I have a class like this :
class Actuator
{
public :
enum class Action
{
disable,
turn_on,
turn_off,
toggle
};
private:
/*Data member*/
public :
/*function member*/
};
In another class i define two 2D arrays of "Action" enum class :
class Constant_Value
{
private:
static constexpr std::array<std::array<Actuator::Action, NUMBER_OF_RELAYS>, NUMBER_OF_ACTUATORS> m_actuator_relay_default_config
{{
{{Actuator::Action::toggle, Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::disable}},
{{Actuator::Action::disable, Actuator::Action::toggle, Actuator::Action::disable, Actuator::Action::disable}},
{{Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::toggle, Actuator::Action::disable}},
{{Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::toggle}}
}};
static constexpr std::array<std::array<Actuator::Action, NUMBER_OF_LEDS>, NUMBER_OF_ACTUATORS> m_actuator_led_default_config
{{
{{Actuator::Action::toggle, Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::disable}},
{{Actuator::Action::disable, Actuator::Action::toggle, Actuator::Action::disable, Actuator::Action::disable}},
{{Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::toggle, Actuator::Action::disable}},
{{Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::toggle}}
}};
struct
{
std::array<std::array<Actuator::Action, NUMBER_OF_RELAYS>, NUMBER_OF_ACTUATORS> actuator_relay_config;
std::array<std::array<Actuator::Action, NUMBER_OF_RELAYS>, NUMBER_OF_ACTUATORS> actuator_led_config;
}m_eeprom_data;
public:
/*Function member*/
};
As name of arrays indicates, the two arrays are default values so i define those as "constexpr" and struct is a editable buffer. In constructure of "Consttant_Value" class I initialize the struct buffer as following :
Constant_Value::Constant_Value()
{
EEPROM.begin(sizeof(m_eeprom_data));
//Check if the EEPROM contains valid data from another run :
if (EEPROM.percentUsed() >= 0)
{
//Load data from eeprom
EEPROM.get(0, m_eeprom_data);
}
else
{
//Prepare default date to write to EEPROM :
m_eeprom_data.actuator_relay_config[0] = m_actuator_relay_default_config[0];
m_eeprom_data.actuator_relay_config[1] = m_actuator_relay_default_config[1];
m_eeprom_data.actuator_relay_config[2] = m_actuator_relay_default_config[2];
m_eeprom_data.actuator_relay_config[3] = m_actuator_relay_default_config[3];
m_eeprom_data.actuator_led_config[0] = m_actuator_led_default_config[0];
m_eeprom_data.actuator_led_config[1] = m_actuator_led_default_config[1];
m_eeprom_data.actuator_led_config[2] = m_actuator_led_default_config[2];
m_eeprom_data.actuator_led_config[3] = m_actuator_led_default_config[3];
// set the EEPROM data ready for writing
EEPROM.put(0, m_eeprom_data);
// write the data to EEPROM
EEPROM.commit();
}
}
When i compile above code following error occur :
/home/ali/.platformio/packages/toolchain-xtensa/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld: .pio/build/esp07/src/Constant_Value.cpp.o:(.text._ZN10my_program14Constant_ValueC2Ev+0x4): undefined reference to `my_program::Constant_Value::m_actuator_relay_default_config'
/home/ali/.platformio/packages/toolchain-xtensa/bin/../lib/gcc/xtensa-lx106-elf/4.8.2/../../../../xtensa-lx106-elf/bin/ld: .pio/build/esp07/src/Constant_Value.cpp.o:(.text._ZN10my_program14Constant_ValueC2Ev+0x8): undefined reference to `my_program::Constant_Value::m_actuator_led_default_config'
collect2: error: ld returned 1 exit status
*** [.pio/build/esp07/firmware.elf] Error 1
When i define a 1D temporary array as data member of Constant_Value class like :
static constexpr std::array<Actuator::Action, NUMBER_OF_RELAYS> test
{{Actuator::Action::toggle, Actuator::Action::disable, Actuator::Action::disable, Actuator::Action::disable}};
and assign it to first element of buffer like :
m_eeprom_data.actuator_relay_config[0] = test;
and comment other assignment lines it will compile successfully.
Your problem is related to staticvariables. The rules for static and therefore also static constexpr variables depend on the particular C++XX standard being used. If the class members are not declared static constexpr the code will work from C++11 on-wards while with it it will only work like this for C++17 and later. For versions prior to C++17 you will have to supply additional out-of-class-definitions.
The standard for static constexpr class members has changed over the years:
Prior to C++17 any static class members did not have a location in memory until the variable was defined outside the class. This way one might define it at some other place, in another source file or a library. If it is used you needed to provide an out-of-class definition.
class SomeClass {
public:
static int count;
};
// Out of class definition
int SomeClass::count = 0;
This also applied to constexpr class members as such class members have to be static and therefore had to be initialised.
In C++17 static inline class members were introduced. So somebody could define a member as inline static and provide a definition inside the class.
class SomeClass {
public:
// Works since C++17
inline static int count = 0;
};
At the same time static constexpr variables were made implicitly inline. This means any static constexpr variable would be implicitly inline static constexpr and one would have to provide a definition inside the class.
So for C++17 compilers and onwards your code is perfectly fine while with compilers prior to C++17 you would have to provide the out-of-class definitions:
constexpr std::array<std::array<Actuator::Action, NUMBER_OF_RELAYS>, NUMBER_OF_ACTUATORS> Constant_Value::m_actuator_relay_default_config;
constexpr std::array<std::array<Actuator::Action, NUMBER_OF_RELAYS>, NUMBER_OF_ACTUATORS> Constant_Value::m_actuator_led_default_config;
constexpr std::array<Actuator::Action, NUMBER_OF_RELAYS> Constant_Value::test;
Test it here.
In your case your compiler seems to be pre-C++17 and not be fully C++17 compliant (or set to C++14 or C++11): The definition of static constexpr members works with an std::array but not with an array of arrays. So something like
constexpr std::array<std::array<Actuator::Action, NUMBER_OF_RELAYS>, NUMBER_OF_ACTUATORS> Constant_Value::m_actuator_relay_default_config;
constexpr std::array<std::array<Actuator::Action, NUMBER_OF_RELAYS>, NUMBER_OF_ACTUATORS> Constant_Value::m_actuator_led_default_config;
should be sufficient with your particular compiler. This behaviour can be observed with GCC compilers compiled with C++14 but e.g. won't compile with Clang C++14. Therefore I would adise you to do so for all three of them so your code does not break with another compiler.
tl;dr: Prior to C++17 you have to proved out-of-class-definitions for all three class members m_actuator_relay_default_config, m_actuator_led_default_config and test. From C++17 onwards your code should compile fine.