C++ invalid conversion from 'const char' to 'char' Arduino Uno

Question

I've already gone through the following links but couldn't get it to work:

Adding to c* invalid conversion from const char* to char*"

Why is conversion from string constant to 'char*' valid in C but invalid in C++

I'm working on arduino Uno. I need to send an argument of type char* to a function. I have the following code:

const char* topic = "SampleTopic";
const char* msg = "Hello";
publish(0, 0, 0, "msgid", topic, msg);

I'm getting this error:

initializing argument 5 of 'void GSM_MQTT::publish(char, char, char, unsigned int, char*, char*)'

void publish(char DUP, char Qos, char RETAIN, unsigned int MessageID, char *Topic, char *Message);

warning: invalid conversion from 'const char*' to 'char*' [-fpermissive]

publish(0, 0, 0, _generateMessageID(), topic, msg);
                                              ^

I've even tried using const std::string& topic = "SampleTopic"; but get this error:

'string' in namespace 'std' does not name a type

Even const char* topic = (char*)"SampleTopic"; and passing it as func(topic) gives the same error.

How can I resolve this ??

Strings and string literals is an area where C and C++ differs. You have to remember that C and C++ are two very distinct and different languages, with different semantic rules and constructs. — Some programmer dude
– Some programmer dude, Commented Jul 29, 2019 at 17:23
As for your std::string error, Arduino may use C++ but IIRC it doesn't use the standard C++ library. Instead it have its own class-library, where you have for example a String class. — Some programmer dude
– Some programmer dude, Commented Jul 29, 2019 at 17:25
Oh and the first error you get doesn't match the first code snippet, please try to create a minimal reproducible example to show us. And you should never convert a const anything to non-const. You could cast const away, but it's almost always a bad idea. The error message, by the way, contains a hint about why you get the warning (you pass a const char * to a function that expects a char *). — Some programmer dude
– Some programmer dude, Commented Jul 29, 2019 at 17:27
If you are sure that publish does not modify the strings pointed to by Topic and Message, you could use an ugly cast to cast away the const: publish(0, 0, 0, (char *)topic, (char *)msg);. If publish is under your control, it would be better to change publish to use const char * parameters. — Ian Abbott
– Ian Abbott, Commented Jul 29, 2019 at 17:28
@IanAbbott using (char*)"Topic Name" but it gives me ISO C++ forbids converting a string constant to 'char*' — mrid
– mrid, Commented Jul 29, 2019 at 17:37

prog-fh · Accepted Answer · 2019-07-29 17:43:28Z

1

I don't known anything about the GSM_MQTT::publish() function but if it requires some modifiable strings (char * without const) for Topic and Message, then you have to provide modifiable strings.

char topic[] = "SampleTopic";
char msg[] = "Hello";

(you have to make sure the storage is sufficient for the modifications)

On the other hand, if this function does not actually require some modifiable strings, then it is probably a mistake in the API and these parameters should have been declared const char *.
If this is actually the case, you can safely use const_cast (or even a traditional C cast) when passing your two const char * arguments.

const char* topic = "SampleTopic";
const char* msg = "Hello";
publish(0, 0, 0, ???msgid???, const_cast<char *>(topic), const_cast<char *>(msg));

Note that in your example you use the string "msgid" where an unsigned int is expected.

edited Jul 29, 2019 at 17:43

answered Jul 29, 2019 at 17:34

prog-fh

17.3k1 gold badge21 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

KIIV Over a year ago

According to the library code these parameters are not modified inside and could be declared const(and it should be if it should be const correct).

Collectives™ on Stack Overflow

C++ invalid conversion from 'const char' to 'char' Arduino Uno

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related