Arduino/C++: Access specific header file of structs from library - c++

Question

(Not sure whether this is exclusively a C/C++ issue)
I’m currently fragmenting elements of a large Arduino project into reusable libraries - so far soo good.
However, a number of methods in the libraries return special structs which are declared in a data-types.h file contained in each library. The problem I have now is I'm unable to import/utilise these structs in my main sketch. I've tried declaring a variable of the DataTypes class in the main library header file and accessing the structs through it, but I get error error: invalid use of 'struct DataTypes::_theStructNameHere_t'
How would I go about accessing these structs from the library in my main sketch to declare as a variable type? I don't want to have to copy the header file which contains the structs from the library into my sketch, and I also don't want to have to create a separate library just for this single header file of structs!
Here's a quick example of what I mean:
Main.cpp:
#include <Arduino.h>
#include <MyLibrary.h>
MyLibrary myLib;
void setup() {
(This is declared in the library) myLib.dataTypes._theStructNameHere_t response = myLib.getASpecialValueWhichIsOfType_theStructNameHere_t()// Gives "error: invalid use of 'struct DataTypes::_theStructNameHere_t'""
// Example usage of the struct:
Serial.print("\n Loop Card Status: ");Serial.print(response.loop_status, HEX);
if (response.number_allocated > 0) {
Serial.print("\n Devices Allocated: ");Serial.print(response.number_allocated, HEX);
} else {
if (response.loop_status != 0x123) {
// Some condition
} else {
// Something else
}
}
}
void loop() {
...
}
Library Structure:
src/
- /data-types/
- - data-types.h
- MyLibrary.cpp
- MyLibrary.h
Library Header MyLibrary.h:
#ifndef _MYLIBRARY_H_
#define _MYLIBRARY_H_
#include <Arduino.h>
#include "./helpers/helpers.h"
...
#include "./data-types/data-types.h"
class MyLibrary {
public:
Uart *_commPort;
Helpers helpers;
...
DataTypes dataTypes;
DataTypes::_theStructNameHere_t getASpecialValueWhichIsOfType_theStructNameHere_t();
...
protected:
private:
};
#endif // _MYLIBRARY_H_
DataTypes Class data-types.h:
#ifndef _RESPONSE_TYPES_H
#define _RESPONSE_TYPES_H
class DataTypes
{
public:
struct _theStructNameHere_t
{
bool successful;
uint8_t loop_status;
uint8_t number_allocated;
uint8_t highest_address;
uint8_t number_inputs;
uint8_t number_outputs;
}
..even more..
private:
}
#endif // _RESPONSE_TYPES_H

Answer 1

I was able to obtain a MCVE from your example:
class DataTypes
{
public:
struct _theStructNameHere_t
{
};
};
class Library
{
public:
DataTypes dataTypes;
DataTypes::_theStructNameHere_t getMyDataType();
};
int main(int argc, char *argv[])
{
Library myLib;
myLib.dataTypes._theStructNameHere_t response;
}
which gives a similar error as your code:
~$ g++ test.cpp
test.cpp: In function 'int main(int, char**)':
test.cpp:20:21: error: invalid use of 'struct DataTypes::_theStructNameHere_t'
myLib.dataTypes._theStructNameHere_t response;
The problem is that you use an instance to access the struct type/name. To fix it, replace
myLib.dataTypes._theStructNameHere_t response = ...;
with
DataTypes::_theStructNameHere_t response = ...;
Notes:
Instead of using classes to create separate namespaces, please consider using namespaces directly. This is a feature of C++ which is available under Arduino.
namespace Library {
namespace DataTypes {
struct _theStructNameHere_t
{
...
};
...
} /*** namespace Library::DataTypes ***/
} /*** namespace Library ***/
Please read StackOverflow guidelines concerning how to ask a good question, in particular the section about Mininimal, Complete and Verifiable Example.
Sooner or later someone will tell you that there is no such thing as C/C++; C is C and C++ is C++; Arduino lives in its own world, even if is based on C++. Thus, you might want to remove C and C++ tags from your question.