CODEWITHSUNDEEP
c++arduinoarduino-unoarduino-c++
mrid
mrid

Reputation: 5796

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

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 ??

Upvotes: 1

Views: 3340

Answers (1)

prog-fh
prog-fh

Reputation: 16785

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.

Upvotes: 1

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