Using C++ static class members

[sigma_shig]

Hi.
I'm trying to use my own C++ classes with ESP-IDF (for VSCode). I have a problem with the compilation of very simple application:
hello_world.cpp:

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#include "Class1.h"

extern "C" void app_main(void)
{
    printf("Hello world!\n");
    Class1* c1 = new Class1();
}

Class1.h

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class Class1{
public:
    static int staticField1;
    Class1();
};

Class1.cpp

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#include "Class1.h"
#include <esp_log.h>

Class1::Class1()
{
    ESP_LOGI("Class1", "Constructor");
    staticField1 = 123;
    ESP_LOGI("Class1", "staticField1 = %d", staticField1);
}

When I tried to compile it, I have an error:

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[6/8] Linking CXX executable hello_world.elf 
FAILED: hello_world.elf 
cmd.exe /C "cd . && C:\Users\igorg\.espressif\tools\xtensa-esp32-elf\esp-2022r1-11.2.0\xtensa-esp32-elf\bin\xtensa-esp32-elf-g++.exe -mlongcalls -Wno-frame-address  @CMakeFiles\hello_world.elf.rsp -o hello_world.elf  && cd ."c:/users/igorg/.espressif/tools/xtensa-esp32-elf/esp-2022r1-11.2.0/xtensa-esp32-elf/bin/../lib/gcc/xtensa-esp32-elf/11.2.0/../../../../xtensa-esp32-elf/bin/ld.exe: esp-idf/main/libmain.a(Class1.cpp.obj):(.literal._ZN6Class1C2Ev+0x8): undefined reference to `Class1::staticField1'
collect2.exe: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.     

The application is compiled successfully when I comment the last 2 lines with staticField1 usage.
How can I use static fields inside of C++ classes?

[Craige Hales]

from https://en.cppreference.com/w/cpp/language/static

class X { static int n; }; // declaration (uses 'static')
int X::n = 1; // definition (does not use 'static')

you've declared it, but not defined it. Look at the end of the long line,

undefined reference to `Class1::staticField1'

[sigma_shig]

Thank you. Yep... It's trivial, but I forgot about it.