?Send a temp String to a Function OR storing a C string, then send to function?

Projects Programming
1

Hey!

If I get anything wrong, let me know. :slight_smile:

I'm wondering what is better. Using a "temporary" String to send text to a function.
-or-
Store the text in a C string and then send that to a function.

I'm using the UTFT library to display stuff to my screen.

The manual says send a String or C string (which is a char array) to the print function.

So if all I want to do is print the same text every time I click a button. Which of these uses less memory and uses the memory the most responsibly?

myGLCD.print("Hello World!", 10, 150);

-or-

const char myText[13] = "Hello World!";

myGLCD.print(myText, 10, 150);

The reason I also ask is because the compiler throws this error for the first example:
ISO C++ forbids converting a string constant to 'char*'

My understanding of String and C string comes from this - The Evils of Arduino Strings | Majenko's Hardware Hacking Blog

But it doesn't cover what to do when you just need to send a simple text to a function.
.

2

can't you use c_str() to convert a String to char*?

#include <string>
#include <iostream>

using namespace std;

int
main()
{
   string SS;     // C++ STL string

   SS = "This is a string";

   cout << SS << endl;

   printf ("%s: %s\n", __func__, SS.c_str());
}
3

Yes. But that's not what I'm asking. And in Arduino, you really shouldn't use String objects. You should use C string variables. But in my two examples, I'm not using a String object per say.

(Maybe read the blog post I shared in my original post? But I'm also still trying to understand this.)

If we had the amount of memory as a regular computer, String vs C string wouldn't matter all that much.

It's because we don't have a boat load of computing power or memory in an Arduino that makes this important.

I'm wondering if I should just send the actual String to the function without creating any String object. (Which causes the compiler to throw a minor error.)

Such as this:

myGLCD.print("Hello World!", 10, 150);

(Notice that I didn't actually use a String object. I just sent the text. If I was gathering text and then sending it, I would use a C string.)

OR

Is it better on saving memory and computing power to do this? Which is to create a C string variable and then pass that to the function (Because using C string is the proper way to use long groups of text in Arduino).

Such as this:

const char myText[13] = "Hello World!";

myGLCD.print(myText, 10, 150);

(Which the compiler doesn't whine at all about.)

But which one uses less memory and less computing power?

.

4

mafokken:
I'm wondering if I should just send the actual String to the function without creating any String object. (Which causes the compiler to throw a minor error.)

myGLCD.print("Hello World!", 10, 150);

This just reflects bad implementation of the library. "Hello World!" is a const char* but print() takes char* hence the warning/error.

const char myText[13] = "Hello World!";

myGLCD.print(myText, 10, 150);

If you don't want to modify the library (which you should), this is what you have to do.

mafokken:
Is it better on saving memory and computing power to do this? Which is to create a C string variable and then pass that to the function (Because using C string is the proper way to use long groups of text in Arduino).

Modern hardware + compiler optimization render this concern pointless.

5

Ok. Thank you. I'm trying to understand that.

Are both of these considered the same then? If not, how are they different? And if you would rather point me to a resource that explains it, that would be awesome.

(I changed it a little bit.)

myGLCD.print("Hello World!", 10, 150);

// vs.

const char *myText = "Hello World!";

myGLCD.print(myText, 10, 150);

.

6

Actually,

const char myText[13] = "Hello World!";
myGLCD.print(myText, 10, 150);

is the same as:

const char* const myText = "Hello World!";
myGLCD.print(myText, 10, 150);

which will generate the same error/warning as:

myGLCD.print("Hello World!", 10, 150);

or

const char* myText = "Hello World!";
myGLCD.print(myText, 10, 150);

Working code is:

char myText[13] = "Hello World!";
myGLCD.print(myText, 10, 150);
7

Ok. From what you said, that would make sense that it should cause a Warning/Error.

But it doesn't throw a warning. Would you know why?

8

mafokken:
Ok. From what you said, that would make sense that it should cause a Warning/Error.

But it doesn't throw a warning. Would you know why?

No idea!.

9 
char myText[13] = "Hello World!";
myGLCD.print(myText, 10, 150);

Doesn't cause an error / warning because it's perfectly acceptable to initialize an char array with a c-string literal. When passed to the print function, myText is treated as a char * which is (presumably) what it's looking for.

10

gfvalvo:

char myText[13] = "Hello World!";

myGLCD.print(myText, 10, 150);



Doesn't cause an error / warning because it's perfectly acceptable to initialize an char array with a c-string literal. When passed to the print function, myText is treated as a char * which is (presumably) what it's looking for.

I think he meant:

const char myText[13] = "Hello World!";
myGLCD.print(myText, 10, 150);

does not cause error/warning.

11

Right.

12

I don't have that library, but:

void myPrint(char *ptr);

const char myText[] = "Hello World!";

void setup() {
  myPrint(myText);
}

void loop() {
}

void myPrint(char *ptr) {
  Serial.println(ptr);
}

C:\Users\GFV\AppData\Local\Temp\arduino_modified_sketch_328401\sketch_jan28b.ino: In function 'void setup()':

C:\Users\GFV\AppData\Local\Temp\arduino_modified_sketch_328401\sketch_jan28b.ino:6:17: warning: invalid conversion from 'const char*' to 'char*' [-fpermissive]

   myPrint(myText);

                 ^