo.cpp.

[ElectricRCAircraftGuy]

I added my instructions here: C++ fmt library installation & setup