How to use fmt library in the header-only mode?

Question

Having a hard time using the header-only mode of fmt library. Here is what I tried in details: I downloaded fmt7.1.3 from https://fmt.dev/latest/index.html, only put the directory fmt-7.1.3/include/fmt in a directory ([trgdir]) and wrote a test.cpp as follow:

#include <iostream>
#include <fmt/format.h>
int main() {
    fmt::format("The answer is {}.", 42);
    return 0;
}

Then in the terminal I use

gcc -I[trgdir] test.cpp

where gcc I defined as

alias gcc='gcc-10 -xc++ -lstdc++ -shared-libgcc -std=c++17 -O2 '

I got the error goes as

Undefined symbols for architecture x86_64:
  "__ZN3fmt2v76detail7vformatB5cxx11ENS0_17basic_string_viewIcEENS0_11format_argsE", referenced from:
      _main in ccEeTo0w.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status

I have checked this post but I still cannot solve my issue. How to use the fmt library without getting "Undefined symbols for architecture x86_64"

Answer 1

You need to define a macro before include, like this:

#define FMT_HEADER_ONLY
#include "fmt/format.h"